"I was led in the autumn of 1784, by the event of a rude experiment,

to think it probable that the formation of dew is attended with the

production of cold. In 1788, a paper on hoar-frost, by Mr. Patrick

Wilson, of Glasgow, was published in the first volume of the

Transactions of the Royal Society of Edinburgh, by which it appeared

that this opinion bad been entertained by that gentleman before it

had occurred to myself. In the course of the same year, Mr. Six, of

Canterbury, mentioned in a paper communicated to the Royal Society

that on clear and dewy nights he always found the mercury lower in a

thermometer laid upon the ground in a meadow in his neighborhood than

it was in a similar thermometer suspended in the air six feet above the

former; and that upon one night the difference amounted to five degrees

of Fahrenheit's scale. Mr. Six, however, did not suppose, agreeably to

the opinion of Mr. Wilson and myself, that the cold was occasioned by

the formation of dew, but imagined that it proceeded partly from the

low temperature of the air, through which the dew, already formed in the

atmosphere, had descended, and partly from the evaporation of moisture

from the ground, on which his thermometer had been placed. The

conjecture of Mr. Wilson and the observations of Mr. Six, together

with many facts which I afterwards learned in the course of reading,

strengthened my opinion; but I made no attempt, before the autumn of

1811, to ascertain by experiment if it were just, though it had in

the mean time almost daily occurred to my thoughts. Happening, in

that season, to be in that country in a clear and calm night, I laid a

thermometer upon grass wet with dew, and suspended a second in the air,

two feet above the other. An hour afterwards the thermometer on the

grass was found to be eight degrees lower, by Fahrenheit's division,

than the one in the air. Similar results having been obtained from

several similar experiments, made during the same autumn, I determined

in the next spring to prosecute the subject with some degree of

steadiness, and with that view went frequently to the house of one of my

friends who lives in Surrey.

"At the end of two months I fancied that I had collected information

worthy of being published; but, fortunately, while preparing an account

of it I met by accident with a small posthumous work by Mr. Six, printed

at Canterbury in 1794, in which are related differences observed on dewy

nights between thermometers placed upon grass and others in the air that

are much greater than those mentioned in the paper presented by him to

the Royal Society in 1788. In this work, too, the cold of the grass is

attributed, in agreement with the opinion of Mr. Wilson, altogether to

the dew deposited upon it. The value of my own observations appearing to

me now much diminished, though they embraced many points left untouched

by Mr. Six, I gave up my intentions of making them known. Shortly after,

however, upon considering the subject more closely, I began to suspect

that Mr. Wilson, Mr. Six, and myself had all committed an error

regarding the cold which accompanies dew as an effect of the formation

of that fluid. I therefore resumed my experiments, and having by means

of them, I think, not only established the justness of my suspicions,

but ascertained the real cause both of dew and of several other natural

appearances which have hitherto received no sufficient explanation, I

venture now to submit to the consideration of the learned an account

of some of my labors, without regard to the order of time in which they

were performed, and of various conclusions which may be drawn from them,

mixed with facts and opinions already published by others:

"There are various occurrences in nature which seem to me strictly

allied to dew, though their relation to it be not always at first sight

perceivable. The statement and explanation of several of these will form

the concluding part of the present essay.

"1. I observed one morning, in winter, that the insides of the panes of

glass in the windows of my bedchamber were all of them moist, but that

those which had been covered by an inside shutter during the night were

much more so than the others which had been uncovered. Supposing that

this diversity of appearance depended upon a difference of temperature,

I applied the naked bulbs of two delicate thermometers to a covered

and uncovered pane; on which I found that the former was three degrees

colder than the latter. The air of the chamber, though no fire was kept

in it, was at this time eleven and one-half degrees warmer than that

without. Similar experiments were made on many other mornings, the

results of which were that the warmth of the internal air exceeded that

of the external from eight to eighteen degrees, the temperature of the

covered panes would be from one to five degrees less than the uncovered;

that the covered were sometimes dewed, while the uncovered were dry;

that at other times both were free from moisture; that the outsides of

the covered and uncovered panes had similar differences with respect to

heat, though not so great as those of the inner surfaces; and that no

variation in the quantity of these differences was occasioned by the

weather's being cloudy or fair, provided the heat of the internal air

exceeded that of the external equally in both of those states of the

atmosphere.

"The remote reason of these differences did not immediately present

itself. I soon, however, saw that the closed shutter shielded the glass

which it covered from the heat that was radiated to the windows by

the walls and furniture of the room, and thus kept it nearer to the

temperature of the external air than those parts could be which, from

being uncovered, received the heat emitted to them by the bodies just

mentioned.

"In making these experiments, I seldom observed the inside of any pane

to be more than a little damped, though it might be from eight to twelve

degrees colder than the general mass of the air in the room; while, in

the open air, I had often found a great dew to form on substances

only three or four degrees colder than the atmosphere. This at first

surprised me; but the cause now seems plain. The air of the chamber had

once been a portion of the external atmosphere, and had afterwards

been heated, when it could receive little accessories to its original

moisture. It constantly required being cooled considerably before it

was even brought back to its former nearness to repletion with water;

whereas the whole external air is commonly, at night, nearly replete

with moisture, and therefore readily precipitates dew on bodies only a

little colder than itself.

"When the air of a room is warmer than the external atmosphere, the

effect of an outside shutter on the temperature of the glass of the

window will be directly opposite to what has just been stated; since

it must prevent the radiation, into the atmosphere, of the heat of the

chamber transmitted through the glass.

"2. Count Rumford appears to have rightly conjectured that the

inhabitants of certain hot countries, who sleep at nights on the tops of

their houses, are cooled during this exposure by the radiation of their

heat to the sky; or, according to his manner of expression, by receiving

frigorific rays from the heavens. Another fact of this kind seems to be

the greater chill which we often experience upon passing at night from

the cover of a house into the air than might have been expected from the

cold of the external atmosphere. The cause, indeed, is said to be the

quickness of transition from one situation to another. But if this were

the whole reason, an equal chill would be felt in the day, when the

difference, in point of heat, between the internal and external air was

the same as at night, which is not the case. Besides, if I can trust my

own observation, the feeling of cold from this cause is more remarkable

in a clear than in a cloudy night, and in the country than in towns. The

following appears to be the manner in which these things are chiefly to

be explained:

"During the day our bodies while in the open air, although not

immediately exposed to the sun's rays, are yet constantly deriving

heat from them by means of the reflection of the atmosphere. This heat,

though it produces little change on the temperature of the air which it

traverses, affords us some compensation for the heat which we radiate to

the heavens. At night, also, if the sky be overcast, some compensation

will be made to us, both in the town and in the country, though in a

less degree than during the day, as the clouds will remit towards the

earth no inconsiderable quantity of heat. But on a clear night, in an

open part of the country, nothing almost can be returned to us from

above in place of the heat which we radiate upward. In towns, however,

some compensation will be afforded even on the clearest nights for the

heat which we lose in the open air by that which is radiated to us from

the sun round buildings.

"To our loss of heat by radiation at times that we derive little

compensation from the radiation of other bodies is probably to be

attributed a great part of the hurtful effects of the night air.

Descartes says that these are not owing to dew, as was the common

opinion of his contemporaries, but to the descent of certain noxious

vapors which have been exhaled from the earth during the heat of the

day, and are afterwards condensed by the cold of a serene night. The

effects in question certainly cannot be occasioned by dew, since that

fluid does not form upon a healthy human body in temperate climates; but

they may, notwithstanding, arise from the same cause that produces dew

on those substances which do not, like the human body, possess the power

of generating heat for the supply of what they lose by radiation or any

other means."(2)

This explanation made it plain why dew forms on a clear night, when

there are no clouds to reflect the radiant heat. Combined with Dalton's

theory that vapor is an independent gas, limited in quantity in any

given space by the temperature of that space, it solved the problem of

the formation of clouds, rain, snow, and hoar-frost. Thus this paper

of Wells's closed the epoch of speculation regarding this field of

meteorology, as Hutton's paper of 1784 had opened it. The fact that the

volume containing Hutton's paper contained also his epoch-making paper

on geology finds curiously a duplication in the fact that Wells's volume

contained also his essay on Albinism, in which the doctrine of natural

selection was for the first time formulated, as Charles Darwin freely

admitted after his own efforts had made the doctrine famous.

ISOTHERMS AND OCEAN CURRENTS

The very next year after Dr. Wells's paper was published there appeared

in France the third volume of the Memoires de Physique et de Chimie de

la Societe d'Arcueil, and a new epoch in meteorology was inaugurated.

The society in question was numerically an inconsequential band, listing

only a dozen members; but every name was a famous one: Arago, Berard,

Berthollet, Biot, Chaptal, De Candolle, Dulong, Gay-Lussac, Humboldt,

Laplace, Poisson, and Thenard--rare spirits every one. Little danger

that the memoirs of such a band would be relegated to the dusty shelves

where most proceedings of societies belong--no milk-for-babes fare would

be served to such a company.

The particular paper which here interests us closes this third and

last volume of memoirs. It is entitled "Des Lignes Isothermes et de

la Distribution de la Chaleursurle Globe." The author is Alexander

Humboldt. Needless to say, the topic is handled in a masterly

manner. The distribution of heat on the surface of the globe, on the

mountain-sides, in the interior of the earth; the causes that regulate

such distribution; the climatic results--these are the topics discussed.

But what gives epochal character to the paper is the introduction of

those isothermal lines circling the earth in irregular course, joining

together places having the same mean annual temperature, and thus laying

the foundation for a science of comparative climatology.

It is true the attempt to study climates comparatively was not new.

Mairan had attempted it in those papers in which he developed his

bizarre ideas as to central emanations of heat. Euler had brought

his profound mathematical genius to bear on the topic, evolving the

"extraordinary conclusion that under the equator at midnight the

cold ought to be more rigorous than at the poles in winter." And

in particular Richard Kirwan, the English chemist, had combined the

mathematical and the empirical methods and calculated temperatures for

all latitudes. But Humboldt differs from all these predecessors in that

he grasps the idea that the basis of all such computations should be

not theory, but fact. He drew his isothermal lines not where some occult

calculation would locate them on an ideal globe, but where practical

tests with the thermometer locate them on our globe as it is. London,

for example, lies in the same latitude as the southern extremity of

Hudson Bay; but the isotherm of London, as Humboldt outlines it, passes

through Cincinnati.

Of course such deviations of climatic conditions between places in the

same latitude had long been known. As Humboldt himself observes,

the earliest settlers of America were astonished to find themselves

subjected to rigors of climate for which their European experience had

not at all prepared them. Moreover, sagacious travellers, in particular

Cook's companion on his second voyage, young George Forster, had

noted as a general principle that the western borders of continents

in temperate regions are always warmer than corresponding latitudes of

their eastern borders; and of course the general truth of temperatures

being milder in the vicinity of the sea than in the interior of

continents had long been familiar. But Humboldt's isothermal lines for

the first time gave tangibility to these ideas, and made practicable a

truly scientific study of comparative climatology.

In studying these lines, particularly as elaborated by further

observations, it became clear that they are by no means haphazard in

arrangement, but are dependent upon geographical conditions which in

most cases are not difficult to determine. Humboldt himself pointed out

very clearly the main causes that tend to produce deviations from the

average--or, as Dove later on called it, the normal--temperature of any

given latitude. For example, the mean annual temperature of a region

(referring mainly to the northern hemisphere) is raised by the proximity

of a western coast; by a divided configuration of the continent into

peninsulas; by the existence of open seas to the north or of radiating

continental surfaces to the south; by mountain ranges to shield from

cold winds; by the infrequency of swamps to become congealed; by the

absence of woods in a dry, sandy soil; and by the serenity of sky in the

summer months and the vicinity of an ocean current bringing water which

is of a higher temperature than that of the surrounding sea.

Conditions opposite to these tend, of course, correspondingly to lower

the temperature. In a word, Humboldt says the climatic distribution of

heat depends on the relative distribution of land and sea, and on the

"hypsometrical configuration of the continents"; and he urges that

"great meteorological phenomena cannot be comprehended when considered

independently of geognostic relations"--a truth which, like most other

general principles, seems simple enough once it is pointed out.

With that broad sweep of imagination which characterized him, Humboldt

speaks of the atmosphere as the "aerial ocean, in the lower strata

and on the shoals of which we live," and he studies the atmospheric

phenomena always in relation to those of that other ocean of water. In

each of these oceans there are vast permanent currents, flowing

always in determinate directions, which enormously modify the climatic

conditions of every zone. The ocean of air is a vast maelstrom, boiling

up always under the influence of the sun's heat at the equator, and

flowing as an upper current towards either pole, while an undercurrent

from the poles, which becomes the trade-winds, flows towards the equator

to supply its place.

But the superheated equatorial air, becoming chilled, descends to the

surface in temperate latitudes, and continues its poleward journey as

the anti-trade-winds. The trade-winds are deflected towards the west,

because in approaching the equator they constantly pass over surfaces of

the earth having a greater and greater velocity of rotation, and so, as

it were, tend to lag behind--an explanation which Hadley pointed out in

1735, but which was not accepted until Dalton independently worked it

out and promulgated it in 1793. For the opposite reason, the anti-trades

are deflected towards the east; hence it is that the western, borders

of continents in temperate zones are bathed in moist sea-breezes, while

their eastern borders lack this cold-dispelling influence.

In the ocean of water the main currents run as more sharply

circumscribed streams--veritable rivers in the sea. Of these the best

known and most sharply circumscribed is the familiar Gulf Stream,

which has its origin in an equatorial current, impelled westward by

trade-winds, which is deflected northward in the main at Cape St. Roque,

entering the Caribbean Sea and Gulf of Mexico, to emerge finally through

the Strait of Florida, and journey off across the Atlantic to warm the

shores of Europe.

Such, at least, is the Gulf Stream as Humboldt understood it. Since his

time, however, ocean currents in general, and this one in particular,

have been the subject of no end of controversy, it being hotly disputed

whether either causes or effects of the Gulf Stream are just what

Humboldt, in common with others of his time, conceived them to be. About

the middle of the century Lieutenant M. F. Maury, the distinguished

American hydrographer and meteorologist, advocated a theory of

gravitation as the chief cause of the currents, claiming that difference

in density, due to difference in temperature and saltness, would

sufficiently account for the oceanic circulation. This theory gained

great popularity through the wide circulation of Maury's Physical

Geography of the Sea, which is said to have passed through more editions

than any other scientific book of the period; but it was ably and

vigorously combated by Dr. James Croll, the Scottish geologist, in his

Climate and Time, and latterly the old theory that ocean currents are

due to the trade-winds has again come into favor. Indeed, very recently

a model has been constructed, with the aid of which it is said to have

been demonstrated that prevailing winds in the direction of the actual

trade-winds would produce such a current as the Gulf Stream.

Meantime, however, it is by no means sure that gravitation does not

enter into the case to the extent of producing an insensible general

oceanic circulation, independent of the Gulf Stream and similar marked

currents, and similar in its larger outlines to the polar-equatorial

circulation of the air. The idea of such oceanic circulation was first

suggested in detail by Professor Lenz, of St. Petersburg, in 1845, but

it was not generally recognized until Dr. Carpenter independently hit

upon the idea more than twenty years later. The plausibility of the

conception is obvious; yet the alleged fact of such circulation has been

hotly disputed, and the question is still sub judice.

But whether or not such general circulation of ocean water takes place,

it is beyond dispute that the recognized currents carry an enormous

quantity of heat from the tropics towards the poles. Dr. Croll, who has

perhaps given more attention to the physics of the subject than almost

any other person, computes that the Gulf Stream conveys to the North

Atlantic one-fourth as much heat as that body receives directly from the

sun, and he argues that were it not for the transportation of heat by

this and similar Pacific currents, only a narrow tropical region of the

globe would be warm enough for habitation by the existing faunas. Dr.

Croll argues that a slight change in the relative values of northern

and southern trade-winds (such as he believes has taken place at various

periods in the past) would suffice to so alter the equatorial current

which now feeds the Gulf Stream that its main bulk would be deflected

southward instead of northward, by the angle of Cape St. Roque. Thus the

Gulf Stream would be nipped in the bud, and, according to Dr. Croll's

estimates, the results would be disastrous for the northern hemisphere.

The anti-trades, which now are warmed by the Gulf Stream, would then

blow as cold winds across the shores of western Europe, and in all

probability a glacial epoch would supervene throughout the northern

hemisphere.

The same consequences, so far as Europe is concerned at least, would

apparently ensue were the Isthmus of Panama to settle into the sea,

allowing the Caribbean current to pass into the Pacific. But the

geologist tells us that this isthmus rose at a comparatively recent

geological period, though it is hinted that there had been some time

previously a temporary land connection between the two continents. Are

we to infer, then, that the two Americas in their unions and disunions

have juggled with the climate of the other hemisphere? Apparently so, if

the estimates made of the influence of the Gulf Stream be tenable. It is

a far cry from Panama to Russia. Yet it seems within the possibilities

that the meteorologist may learn from the geologist of Central America

something that will enable him to explain to the paleontologist of

Europe how it chanced that at one time the mammoth and rhinoceros roamed

across northern Siberia, while at another time the reindeer and musk-ox

browsed along the shores of the Mediterranean.

Possibilities, I said, not probabilities. Yet even the faint glimmer of

so alluring a possibility brings home to one with vividness the truth

of Humboldt's perspicuous observation that meteorology can be properly

comprehended only when studied in connection with the companion

sciences. There are no isolated phenomena in nature.

CYCLONES AND ANTI-CYCLONES

Yet, after all, it is not to be denied that the chief concern of the

meteorologist must be with that other medium, the "ocean of air, on

the shoals of which we live." For whatever may be accomplished by water

currents in the way of conveying heat, it is the wind currents that

effect the final distribution of that heat. As Dr. Croll has urged, the

waters of the Gulf Stream do not warm the shores of Europe by direct

contact, but by warming the anti-trade-winds, which subsequently blow

across the continent. And everywhere the heat accumulated by water

becomes effectual in modifying climate, not so much by direct radiation

as by diffusion through the medium of the air.

This very obvious importance of aerial currents led to their practical

study long before meteorology had any title to the rank of science, and

Dalton's explanation of the trade-winds had laid the foundation for a

science of wind dynamics before the beginning of the nineteenth century.

But no substantial further advance in this direction was effected until

about 1827, when Heinrich W. Dove, of Konigsberg, afterwards to be known

as perhaps the foremost meteorologist of his generation, included

the winds among the subjects of his elaborate statistical studies in

climatology.

Dove classified the winds as permanent, periodical, and variable. His

great discovery was that all winds, of whatever character, and not

merely the permanent winds, come under the influence of the earth's

rotation in such a way as to be deflected from their course, and hence

to take on a gyratory motion--that, in short, all local winds are minor

eddies in the great polar-equatorial whirl, and tend to reproduce in

miniature the character of that vast maelstrom. For the first time,

then, temporary or variable winds were seen to lie within the province

of law.

A generation later, Professor William Ferrel, the American

meteorologist, who had been led to take up the subject by a perusal of

Maury's discourse on ocean winds, formulated a general mathematical law,

to the effect that any body moving in a right line along the surface of

the earth in any direction tends to have its course deflected, owing to

the earth's rotation, to the right hand in the northern and to the left

hand in the southern hemisphere. This law had indeed been stated as

early as 1835 by the French physicist Poisson, but no one then thought

of it as other than a mathematical curiosity; its true significance was

only understood after Professor Ferrel had independently rediscovered it

(just as Dalton rediscovered Hadley's forgotten law of the trade-winds)

and applied it to the motion of wind currents.

Then it became clear that here is a key to the phenomena of atmospheric

circulation, from the great polar-equatorial maelstrom which manifests

itself in the trade-winds to the most circumscribed riffle which is

announced as a local storm. And the more the phenomena were studied,

the more striking seemed the parallel between the greater maelstrom

and these lesser eddies. Just as the entire atmospheric mass of each

hemisphere is seen, when viewed as a whole, to be carried in a great

whirl about the pole of that hemisphere, so the local disturbances

within this great tide are found always to take the form of whirls about

a local storm-centre--which storm-centre, meantime, is carried along

in the major current, as one often sees a little whirlpool in the water

swept along with the main current of the stream. Sometimes, indeed, the

local eddy, caught as it were in an ancillary current of the great

polar stream, is deflected from its normal course and may seem to travel

against the stream; but such deviations are departures from the rule. In

the great majority of cases, for example, in the north temperate zone, a

storm-centre (with its attendant local whirl) travels to the northeast,

along the main current of the anti-trade-wind, of which it is a part;

and though exceptionally its course may be to the southeast instead, it

almost never departs so widely from the main channel as to progress to

the westward. Thus it is that storms sweeping over the United States can

be announced, as a rule, at the seaboard in advance of their coming by

telegraphic communication from the interior, while similar storms

come to Europe off the ocean unannounced. Hence the more practical

availability of the forecasts of weather bureaus in the former country.

But these local whirls, it must be understood, are local only in a very

general sense of the word, inasmuch as a single one may be more than

a thousand miles in diameter, and a small one is two or three hundred

miles across. But quite without regard to the size of the whirl, the air

composing it conducts itself always in one of two ways. It never whirls

in concentric circles; it always either rushes in towards the centre in

a descending spiral, in which case it is called a cyclone, or it spreads

out from the centre in a widening spiral, in which case it is called an

anti-cyclone. The word cyclone is associated in popular phraseology with

a terrific storm, but it has no such restriction in technical usage. A

gentle zephyr flowing towards a "storm-centre" is just as much a

cyclone to the meteorologist as is the whirl constituting a West-Indian

hurricane. Indeed, it is not properly the wind itself that is called the

cyclone in either case, but the entire system of whirls--including the

storm-centre itself, where there may be no wind at all.

What, then, is this storm-centre? Merely an area of low barometric

pressure--an area where the air has become lighter than the air of

surrounding regions. Under influence of gravitation the air seeks its

level just as water does; so the heavy air comes flowing in from

all sides towards the low-pressure area, which thus becomes a

"storm-centre." But the inrushing currents never come straight to their

mark. In accordance with Ferrel's law, they are deflected to the right,

and the result, as will readily be seen, must be a vortex current, which

whirls always in one direction--namely, from left to right, or in the

direction opposite to that of the hands of a watch held with its face

upward. The velocity of the cyclonic currents will depend largely upon

the difference in barometric pressure between the storm-centre and the

confines of the cyclone system. And the velocity of the currents will

determine to some extent the degree of deflection, and hence the exact

path of the descending spiral in which the wind approaches the centre.

But in every case and in every part of the cyclone system it is true, as

Buys Ballot's famous rule first pointed out, that a person standing with

his back to the wind has the storm-centre at his left.

The primary cause of the low barometric pressure which marks the

storm-centre and establishes the cyclone is expansion of the air through

excess of temperature. The heated air, rising into cold upper regions,

has a portion of its vapor condensed into clouds, and now a new dynamic

factor is added, for each particle of vapor, in condensing, gives up its

modicum of latent heat. Each pound of vapor thus liberates, according

to Professor Tyndall's estimate, enough heat to melt five pounds of cast

iron; so the amount given out where large masses of cloud are forming

must enormously add to the convection currents of the air, and hence to

the storm-developing power of the forming cyclone. Indeed, one school

of meteorologists, of whom Professor Espy was the leader, has held that,

without such added increment of energy constantly augmenting the dynamic

effects, no storm could long continue in violent action. And it is

doubted whether any storm could ever attain, much less continue, the

terrific force of that most dreaded of winds of temperate zones, the

tornado--a storm which obeys all the laws of cyclones, but differs from

ordinary cyclones in having a vortex core only a few feet or yards in

diameter--without the aid of those great masses of condensing vapor

which always accompany it in the form of storm-clouds.

The anti-cyclone simply reverses the conditions of the cyclone. Its

centre is an area of high pressure, and the air rushes out from it in

all directions towards surrounding regions of low pressure. As before,

all parts of the current will be deflected towards the right, and

the result, clearly, is a whirl opposite in direction to that of the

cyclone. But here there is a tendency to dissipation rather than to

concentration of energy, hence, considered as a storm-generator, the

anti-cyclone is of relative insignificance.

In particular the professional meteorologist who conducts a "weather

bureau"--as, for example, the chief of the United States signal-service

station in New York--is so preoccupied with the observation of this

phenomenon that cyclone-hunting might be said to be his chief pursuit.

It is for this purpose, in the main, that government weather bureaus

or signal-service departments have been established all over the world.

Their chief work is to follow up cyclones, with the aid of telegraphic

reports, mapping their course and recording the attendant meteorological

conditions. Their so-called predictions or forecasts are essentially

predications, gaining locally the effect of predictions because the

telegraph outstrips the wind.

At only one place on the globe has it been possible as yet for the

meteorologist to make long-time forecasts meriting the title of

predictions. This is in the middle Ganges Valley of northern India.

In this country the climatic conditions are largely dependent upon the

periodical winds called monsoons, which blow steadily landward from

April to October, and seaward from October to April. The summer monsoons

bring the all-essential rains; if they are delayed or restricted

in extent, there will be drought and consequent famine. And such

restriction of the monsoon is likely to result when there has been an

unusually deep or very late snowfall on the Himalayas, because of the

lowering of spring temperature by the melting snow. Thus here it is

possible, by observing the snowfall in the mountains, to predict with

some measure of success the average rainfall of the following summer.

The drought of 1896, with the consequent famine and plague that

devastated India the following winter, was thus predicted some months in

advance.

This is the greatest present triumph of practical meteorology. Nothing

like it is yet possible anywhere in temperate zones. But no one can

say what may not be possible in times to come, when the data now being

gathered all over the world shall at last be co-ordinated, classified,

and made the basis of broad inductions. Meteorology is pre-eminently a

science of the future.

VI. MODERN THEORIES OF HEAT AND LIGHT

THE eighteenth-century philosopher made great strides in his studies

of the physical properties of matter and the application of these

properties in mechanics, as the steam-engine, the balloon, the optic

telegraph, the spinning-jenny, the cotton-gin, the chronometer, the

perfected compass, the Leyden jar, the lightning-rod, and a host of

minor inventions testify. In a speculative way he had thought out more

or less tenable conceptions as to the ultimate nature of matter, as

witness the theories of Leibnitz and Boscovich and Davy, to which we

may recur. But he had not as yet conceived the notion of a distinction

between matter and energy, which is so fundamental to the physics of a

later epoch. He did not speak of heat, light, electricity, as forms

of energy or "force"; he conceived them as subtile forms of matter--as

highly attenuated yet tangible fluids, subject to gravitation and

chemical attraction; though he had learned to measure none of them but

heat with accuracy, and this one he could test only within narrow limits

until late in the century, when Josiah Wedgwood, the famous potter,

taught him to gauge the highest temperatures with the clay pyrometer.

He spoke of the matter of heat as being the most universally distributed

fluid in nature; as entering in some degree into the composition of

nearly all other substances; as being sometimes liquid, sometimes

condensed or solid, and as having weight that could be detected with

the balance. Following Newton, he spoke of light as a "corpuscular

emanation" or fluid, composed of shining particles which possibly are

transmutable into particles of heat, and which enter into chemical

combination with the particles of other forms of matter. Electricity

he considered a still more subtile kind of matter-perhaps an attenuated

form of light. Magnetism, "vital fluid," and by some even a "gravic

fluid," and a fluid of sound were placed in the same scale; and, taken

together, all these supposed subtile forms of matter were classed as

"imponderables."

This view of the nature of the "imponderables" was in some measure a

retrogression, for many seventeenth-century philosophers, notably

Hooke and Huygens and Boyle, had held more correct views; but the

materialistic conception accorded so well with the eighteenth-century

tendencies of thought that only here and there a philosopher like Euler

called it in question, until well on towards the close of the century.

Current speech referred to the materiality of the "imponderables"

unquestioningly. Students of meteorology--a science that was just

dawning--explained atmospheric phenomena on the supposition that heat,

the heaviest imponderable, predominated in the lower atmosphere, and

that light, electricity, and magnetism prevailed in successively higher

strata. And Lavoisier, the most philosophical chemist of the century,

retained heat and light on a par with oxygen, hydrogen, iron, and the

rest, in his list of elementary substances.

COUNT RUMFORD AND THE VIBRATORY THEORY OF HEAT

But just at the close of the century the confidence in the status of

the imponderables was rudely shaken in the minds of philosophers by the

revival of the old idea of Fra Paolo and Bacon and Boyle, that heat,

at any rate, is not a material fluid, but merely a mode of motion or

vibration among the particles of "ponderable" matter. The new champion

of the old doctrine as to the nature of heat was a very distinguished

philosopher and diplomatist of the time, who, it may be worth recalling,

was an American. He was a sadly expatriated American, it is true, as his

name, given all the official appendages, will amply testify; but he had

been born and reared in a Massachusetts village none the less, and

he seems always to have retained a kindly interest in the land of his

nativity, even though he lived abroad in the service of other powers

during all the later years of his life, and was knighted by England,

ennobled by Bavaria, and honored by the most distinguished scientific

bodies of Europe. The American, then, who championed the vibratory

theory of heat, in opposition to all current opinion, in this closing

era of the eighteenth century, was Lieutenant-General Sir Benjamin

Thompson, Count Rumford, F.R.S.

Rumford showed that heat may be produced in indefinite quantities by

friction of bodies that do not themselves lose any appreciable matter

in the process, and claimed that this proves the immateriality of heat.

Later on he added force to the argument by proving, in refutation of the

experiments of Bowditch, that no body either gains or loses weight in

virtue of being heated or cooled. He thought he had proved that heat is

only a form of motion.

His experiment for producing indefinite quantities of heat by friction

is recorded by him in his paper entitled, "Inquiry Concerning the Source

of Heat Excited by Friction."

"Being engaged, lately, in superintending the boring of cannon in the

workshops of the military arsenal at Munich," he says, "I was struck

with the very considerable degree of heat which a brass gun acquires in

a short time in being bored; and with the still more intense heat (much

greater than that of boiling water, as I found by experiment) of the

metallic chips separated from it by the borer.

"Taking a cannon (a brass six-pounder), cast solid, and rough, as it

came from the foundry, and fixing it horizontally in a machine used

for boring, and at the same time finishing the outside of the cannon by

turning, I caused its extremity to be cut off; and by turning down

the metal in that part, a solid cylinder was formed, 7 3/4 inches in

diameter and 9 8/10 inches long; which, when finished, remained joined

to the rest of the metal (that which, properly speaking, constituted the

cannon) by a small cylindrical neck, only 2 1/5 inches in diameter and 3

8/10 inches long.

"This short cylinder, which was supported in its horizontal position,

and turned round its axis by means of the neck by which it remained

united to the cannon, was now bored with the horizontal borer used in

boring cannon.

"This cylinder being designed for the express purpose of generating heat

by friction, by having a blunt borer forced against its solid bottom at

the same time that it should be turned round its axis by the force of

horses, in order that the heat accumulated in the cylinder might from

time to time be measured, a small, round hole 0.37 of an inch only in

diameter and 4.2 inches in depth, for the purpose of introducing a small

cylindrical mercurial thermometer, was made in it, on one side, in a

direction perpendicular to the axis of the cylinder, and ending in the

middle of the solid part of the metal which formed the bottom of the

bore.

"At the beginning of the experiment, the temperature of the air in the

shade, as also in the cylinder, was just sixty degrees Fahrenheit. At

the end of thirty minutes, when the cylinder had made 960 revolutions

about its axis, the horses being stopped, a cylindrical mercury

thermometer, whose bulb was 32/100 of an inch in diameter and 3 1/4

inches in length, was introduced into the hole made to receive it in

the side of the cylinder, when the mercury rose almost instantly to one

hundred and thirty degrees.

"In order, by one decisive experiment, to determine whether the air

of the atmosphere had any part or not in the generation of the heat, I

contrived to repeat the experiment under circumstances in which it was

evidently impossible for it to produce any effect whatever. By means

of a piston exactly fitted to the mouth of the bore of the cylinder,

through the middle of which piston the square iron bar, to the end of

which the blunt steel borer was fixed, passed in a square hole made

perfectly air-tight, the excess of the external air, to the inside of

the bore of the cylinder, was effectually prevented. I did not find,

however, by this experiment that the exclusion of the air diminished in

the smallest degree the quantity of heat excited by the friction.

"There still remained one doubt, which, though it appeared to me to be

so slight as hardly to deserve any attention, I was, however, desirous

to remove. The piston which choked the mouth of the bore of the

cylinder, in order that it might be air-tight, was fitted into it with

so much nicety, by means of its collars of leather, and pressed against

it with so much force, that, notwithstanding its being oiled, it

occasioned a considerable degree of friction when the hollow cylinder

was turned round its axis. Was not the heat produced, or at least some

part of it, occasioned by this friction of the piston? and, as the

external air had free access to the extremity of the bore, where it came

into contact with the piston, is it not possible that this air may have

had some share in the generation of the heat produced?

"A quadrangular oblong deal box, water-tight, being provided with

holes or slits in the middle of each of its ends, just large enough to

receive, the one the square iron rod to the end of which the blunt steel

borer was fastened, the other the small cylindrical neck which joined

the hollow cylinder to the cannon; when this box (which was occasionally

closed above by a wooden cover or lid moving on hinges) was put into

its place--that is to say, when, by means of the two vertical opening

or slits in its two ends, the box was fixed to the machinery in such

a manner that its bottom being in the plane of the horizon, its axis

coincided with the axis of the hollow metallic cylinder, it is evident,

from the description, that the hollow, metallic cylinder would occupy

the middle of the box, without touching it on either side; and that,

on pouring water into the box and filling it to the brim, the cylinder

would be completely covered and surrounded on every side by that fluid.

And, further, as the box was held fast by the strong, square iron rod

which passed in a square hole in the centre of one of its ends, while

the round or cylindrical neck which joined the hollow cylinder to the

end of the cannon could turn round freely on its axis in the round hole

in the centre of the other end of it, it is evident that the machinery

could be put in motion without the least danger of forcing the box out

of its place, throwing the water out of it, or deranging any part of the

apparatus."

Everything being thus ready, the box was filled with cold water, having

been made water-tight by means of leather collars, and the machinery put

in motion. "The result of this beautiful experiment," says Rumford, "was

very striking, and the pleasure it afforded me amply repaid me for

all the trouble I had had in contriving and arranging the complicated

machinery used in making it. The cylinder, revolving at the rate of

thirty-two times in a minute, had been in motion but a short time when I

perceived, by putting my hand into the water and touching the outside

of the cylinder, that heat was generated, and it was not long before the

water which surrounded the cylinder began to be sensibly warm.

"At the end of one hour I found, by plunging a thermometer into the

box,... that its temperature had been raised no less than forty-seven

degrees Fahrenheit, being now one hundred and seven degrees Fahrenheit.

... One hour and thirty minutes after the machinery had been put in

motion the heat of the water in the box was one hundred and forty-two

degrees. At the end of two hours... it was raised to one hundred and

seventy-eight degrees; and at two hours and thirty minutes it ACTUALLY

BOILED!

"It would be difficult to describe the surprise and astonishment

expressed in the countenances of the bystanders on seeing so large a

quantity of cold water heated, and actually made to boil, without any

fire. Though there was, in fact, nothing that could justly be considered

as a surprise in this event, yet I acknowledge fairly that it afforded

me a degree of childish pleasure which, were I ambitious of the

reputation of a GRAVE PHILOSOPHER, I ought most certainly rather to hide

than to discover...."

Having thus dwelt in detail on these experiments, Rumford comes now to

the all-important discussion as to the significance of them--the

subject that had been the source of so much speculation among the

philosophers--the question as to what heat really is, and if there

really is any such thing (as many believed) as an igneous fluid, or a

something called caloric.

"From whence came this heat which was continually given off in this

manner, in the foregoing experiments?" asks Rumford. "Was it furnished

by the small particles of metal detached from the larger solid masses

on their being rubbed together? This, as we have already seen, could not

possibly have been the case.

"Was it furnished by the air? This could not have been the case; for,

in three of the experiments, the machinery being kept immersed in water,

the access of the air of the atmosphere was completely prevented.

"Was it furnished by the water which surrounded the machinery? That this

could not have been the case is evident: first, because this water was

continually RECEIVING heat from the machinery, and could not, at the

same time, be GIVING TO and RECEIVING HEAT FROM the same body; and,

secondly, because there was no chemical decomposition of any part of

this water. Had any such decomposition taken place (which, indeed, could

not reasonably have been expected), one of its component elastic fluids

(most probably hydrogen) must, at the same time, have been set at

liberty, and, in making its escape into the atmosphere, would have been

detected; but, though I frequently examined the water to see if any

air-bubbles rose up through it, and had even made preparations for

catching them if they should appear, I could perceive none; nor was

there any sign of decomposition of any kind whatever, or other chemical

process, going on in the water.

"Is it possible that the heat could have been supplied by means of the

iron bar to the end of which the blunt steel borer was fixed? Or by the

small neck of gun-metal by which the hollow cylinder was united to the

cannon? These suppositions seem more improbable even than either of

the before-mentioned; for heat was continually going off, or OUT OF THE

MACHINERY, by both these passages during the whole time the experiment

lasted.

"And in reasoning on this subject we must not forget to consider that

most remarkable circumstance, that the source of the heat generated by

friction in these experiments appeared evidently to be INEXHAUSTIBLE.

"It is hardly necessary to add that anything which any INSULATED body,

or system of bodies, can continue to furnish WITHOUT LIMITATION cannot

possibly be a MATERIAL substance; and it appears to me to be extremely

difficult, if not quite impossible, to form any distinct idea of

anything capable of being excited and communicated, in the manner

the heat was excited and communicated in these experiments, except in

MOTION."(1)

THOMAS YOUNG AND THE WAVE THEORY OF LIGHT

But contemporary judgment, while it listened respectfully to Rumford,

was little minded to accept his verdict. The cherished beliefs of a

generation are not to be put down with a single blow. Where many minds

have a similar drift, however, the first blow may precipitate a

general conflict; and so it was here. Young Humphry Davy had duplicated

Rumford's experiments, and reached similar conclusions; and soon others

fell into line. Then, in 1800, Dr. Thomas Young--"Phenomenon Young" they

called him at Cambridge, because he was reputed to know everything--took

up the cudgels for the vibratory theory of light, and it began to be

clear that the two "imponderables," heat and light, must stand or

fall together; but no one as yet made a claim against the fluidity of

electricity.

Before we take up the details of the assault made by Young upon the

old doctrine of the materiality of light, we must pause to consider the

personality of Young himself. For it chanced that this Quaker physician

was one of those prodigies who come but few times in a century, and

the full list of whom in the records of history could be told on one's

thumbs and fingers. His biographers tell us things about him that read

like the most patent fairy-tales. As a mere infant in arms he had been

able to read fluently. Before his fourth birthday came he had read the

Bible twice through, as well as Watts's Hymns--poor child!--and when

seven or eight he had shown a propensity to absorb languages much as

other children absorb nursery tattle and Mother Goose rhymes. When

he was fourteen, a young lady visiting the household of his tutor

patronized the pretty boy by asking to see a specimen of his penmanship.

The pretty boy complied readily enough, and mildly rebuked his

interrogator by rapidly writing some sentences for her in fourteen

languages, including such as, Arabian, Persian, and Ethiopic.

Meantime languages had been but an incident in the education of the lad.

He seems to have entered every available field of thought--mathematics,

physics, botany, literature, music, painting, languages, philosophy,

archaeology, and so on to tiresome lengths--and once he had entered any

field he seldom turned aside until he had reached the confines of the

subject as then known and added something new from the recesses of his

own genius. He was as versatile as Priestley, as profound as Newton

himself. He had the range of a mere dilettante, but everywhere the full

grasp of the master. He took early for his motto the saying that what

one man has done, another man may do. Granting that the other man has

the brain of a Thomas Young, it is a true motto.

Such, then, was the young Quaker who came to London to follow out

the humdrum life of a practitioner of medicine in the year 1801. But

incidentally the young physician was prevailed upon to occupy the

interims of early practice by fulfilling the duties of the chair of

Natural Philosophy at the Royal Institution, which Count Rumford

had founded, and of which Davy was then Professor of Chemistry--the

institution whose glories have been perpetuated by such names as Faraday

and Tyndall, and which the Briton of to-day speaks of as the "Pantheon

of Science." Here it was that Thomas Young made those studies which have

insured him a niche in the temple of fame not far removed from that of

Isaac Newton.

As early as 1793, when he was only twenty, Young had begun to

Communicate papers to the Royal Society of London, which were adjudged

worthy to be printed in full in the Philosophical Transactions; so it

is not strange that he should have been asked to deliver the Bakerian

lecture before that learned body the very first year after he came to

London. The lecture was delivered November 12, 1801. Its subject was

"The Theory of Light and Colors," and its reading marks an epoch in

physical science; for here was brought forward for the first time

convincing proof of that undulatory theory of light with which every

student of modern physics is familiar--the theory which holds that light

is not a corporeal entity, but a mere pulsation in the substance of

an all-pervading ether, just as sound is a pulsation in the air, or in

liquids or solids.

Young had, indeed, advocated this theory at an earlier date, but it was

not until 1801 that he hit upon the idea which enabled him to bring it

to anything approaching a demonstration. It was while pondering over the

familiar but puzzling phenomena of colored rings into which white

light is broken when reflected from thin films--Newton's rings, so

called--that an explanation occurred to him which at once put the entire

undulatory theory on a new footing. With that sagacity of insight which

we call genius, he saw of a sudden that the phenomena could be explained

by supposing that when rays of light fall on a thin glass, part of the

rays being reflected from the upper surface, other rays, reflected from

the lower surface, might be so retarded in their course through the

glass that the two sets would interfere with one another, the forward

pulsation of one ray corresponding to the backward pulsation of another,

thus quite neutralizing the effect. Some of the component pulsations of

the light being thus effaced by mutual interference, the remaining

rays would no longer give the optical effect of white light; hence the

puzzling colors.

Here is Young's exposition of the subject:

Of the Colors of Thin Plates

"When a beam of light falls upon two refracting surfaces, the partial

reflections coincide perfectly in direction; and in this case the

interval of retardation taken between the surfaces is to their radius as

twice the cosine of the angle of refraction to the radius.

"Let the medium between the surfaces be rarer than the surrounding

mediums; then the impulse reflected at the second surface, meeting a

subsequent undulation at the first, will render the particles of the

rarer medium capable of wholly stopping the motion of the denser and

destroying the reflection, while they themselves will be more strongly

propelled than if they had been at rest, and the transmitted light will

be increased. So that the colors by reflection will be destroyed, and

those by transmission rendered more vivid, when the double thickness or

intervals of retardation are any multiples of the whole breadth of

the undulations; and at intermediate thicknesses the effects will be

reversed according to the Newtonian observation.

"If the same proportions be found to hold good with respect to thin

plates of a denser medium, which is, indeed, not improbable, it will be

necessary to adopt the connected demonstrations of Prop. IV., but, at

any rate, if a thin plate be interposed between a rarer and a denser

medium, the colors by reflection and transmission may be expected to

change places."

OF THE COLORS OF THICK PLATES

"When a beam of light passes through a refracting surface, especially

if imperfectly polished, a portion of it is irregularly scattered, and

makes the surface visible in all directions, but most conspicuously

in directions not far distant from that of the light itself; and if a

reflecting surface be placed parallel to the refracting surface, this

scattered light, as well as the principal beam, will be reflected, and

there will be also a new dissipation of light, at the return of the beam

through the refracting surface. These two portions of scattered light

will coincide in direction; and if the surfaces be of such a form as to

collect the similar effects, will exhibit rings of colors. The interval

of retardation is here the difference between the paths of the principal

beam and of the scattered light between the two surfaces; of course,

wherever the inclination of the scattered light is equal to that of the

beam, although in different planes, the interval will vanish and all the

undulations will conspire. At other inclinations, the interval will be

the difference of the secants from the secant of the inclination, or

angle of refraction of the principal beam. From these causes, all the

colors of concave mirrors observed by Newton and others are necessary

consequences; and it appears that their production, though somewhat

similar, is by no means as Newton imagined, identical with the

production of thin plates."(2)

By following up this clew with mathematical precision, measuring the

exact thickness of the plate and the space between the different rings

of color, Young was able to show mathematically what must be the length

of pulsation for each of the different colors of the spectrum. He

estimated that the undulations of red light, at the extreme lower end

of the visible spectrum, must number about thirty-seven thousand six

hundred and forty to the inch, and pass any given spot at a rate of four

hundred and sixty-three millions of millions of undulations in a second,

while the extreme violet numbers fifty-nine thousand seven hundred and

fifty undulations to the inch, or seven hundred and thirty-five millions

of millions to the second.

The Colors of Striated Surfaces

Young similarly examined the colors that are produced by scratches on

a smooth surface, in particular testing the light from "Mr. Coventry's

exquisite micrometers," which consist of lines scratched on glass at

measured intervals. These microscopic tests brought the same results as

the other experiments. The colors were produced at certain definite

and measurable angles, and the theory of interference of undulations

explained them perfectly, while, as Young affirmed with confidence, no

other hypothesis hitherto advanced would explain them at all. Here are

his words:

"Let there be in a given plane two reflecting points very near each

other, and let the plane be so situated that the reflected image of a

luminous object seen in it may appear to coincide with the points; then

it is obvious that the length of the incident and reflected ray, taken

together, is equal with respect to both points, considering them as

capable of reflecting in all directions. Let one of the points be

now depressed below the given plane; then the whole path of the

light reflected from it will be lengthened by a line which is to the

depression of the point as twice the cosine of incidence to the radius.

"If, therefore, equal undulations of given dimensions be reflected

from two points, situated near enough to appear to the eye but as one,

whenever this line is equal to half the breadth of a whole undulation

the reflection from the depressed point will so interfere with the

reflection from the fixed point that the progressive motion of the one

will coincide with the retrograde motion of the other, and they will

both be destroyed; but when this line is equal to the whole breadth of

an undulation, the effect will be doubled, and when to a breadth and

a half, again destroyed; and thus for a considerable number of

alternations, and if the reflected undulations be of a different kind,

they will be variously affected, according to their proportions to the

various length of the line which is the difference between the lengths

of their two paths, and which may be denominated the interval of a

retardation.

"In order that the effect may be the more perceptible, a number of pairs

of points must be united into two parallel lines; and if several such

pairs of lines be placed near each other, they will facilitate the

observation. If one of the lines be made to revolve round the other as

an axis, the depression below the given plane will be as the sine of the

inclination; and while the eye and the luminous object remain fixed the

difference of the length of the paths will vary as this sine.

"The best subjects for the experiment are Mr. Coventry's exquisite

micrometers; such of them as consist of parallel lines drawn on glass,

at a distance of one-five-hundredth of an inch, are the most convenient.

Each of these lines appears under a microscope to consist of two or more

finer lines, exactly parallel, and at a distance of somewhat more than

a twentieth more than the adjacent lines. I placed one of these so as to

reflect the sun's light at an angle of forty-five degrees, and fixed

it in such a manner that while it revolved round one of the lines as an

axis, I could measure its angular motion; I found that the longest red

color occurred at the inclination 10 1/4 degrees, 20 3/4 degrees, 32

degrees, and 45 degrees; of which the sines are as the numbers 1, 2, 3,

and 4. At all other angles also, when the sun's light was reflected from

the surface, the color vanished with the inclination, and was equal at

equal inclinations on either side.

This experiment affords a very strong confirmation of the theory. It is

impossible to deduce any explanation of it from any hypothesis hitherto

advanced; and I believe it would be difficult to invent any other

that would account for it. There is a striking analogy between this

separation of colors and the production of a musical note by successive

echoes from equidistant iron palisades, which I have found to correspond

pretty accurately with the known velocity of sound and the distances of

the surfaces.

"It is not improbable that the colors of the integuments of some

insects, and of some other natural bodies, exhibiting in different

lights the most beautiful versatility, may be found to be of this

description, and not to be derived from thin plates. In some cases a

single scratch or furrow may produce similar effects, by the reflection

of its opposite edges."(3)

This doctrine of interference of undulations was the absolutely novel

part of Young's theory. The all-compassing genius of Robert Hooke had,

indeed, very nearly apprehended it more than a century before, as Young

himself points out, but no one else bad so much as vaguely conceived

it; and even with the sagacious Hooke it was only a happy guess, never

distinctly outlined in his own mind, and utterly ignored by all

others. Young did not know of Hooke's guess until he himself had fully

formulated the theory, but he hastened then to give his predecessor

all the credit that could possibly be adjudged his due by the most

disinterested observer. To Hooke's contemporary, Huygens, who was the

originator of the general doctrine of undulation as the explanation of

light, Young renders full justice also. For himself he claims only the

merit of having demonstrated the theory which these and a few others of

his predecessors had advocated without full proof.

The following year Dr. Young detailed before the Royal Society

other experiments, which threw additional light on the doctrine of

interference; and in 1803 he cited still others, which, he affirmed,

brought the doctrine to complete demonstration. In applying this

demonstration to the general theory of light, he made the striking

suggestion that "the luminiferous ether pervades the substance of all

material bodies with little or no resistance, as freely, perhaps, as the

wind passes through a grove of trees." He asserted his belief also that

the chemical rays which Ritter had discovered beyond the violet end of

the visible spectrum are but still more rapid undulations of the same

character as those which produce light. In his earlier lecture he had

affirmed a like affinity between the light rays and the rays of

radiant heat which Herschel detected below the red end of the spectrum,

suggesting that "light differs from heat only in the frequency of its

undulations or vibrations--those undulations which are within certain

limits with respect to frequency affecting the optic nerve and

constituting light, and those which are slower and probably stronger

constituting heat only." From the very outset he had recognized the

affinity between sound and light; indeed, it had been this affinity that

led him on to an appreciation of the undulatory theory of light.

But while all these affinities seemed so clear to the great

co-ordinating brain of Young, they made no such impression on the minds

of his contemporaries. The immateriality of light had been substantially

demonstrated, but practically no one save its author accepted the

demonstration. Newton's doctrine of the emission of corpuscles was too

firmly rooted to be readily dislodged, and Dr. Young had too many other

interests to continue the assault unceasingly. He occasionally wrote

something touching on his theory, mostly papers contributed to

the Quarterly Review and similar periodicals, anonymously or

under pseudonym, for he had conceived the notion that too great

conspicuousness in fields outside of medicine would injure his practice

as a physician. His views regarding light (including the original papers

from the Philosophical Transactions of the Royal Society) were again

given publicity in full in his celebrated volume on natural philosophy,

consisting in part of his lectures before the Royal Institution,

published in 1807; but even then they failed to bring conviction to

the philosophic world. Indeed, they did not even arouse a controversial

spirit, as his first papers had done.

ARAGO AND FRESNEL CHAMPION THE WAVE THEORY

So it chanced that when, in 1815, a young French military engineer,

named Augustin Jean Fresnel, returning from the Napoleonic wars,

became interested in the phenomena of light, and made some experiments

concerning diffraction which seemed to him to controvert the accepted

notions of the materiality of light, he was quite unaware that his

experiments had been anticipated by a philosopher across the Channel.

He communicated his experiments and results to the French Institute,

supposing them to be absolutely novel. That body referred them to a

committee, of which, as good fortune would have it, the dominating

member was Dominique Francois Arago, a man as versatile as Young

himself, and hardly less profound, if perhaps not quite so original.

Arago at once recognized the merit of Fresnel's work, and soon became a

convert to the theory. He told Fresnel that Young had anticipated him

as regards the general theory, but that much remained to be done, and

he offered to associate himself with Fresnel in prosecuting the

investigation. Fresnel was not a little dashed to learn that his

original ideas had been worked out by another while he was a lad, but he

bowed gracefully to the situation and went ahead with unabated zeal.

The championship of Arago insured the undulatory theory a hearing

before the French Institute, but by no means sufficed to bring about

its general acceptance. On the contrary, a bitter feud ensued, in which

Arago was opposed by the "Jupiter Olympus of the Academy," Laplace, by

the only less famous Poisson, and by the younger but hardly less able

Biot. So bitterly raged the feud that a life-long friendship between

Arago and Biot was ruptured forever. The opposition managed to delay the

publication of Fresnel's papers, but Arago continued to fight with his

customary enthusiasm and pertinacity, and at last, in 1823, the Academy

yielded, and voted Fresnel into its ranks, thus implicitly admitting the

value of his work.

It is a humiliating thought that such controversies as this must mar

the progress of scientific truth; but fortunately the story of the

introduction of the undulatory theory has a more pleasant side. Three

men, great both in character and in intellect, were concerned in

pressing its claims--Young, Fresnel, and Arago--and the relations of

these men form a picture unmarred by any of those petty jealousies that

so often dim the lustre of great names. Fresnel freely acknowledged

Young's priority so soon as his attention was called to it; and Young

applauded the work of the Frenchman, and aided with his counsel in the

application of the undulatory theory to the problems of polarization of

light, which still demanded explanation, and which Fresnel's fertility

of experimental resource and profundity of mathematical insight sufficed

in the end to conquer.

After Fresnel's admission to the Institute in 1823 the opposition

weakened, and gradually the philosophers came to realize the merits of

a theory which Young had vainly called to their attention a full

quarter-century before. Now, thanks largely to Arago, both Young and

Fresnel received their full meed of appreciation. Fresnel was given the

Rumford medal of the Royal Society of England in 1825, and chosen one of

the foreign members of the society two years later, while Young in turn

was elected one of the eight foreign members of the French Academy. As

a fitting culmination of the chapter of felicities between the three

friends, it fell to the lot of Young, as Foreign Secretary of the

Royal Society, to notify Fresnel of the honors shown him by England's

representative body of scientists; while Arago, as Perpetual Secretary

of the French Institute, conveyed to Young in the same year the

notification that he had been similarly honored by the savants of

France.

A few months later Fresnel was dead, and Young survived him only two

years. Both died prematurely, but their great work was done, and

the world will remember always and link together these two names in

connection with a theory which in its implications and importance ranks

little below the theory of universal gravitation.

VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM

GALVANI AND VOLTA

The full importance of Young's studies of light might perhaps have

gained earlier recognition had it not chanced that, at the time when

they were made, the attention of the philosophic world was turned with

the fixity and fascination of a hypnotic stare upon another field, which

for a time brooked no rival. How could the old, familiar phenomenon,

light, interest any one when the new agent, galvanism, was in view? As

well ask one to fix attention on a star while a meteorite blazes across

the sky.

Galvanism was so called precisely as the Roentgen ray was christened at

a later day--as a safe means of begging the question as to the nature of

the phenomena involved. The initial fact in galvanism was the discovery

of Luigi Galvani (1737-1798), a physician of Bologna, in 1791, that

by bringing metals in contact with the nerves of a frog's leg violent

muscular contractions are produced. As this simple little experiment led

eventually to the discovery of galvanic electricity and the invention

of the galvanic battery, it may be regarded as the beginning of modern

electricity.

The story is told that Galvani was led to his discovery while preparing

frogs' legs to make a broth for his invalid wife. As the story runs, he

had removed the skins from several frogs' legs, when, happening to touch

the exposed muscles with a scalpel which had lain in close proximity to

an electrical machine, violent muscular action was produced. Impressed

with this phenomenon, he began a series of experiments which finally

resulted in his great discovery. But be this story authentic or not, it

is certain that Galvani experimented for several years upon frogs' legs

suspended upon wires and hooks, until he finally constructed his arc

of two different metals, which, when arranged so that one was placed

in contact with a nerve and the other with a muscle, produced violent

contractions.

These two pieces of metal form the basic principle of the modern

galvanic battery, and led directly to Alessandro Volta's invention

of his "voltaic pile," the immediate ancestor of the modern galvanic

battery. Volta's experiments were carried on at the same time as those

of Galvani, and his invention of his pile followed close upon Galvani's

discovery of the new form of electricity. From these facts the new form

of electricity was sometimes called "galvanic" and sometimes "voltaic"

electricity, but in recent years the term "galvanism" and "galvanic

current" have almost entirely supplanted the use of the term voltaic.

It was Volta who made the report of Galvani's wonderful discovery to

the Royal Society of London, read on January 31, 1793. In this letter he

describes Galvani's experiments in detail and refers to them in glowing

terms of praise. He calls it one of the "most beautiful and important

discoveries," and regarded it as the germ or foundation upon which other

discoveries were to be made. The prediction proved entirely correct,

Volta himself being the chief discoverer.

Working along lines suggested by Galvani's discovery, Volta constructed

an apparatus made up of a number of disks of two different kinds of

metal, such as tin and silver, arranged alternately, a piece of some

moist, porous substance, like paper or felt, being interposed between

each pair of disks. With this "pile," as it was called, electricity

was generated, and by linking together several such piles an electric

battery could be formed.

This invention took the world by storm. Nothing like the enthusiasm it

created in the philosophic world had been known since the invention

of the Leyden jar, more than half a century before. Within a few weeks

after Volta's announcement, batteries made according to his plan were

being experimented with in every important laboratory in Europe.

As the century closed, half the philosophic world was speculating as to

whether "galvanic influence" were a new imponderable, or only a form of

electricity; and the other half was eagerly seeking to discover what new

marvels the battery might reveal. The least imaginative man could see

that here was an invention that would be epoch-making, but the most

visionary dreamer could not even vaguely adumbrate the real measure of

its importance.

It was evident at once that almost any form of galvanic battery,

despite imperfections, was a more satisfactory instrument for generating

electricity than the frictional machine hitherto in use, the advantage

lying in the fact that the current from the galvanic battery could

be controlled practically at will, and that the apparatus itself

was inexpensive and required comparatively little attention. These

advantages were soon made apparent by the practical application of the

electric current in several fields.

It will be recalled that despite the energetic endeavors of such

philosophers as Watson, Franklin, Galvani, and many others, the field

of practical application of electricity was very limited at the close of

the eighteenth century. The lightning-rod had come into general use, to

be sure, and its value as an invention can hardly be overestimated. But

while it was the result of extensive electrical discoveries, and is

a most practical instrument, it can hardly be called one that puts

electricity to practical use, but simply acts as a means of warding

off the evil effects of a natural manifestation of electricity. The

invention, however, had all the effects of a mechanism which turned

electricity to practical account. But with the advent of the new kind of

electricity the age of practical application began.

DAVY AND ELECTRIC LIGHT

Volta's announcement of his pile was scarcely two months old when two

Englishmen, Messrs. Nicholson and Carlisle, made the discovery that

the current from the galvanic battery had a decided effect upon certain

chemicals, among other things decomposing water into its elements,

hydrogen and oxygen. On May 7, 1800, these investigators arranged the

ends of two brass wires connected with the poles of a voltaic pile,

composed of alternate silver and zinc plates, so that the current coming

from the pile was discharged through a small quantity of "New River

water." "A fine stream of minute bubbles immediately began to flow from

the point of the lower wire in the tube which communicated with the

silver," wrote Nicholson, "and the opposite point of the upper wire

became tarnished, first deep orange and then black...." The product of

gas during two hours and a half was two-thirtieths of a cubic inch.

"It was then mixed with an equal quantity of common air," continues

Nicholson, "and exploded by the application of a lighted waxen thread."

This demonstration was the beginning of the very important science of

electro-chemistry.

The importance of this discovery was at once recognized by Sir Humphry

Davy, who began experimenting immediately in this new field. He

constructed a series of batteries in various combinations, with which

he attacked the "fixed alkalies," the composition of which was then

unknown. Very shortly he was able to decompose potash into bright

metallic globules, resembling quicksilver. This new substance he named

"potassium." Then in rapid succession the elementary substances sodium,

calcium, strontium, and magnesium were isolated.

It was soon discovered, also, that the new electricity, like the old,

possessed heating power under certain conditions, even to the fusing of

pieces of wire. This observation was probably first made by Frommsdorff,

but it was elaborated by Davy, who constructed a battery of two thousand

cells with which he produced a bright light from points of carbon--the

prototype of the modern arc lamp. He made this demonstration before the

members of the Royal Institution in 1810. But the practical utility of

such a light for illuminating purposes was still a thing of the future.

The expense of constructing and maintaining such an elaborate battery,

and the rapid internal destruction of its plates, together with the

constant polarization, rendered its use in practical illumination out of

the question. It was not until another method of generating electricity

was discovered that Davy's demonstration could be turned to practical

account.

In Davy's own account of his experiment he says:

"When pieces of charcoal about an inch long and one-sixth of an inch in

diameter were brought near each other (within the thirtieth or fortieth

of an inch), a bright spark was produced, and more than half the volume

of the charcoal became ignited to whiteness; and, by withdrawing the

points from each other, a constant discharge took place through the

heated air, in a space equal to at least four inches, producing a most

brilliant ascending arch of light, broad and conical in form in the

middle. When any substance was introduced into this arch, it instantly

became ignited; platina melted as readily in it as wax in a common

candle; quartz, the sapphire, magnesia, lime, all entered into fusion;

fragments of diamond and points of charcoal and plumbago seemed to

evaporate in it, even when the connection was made in the receiver of an

air-pump; but there was no evidence of their having previously undergone

fusion. When the communication between the points positively and

negatively electrified was made in the air rarefied in the receiver of

the air-pump, the distance at which the discharge took place increased

as the exhaustion was made; and when the atmosphere in the vessel

supported only one-fourth of an inch of mercury in the barometrical

gauge, the sparks passed through a space of nearly half an inch; and, by

withdrawing the points from each other, the discharge was made through

six or seven inches, producing a most brilliant coruscation of purple

light; the charcoal became intensely ignited, and some platina wire

attached to it fused with brilliant scintillations and fell in large

globules upon the plate of the pump. All the phenomena of

chemical decomposition were produced with intense rapidity by this

combination."(1)

But this experiment demonstrated another thing besides the possibility

of producing electric light and chemical decomposition, this being the

heating power capable of being produced by the electric current. Thus

Davy's experiment of fusing substances laid the foundation of the modern

electric furnaces, which are of paramount importance in several great

commercial industries.

While some of the results obtained with Davy's batteries were

practically as satisfactory as could be obtained with modern cell

batteries, the batteries themselves were anything but satisfactory. They

were expensive, required constant care and attention, and, what was more

important from an experimental standpoint at least, were not constant in

their action except for a very limited period of time, the current soon

"running down." Numerous experimenters, therefore, set about devising a

satisfactory battery, and when, in 1836, John Frederick Daniell produced

the cell that bears his name, his invention was epoch-making in the

history of electrical progress. The Royal Society considered it of

sufficient importance to bestow the Copley medal upon the inventor,

whose device is the direct parent of all modern galvanic cells. From the

time of the advent of the Daniell cell experiments in electricity were

rendered comparatively easy. In the mean while, however, another great

discovery was made.

ELECTRICITY AND MAGNETISM

For many years there had been a growing suspicion, amounting in

many instances to belief in the close relationship existing between

electricity and magnetism. Before the winter of 1815, however, it was

a belief that was surmised but not demonstrated. But in that year it

occurred to Jean Christian Oersted, of Denmark, to pass a current of

electricity through a wire held parallel with, but not quite touching, a

suspended magnetic needle. The needle was instantly deflected and swung

out of its position.

"The first experiments in connection with the subject which I am

undertaking to explain," wrote Oersted, "were made during the course

of lectures which I held last winter on electricity and magnetism. From

those experiments it appeared that the magnetic needle could be moved

from its position by means of a galvanic battery--one with a closed

galvanic circuit. Since, however, those experiments were made with an

apparatus of small power, I undertook to repeat and increase them with a

large galvanic battery.

"Let us suppose that the two opposite ends of the galvanic apparatus are

joined by a metal wire. This I shall always call the conductor for

the sake of brevity. Place a rectilinear piece of this conductor in

a horizontal position over an ordinary magnetic needle so that it is

parallel to it. The magnetic needle will be set in motion and will

deviate towards the west under that part of the conductor which comes

from the negative pole of the galvanic battery. If the wire is not more

than four-fifths of an inch distant from the middle of this needle, this

deviation will be about forty-five degrees. At a greater distance

the angle of deviation becomes less. Moreover, the deviation varies

according to the strength of the battery. The conductor can be moved

towards the east or west, so long as it remains parallel to the needle,

without producing any other result than to make the deviation smaller.

"The conductor can consist of several combined wires or metal coils. The

nature of the metal does not alter the result except, perhaps, to make

it greater or less. We have used wires of platinum, gold, silver, brass,

and iron, and coils of lead, tin, and quicksilver with the same result.

If the conductor is interrupted by water, all effect is not cut off,

unless the stretch of water is several inches long.

"The conductor works on the magnetic needle through glass, metals, wood,

water, and resin, through clay vessels and through stone, for when we

placed a glass plate, a metal plate, or a board between the conductor

and the needle the effect was not cut off; even the three together

seemed hardly to weaken the effect, and the same was the case with an

earthen vessel, even when it was full of water. Our experiments also

demonstrated that the said effects were not altered when we used a

magnetic needle which was in a brass case full of water.

"When the conductor is placed in a horizontal plane under the magnetic

needle all the effects we have described take place in precisely the

same way, but in the opposite direction to what took place when the

conductor was in a horizontal plane above the needle.

"If the conductor is moved in a horizontal plane so that it gradually

makes ever-increasing angles with the magnetic meridian, the deviation

of the magnetic needle from the magnetic meridian is increased when the

wire is turned towards the place of the needle; it decreases, on the

other hand, when it is turned away from that place.

"A needle of brass which is hung in the same way as the magnetic needle

is not set in motion by the influence of the conductor. A needle of

glass or rubber likewise remains static under similar experiments. Hence

the electrical conductor affects only the magnetic parts of a substance.

That the electrical current is not confined to the conducting wire,

but is comparatively widely diffused in the surrounding space, is

sufficiently demonstrated from the foregoing observations."(2)

The effect of Oersted's demonstration is almost incomprehensible. By it

was shown the close relationship between magnetism and electricity. It

showed the way to the establishment of the science of electrodynamics;

although it was by the French savant Andre Marie Ampere (1775-1836) that

the science was actually created, and this within the space of one week

after hearing of Oersted's experiment in deflecting the needle. Ampere

first received the news of Oersted's experiment on September 11, 1820,

and on the 18th of the same month he announced to the Academy the

fundamental principles of the science of electro-dynamics--seven days of

rapid progress perhaps unequalled in the history of science.

Ampere's distinguished countryman, Arago, a few months later, gave

the finishing touches to Oersted's and Ampere's discoveries, by

demonstrating conclusively that electricity not only influenced a

magnet, but actually produced magnetism under proper circumstances--a

complemental fact most essential in practical mechanics.

Some four years after Arago's discovery, Sturgeon made the first

"electro-magnet" by winding a soft iron core with wire through which

a current of electricity was passed. This study of electro-magnets was

taken up by Professor Joseph Henry, of Albany, New York, who succeeded

in making magnets of enormous lifting power by winding the iron core

with several coils of wire. One of these magnets, excited by a single

galvanic cell of less than half a square foot of surface, and containing

only half a pint of dilute acids, sustained a weight of six hundred and

fifty pounds.

Thus by Oersted's great discovery of the intimate relationship of

magnetism and electricity, with further elaborations and discoveries by

Ampere, Volta, and Henry, and with the invention of Daniell's cell, the

way was laid for putting electricity to practical use. Soon followed the

invention and perfection of the electro-magnetic telegraph and a host of

other but little less important devices.

FARADAY AND ELECTRO-MAGNETIC INDUCTION

With these great discoveries and inventions at hand, electricity became

no longer a toy or a "plaything for philosophers," but of enormous

and growing importance commercially. Still, electricity generated

by chemical action, even in a very perfect cell, was both feeble and

expensive, and, withal, only applicable in a comparatively limited

field. Another important scientific discovery was necessary before such

things as electric traction and electric lighting on a large scale were

to become possible; but that discovery was soon made by Sir Michael

Faraday.

Faraday, the son of a blacksmith and a bookbinder by trade, had

interested Sir Humphry Davy by his admirable notes on four of Davy's

lectures, which he had been able to attend. Although advised by the

great scientist to "stick to his bookbinding" rather than enter the

field of science, Faraday became, at twenty-two years of age, Davy's

assistant in the Royal Institution. There, for several years, he devoted

all his spare hours to scientific investigations and experiments,

perfecting himself in scientific technique.

A few years later he became interested, like all the scientists of

the time, in Arago's experiment of rotating a copper disk underneath a

suspended compass-needle. When this disk was rotated rapidly, the

needle was deflected, or even rotated about its axis, in a manner quite

inexplicable. Faraday at once conceived the idea that the cause of this

rotation was due to electricity, induced in the revolving disk--not only

conceived it, but put his belief in writing. For several years, however,

he was unable to demonstrate the truth of his assumption, although he

made repeated experiments to prove it. But in 1831 he began a series

of experiments that established forever the fact of electro-magnetic

induction.

In his famous paper, read before the Royal Society in 1831, Faraday

describes the method by which he first demonstrated electro-magnetic

induction, and then explained the phenomenon of Arago's revolving disk.

"About twenty-six feet of copper wire, one-twentieth of an inch in

diameter, were wound round a cylinder of wood as a helix," he said,

"the different spires of which were prevented from touching by a thin

interposed twine. This helix was covered with calico, and then a

second wire applied in the same manner. In this way twelve helices were

"superposed, each containing an average length of wire of twenty-seven

feet, and all in the same direction. The first, third, fifth, seventh,

ninth, and eleventh of these helices were connected at their extremities

end to end so as to form one helix; the others were connected in a

similar manner; and thus two principal helices were produced, closely

interposed, having the same direction, not touching anywhere, and each

containing one hundred and fifty-five feet in length of wire.

One of these helices was connected with a galvanometer, the other with

a voltaic battery of ten pairs of plates four inches square, with double

coppers and well charged; yet not the slightest sensible deflection of

the galvanometer needle could be observed.

"A similar compound helix, consisting of six lengths of copper and six

of soft iron wire, was constructed. The resulting iron helix contained

two hundred and eight feet; but whether the current from the trough was

passed through the copper or the iron helix, no effect upon the other

could be perceived at the galvanometer.

"In these and many similar experiments no difference in action of any

kind appeared between iron and other metals.

"Two hundred and three feet of copper wire in one length were passed

round a large block of wood; other two hundred and three feet of similar

wire were interposed as a spiral between the turns of the first, and

metallic contact everywhere prevented by twine. One of these helices was

connected with a galvanometer and the other with a battery of a hundred

pairs of plates four inches square, with double coppers and well

charged. When the contact was made, there was a sudden and very slight

effect at the galvanometer, and there was also a similar slight effect

when the contact with the battery was broken. But whilst the voltaic

current was continuing to pass through the one helix, no galvanometrical

appearances of any effect like induction upon the other helix could be

perceived, although the active power of the battery was proved to be

great by its heating the whole of its own helix, and by the brilliancy

of the discharge when made through charcoal.

"Repetition of the experiments with a battery of one hundred and twenty

pairs of plates produced no other effects; but it was ascertained, both

at this and at the former time, that the slight deflection of the needle

occurring at the moment of completing the connection was always in one

direction, and that the equally slight deflection produced when the

contact was broken was in the other direction; and, also, that these

effects occurred when the first helices were used.

"The results which I had by this time obtained with magnets led me

to believe that the battery current through one wire did, in reality,

induce a similar current through the other wire, but that it continued

for an instant only, and partook more of the nature of the electrical

wave passed through from the shock of a common Leyden jar than of that

from a voltaic battery, and, therefore, might magnetize a steel needle

although it scarcely affected the galvanometer.

"This expectation was confirmed; for on substituting a small hollow

helix, formed round a glass tube, for the galvanometer, introducing

a steel needle, making contact as before between the battery and the

inducing wire, and then removing the needle before the battery contact

was broken, it was found magnetized.

"When the battery contact was first made, then an unmagnetized needle

introduced, and lastly the battery contact broken, the needle was found

magnetized to an equal degree apparently with the first; but the poles

were of the contrary kinds."(3)

To Faraday these experiments explained the phenomenon of Arago's

rotating disk, the disk inducing the current from the magnet, and, in

reacting, deflecting the needle. To prove this, he constructed a disk

that revolved between the poles of an electro-magnet, connecting the

axis and the edge of the disk with a galvanometer. "... A disk of

copper, twelve inches in diameter, fixed upon a brass axis," he says,

"was mounted in frames so as to be revolved either vertically or

horizontally, its edge being at the same time introduced more or less

between the magnetic poles. The edge of the plate was well amalgamated

for the purpose of obtaining good but movable contact; a part round the

axis was also prepared in a similar manner.

"Conductors or collectors of copper and lead were constructed so as to

come in contact with the edge of the copper disk, or with other forms

of plates hereafter to be described. These conductors we're about four

inches long, one-third of an inch wide, and one-fifth of an inch thick;

one end of each was slightly grooved, to allow of more exact adaptation

to the somewhat convex edge of the plates, and then amalgamated. Copper

wires, one-sixteenth of an inch in thickness, attached in the ordinary

manner by convolutions to the other ends of these conductors, passed

away to the galvanometer.

"All these arrangements being made, the copper disk was adjusted, the

small magnetic poles being about one-half an inch apart, and the edge

of the plate inserted about half their width between them. One of the

galvanometer wires was passed twice or thrice loosely round the brass

axis of the plate, and the other attached to a conductor, which itself

was retained by the hand in contact with the amalgamated edge of the

disk at the part immediately between the magnetic poles. Under these

circumstances all was quiescent, and the galvanometer exhibited no

effect. But the instant the plate moved the galvanometer was influenced,

and by revolving the plate quickly the needle could be deflected ninety

degrees or more."(4)

This rotating disk was really a dynamo electric machine in miniature,

the first ever constructed, but whose direct descendants are the

ordinary dynamos. Modern dynamos range in power from little machines

operating machinery requiring only fractions of a horsepower to great

dynamos operating street-car lines and lighting cities; but all

are built on the same principle as Faraday's rotating disk. By this

discovery the use of electricity as a practical and economical motive

power became possible.

STORAGE BATTERIES

When the discoveries of Faraday of electro-magnetic induction had made

possible the means of easily generating electricity, the next natural

step was to find a means of storing it or accumulating it. This,

however, proved no easy matter, and as yet a practical storage or

secondary battery that is neither too cumbersome, too fragile, nor too

weak in its action has not been invented. If a satisfactory storage

battery could be made, it is obvious that its revolutionary effects

could scarcely be overestimated. In the single field of aeronautics, it

would probably solve the question of aerial navigation. Little wonder,

then, that inventors have sought so eagerly for the invention of

satisfactory storage batteries. As early as 1803 Ritter had attempted to

make such a secondary battery. In 1843 Grove also attempted it. But it

was not until 1859, when Gaston Planche produced his invention, that

anything like a reasonably satisfactory storage battery was made.

Planche discovered that sheets of lead immersed in dilute sulphuric acid

were very satisfactory for the production of polarization effects. He

constructed a battery of sheets of lead immersed in sulphuric acid, and,

after charging these for several hours from the cells of an ordinary

Bunsen battery, was able to get currents of great strength and

considerable duration. This battery, however, from its construction of

lead, was necessarily heavy and cumbersome. Faure improved it somewhat

by coating the lead plates with red-lead, thus increasing the capacity

of the cell. Faure's invention gave a fresh impetus to inventors, and

shortly after the market was filled with storage batteries of various

kinds, most of them modifications of Planche's or Faure's. The ardor

of enthusiastic inventors soon flagged, however, for all these storage

batteries proved of little practical account in the end, as compared

with other known methods of generating power.

Three methods of generating electricity are in general use: static or

frictional electricity is generated by "plate" or "static" machines;

galvanic, generated by batteries based on Volta's discovery; and

induced, or faradic, generated either by chemical or mechanical action.

There is still another kind, thermo-electricity, that may be generated

in a most simple manner. In 1821 Seebecle, of Berlin, discovered that

when a circuit was formed of two wires of different metals, if there

be a difference in temperature at the juncture of these two metals

an electrical current will be established. In this way heat may

be transmitted directly into the energy of the current without the

interposition of the steam-engine. Batteries constructed in this way

are of low resistance, however, although by arranging several of them

in "series," currents of considerable strength can be generated. As yet,

however, they are of little practical importance.