I wish I could, as you desire, give you a better explanation of the
phenomenon in question, since you seem not quite satisfied with your
own; but I think we want more and a greater variety of experiments,
in different circumstances, to enable us to form a thoroughly
satisfactory hypothesis. Not that I make the least doubt of the facts
already related, as I know both Lord Charles Cavendish, and Dr.
Heberden to be very accurate experimenters: but I wish to know the
event of the trials proposed in your six queries; and also, whether
in the same place where the lower vessel receives nearly twice the
quantity of water that is received by the upper, a third vessel
placed at half the height will receive a quantity proportionable. I
will however endeavour to explain to you what occurred to me, when I
first heard of the fact.
I suppose it will be generally allowed, on a little consideration
of the subject, that scarce any drop of water was, when it began to
fall from the clouds, of a magnitude equal to that it has acquired,
when it arrives at the earth; the same of the several pieces of
hail; because they are often so large and so weighty, that we
cannot conceive a possibility of their being suspended in the air,
and remaining at rest there, for any time, how small soever; nor do
we conceive any means of forming them so large, before they set out
to fall. It seems then, that each beginning drop, and particle of
hail, receives continual addition in its progress downwards. This
may be several ways: by the union of numbers in their course, so
that what was at first only descending mist, becomes a shower; or
by each particle, in its descent through air that contains a great
quantity of dissolved water, striking against, attaching to itself,
and carrying down with it such particles of that dissolved water, as
happen to be in its way; or attracting to itself such as do not lie
directly in its course by its different state with regard either to
common or electric fire; or by all these causes united.
In the first case, by the uniting of numbers, larger drops might be
made, but the quantity falling in the same place would be the same at
all heights; unless, as you mention, the whole should be contracted
in falling, the lines described by all the drops converging, so
that what set out to fall from a cloud of many thousand acres,
should reach the earth in perhaps a third of that extent, of which I
somewhat doubt. In the other cases we have two experiments.
1. A dry glass bottle filled with very cold water, in a warm day,
will presently collect from the seemingly dry air that surrounds
it a quantity of water, that shall cover its surface and run down
its sides, which perhaps is done by the power wherewith the cold
water attracts the fluid, common fire that had been united with the
dissolved water in the air, and drawing the fire through the glass
into itself, leaves the water on the outside.
2. An electrified body left in a room for some time, will be more
covered with dust than other bodies in the same room not electrified,
which dust seems to be attracted from the circumambient air.
Now we know that the rain, even in our hottest days, comes from a
very cold region. Its falling sometimes in the form of ice, shows
this clearly; and perhaps even the rain is snow or ice, when it first
moves downwards, though thawed in falling: and we know that the
drops of rain are often electrified: but those causes of addition to
each drop of water, or piece of hail, one would think could not long
continue to produce the same effect; since the air, through which the
drops fall, must soon be stripped of its previously dissolved water,
so as to be no longer capable of augmenting them. Indeed very heavy
showers, of either, are never of long continuance; but moderate rains
often continue so long as to puzzle this hypothesis: so that upon the
whole I think, as I intimated before, that we are yet hardly ripe for
making one. ****
B. FRANKLIN.
FOOTNOTE:
[25] This letter is taken from the Memoirs of the Literary and
Philosophical Society of Manchester, Vol. II. page 126. It was
communicated by the person to whom it is addressed, and was read
in the Society, January 21, 1784, as an appendix to a paper by Dr.
Percival on the same subject. _Editor._
TO MR. NAIRNE, OF LONDON[26].
_Proposing a slowly sensible Hygrometer for certain Purposes._
_Passy, near Paris, Nov. 13, 1780._
SIR,
The qualities hitherto sought in a hygrometer, or instrument to
discover the degrees of moisture and dryness in the air, seem to
have been, an aptitude to receive humidity readily from a moist
air, and to part with it is as readily to a dry air. Different
substances have been found to possess more or less of this quality;
but when we shall have found the substance that has it in the
greatest perfection, there will still remain some uncertainty in the
conclusions to be drawn from the degree shown by the instrument,
arising from the actual state of the instrument itself as to heat and
cold. Thus, if two bottles or vessels of glass or metal being filled,
the one with cold and the other with hot water, are brought into
a room, the moisture of the air in the room will attach itself in
quantities to the surface of the cold vessel, while if you actually
wet the surface of the hot vessel, the moisture will immediately quit
it, and be absorbed by the same air. And thus, in a sudden change of
the air from cold to warm, the instrument remaining longer cold may
condense and absorb more moisture, and mark the air as having become
more humid than it is in reality, and the contrary in a change from
warm to cold.
But if such a suddenly changing instrument could be freed from
these imperfections, yet when the design is to discover the
different degrees of humidity in the air of different countries,
I apprehend the quick sensibility of the instrument to be rather
a disadvantage; since, to draw the desired conclusions from it, a
constant and frequent observation day and night in each country will
be necessary for a year or years, and the mean of each different
set of observations is to be found and determined. After all which
some uncertainty will remain respecting the different degrees of
exactitude with which different persons may have made and taken
notes of their observations.
For these reasons, I apprehend that a substance which, though capable
of being distended by moisture and contracted by dryness, is so
slow in receiving and parting with its humidity, that the frequent
changes in the atmosphere have not time to affect it sensibly, and
which therefore should gradually take nearly the medium of all
those changes and preserve it constantly, would be the most proper
substance of which to make such an hygrometer.
Such an instrument, you, my dear sir, though without intending it,
have made for me; and I, without desiring or expecting it, have
received from you. It is therefore with propriety that I address to
you the following account of it; and the more, as you have both a
head to contrive and a hand to execute the means of perfecting it.
And I do this with greater pleasure, as it affords me the opportunity
of renewing that antient correspondence and acquaintance with you,
which to me was always so pleasing and so instructive.
You may possibly remember, that in or about the year 1758, you made
for me a set of artificial magnets, six in number, each five and
a half inches long, half an inch broad, and one eighth of an inch
thick. These, with two pieces of soft iron, which together equalled
one of the magnets, were inclosed in a little box of mahogany wood,
the grain of which ran with, and not across, the length of the box:
and the box was closed by a little shutter of the same wood, the
grain of which ran across the box; and the ends of this shutting
piece were bevelled so as to fit and slide in a kind of dovetail
groove when the box was to be shut or opened.
I had been of opinion, that good mahogany wood was not affected by
moisture so as to change its dimensions, and that it was always to
be found as the tools of the workman left it. Indeed the difference
at different times in the same country is so small as to be scarcely
in a common way observable. Hence the box, which was made so as to
allow sufficient room for the magnets to slide out and in freely,
and, when in, afforded them so much play that by shaking the box one
could make them strike the opposite sides alternately, continued in
the same state all the time I remained in England, which was four
years, without any apparent alteration. I left England in August
1762, and arrived at Philadelphia in October the same year. In a few
weeks after my arrival, being desirous of showing your magnets to a
philosophical friend, I found them so tight in the box, that it was
with difficulty I got them out; and constantly during the two years I
remained there, viz. till November 1764, this difficulty of getting
them out and in continued. The little shutter too, as wood does not
shrink lengthways of the grain, was found too long to enter its
grooves, and, not being used, was mislaid and lost; and I afterwards
had another made that fitted.
In December 1764 I returned to England, and after some time I
observed that my box was become full big enough for my magnets, and
too wide for my new shutter; which was so much too short for its
grooves, that it was apt to fall out; and to make it keep in, I
lengthened it by adding to each end a little coat of sealing-wax.
I continued in England more than ten years, and during all that time,
after the first change, I perceived no alteration. The magnets had
the same freedom in their box, and the little shutter continued with
the added sealing-wax to fit its grooves, till some weeks after my
second return to America.
As I could not imagine any other cause for this change of dimensions
in the box, when in the different countries, I concluded, first
generally that the air of England was moister than that of America.
And this I supposed an effect of its being an island, where every
wind that blew must necessarily pass over some sea before it arrived,
and of course lick up some vapour. I afterwards indeed doubted
whether it might be just only so far as related to the city of
London, where I resided; because there are many causes of moisture in
the city air, which do not exist to the same degree in the country;
such as the brewers' and dyers' boiling caldrons, and the great
number of pots and tea-kettles continually on the fire, sending forth
abundance of vapour; and also the number of animals who by their
breath continually increase it; to which may be added, that even the
vast quantity of sea coals burnt there, do in kindling discharge a
great deal of moisture.
When I was in England, the last time, you also made for me a little
achromatic pocket telescope, the body was brass, and it had a round
case (I think of thin wood) covered with shagrin. All the while
I remained in England, though possibly there might be some small
changes in the dimensions of this case, I neither perceived nor
suspected any. There was always comfortable room for the telescope
to slip in and out. But soon after I arrived in America, which was
in May 1775, the case became too small for the instrument, it was
with much difficulty and various contrivances that I got it out, and
I could never after get it in again, during my stay there, which
was eighteen months. I brought it with me to Europe, but left the
case as useless, imagining that I should find the continental air of
France as dry as that of Pennsylvania, where my magnet box had also
returned a second time to its narrowness, and pinched the pieces, as
heretofore, obliging me too, to scrape the sealing-wax off the ends
of the shutter.
I had not been long in France, before I was surprised to find, that
my box was become as large as it had always been in England, the
magnets entered and came out with the same freedom, and, when in,
I could rattle them against its sides; this has continued to be
the case without sensible variation. My habitation is out of Paris
distant almost a league, so that the moist air of the city cannot be
supposed to have much effect upon the box. I am on a high dry hill,
in a free air, as likely to be dry as any air in France. Whence it
seems probable that the air of England in general may, as well as
that of London, be moister than the air of America, since that of
France is so, and in a part so distant from the sea.
The greater dryness of the air in America appears from some other
observations. The cabinet work formerly sent us from London, which
consisted in thin plates of fine wood glued upon fir, never would
stand with us; the vaneering, as those plates are called, would get
loose and come off; both woods shrinking, and their grains often
crossing, they were forever cracking and flying. And in my electrical
experiments there, it was remarkable, that a mahogany table, on which
my jars stood under the prime conductor to be charged, would often be
so dry, particularly when the wind had been some time at north-west,
which with us is a very drying wind, as to isolate the jars, and
prevent their being charged till I had formed a communication between
their coatings and the earth. I had a like table in London, which I
used for the same purpose all the time I resided there; but it was
never so dry as to refuse conducting the electricity.
Now what I would beg leave to recommend to you, is, that you would
recollect, if you can, the species of mahogany of which you made
my box, for you know there is a good deal of difference in woods
that go under that name; or if that cannot be, that you would take
a number of pieces of the closest and finest grained mahogany that
you can meet with, plane them to the thinness of about a line, and
the width of about two inches across the grain, and fix each of the
pieces in some instrument that you can contrive, which will permit
them to contract and dilate, and will show, in sensible degrees, by
a moveable hand upon a marked scale, the otherwise less sensible
quantities of such contraction and dilatation. If these instruments
are all kept in the same place while making, and are graduated
together while subject to the same degrees of moisture or dryness,
I apprehend you will have so many comparable hygrometers, which,
being sent into different countries, and continued there for some
time, will find and show there the mean of the different dryness
and moisture of the air of those countries, and that with much less
trouble than by any hygrometer hitherto in use.
With great esteem, I am,
Dear Sir, your most obedient,
And most humble servant,
B. FRANKLIN.
FOOTNOTE:
[26] This letter is taken from the Transactions of the American
Philosophical Society, in which it was read, January 26, 1786.
_Editor._
TO DR. P.[27] IN LONDON.
_Relating a curious Instance of the Effect of Oil on Water._
_Philadelphia, Dec. 1, 1762._
SIR,
During our passage to Madeira, the weather being warm, and the cabin
windows constantly open for the benefit of the air, the candles
at night flared and run very much, which was an inconvenience. At
Madeira, we got oil to burn, and with a common glass tumbler or
beaker, slung in wire, and suspended to the cieling of the cabin, and
a little wire hoop for the wick, furnished with corks to float on the
oil, I made an Italian lamp, that gave us very good light all over
the table.--The glass at bottom contained water to about one third of
its height; another third was taken up with oil; the rest was left
empty that the sides of the glass might protect the flame from the
wind. There is nothing remarkable in all this; but what follows is
particular. At supper, looking on the lamp, I remarked, that though
the surface of the oil was perfectly tranquil, and duly preserved
its position and distance with regard to the brim of the glass, the
water under the oil was in great commotion, rising and falling in
irregular waves, which continued during the whole evening. The lamp
was kept burning as a watch light all night, till the oil was spent,
and the water only remained. In the morning I observed, that though
the motion of the ship continued the same, the water was now quiet,
and its surface as tranquil as that of the oil had been the evening
before.
At night again, when oil was put upon it, the water resumed its
irregular motions, rising in high waves almost to the surface of the
oil, but without disturbing the smooth level of that surface. And
this was repeated every day during the voyage.
Since my arrival in America, I have repeated the experiment
frequently thus. I have put a pack-thread round a tumbler, with
strings of the same, from each side, meeting above it in a knot at
about a foot distance from the top of the tumbler. Then putting in
as much water as would fill about one third part of the tumbler, I
lifted it up by the knot, and swung it to and fro in the air; when
the water appeared to keep its place in the tumbler as steadily as if
it had been ice. But pouring gently in upon the water about as much
oil, and then again swinging it in the air as before, the tranquility
before possessed by the water, was transferred to the surface of the
oil, and the water under it was agitated with the same commotions as
at sea.
I have shewn this experiment to a number of ingenious persons. Those
who are but slightly acquainted with the principles of hydrostatics,
&c. are apt to fancy immediately that they understand it, and readily
attempt to explain it; but their explanations have been different,
and to me not very intelligible. Others, more deeply skilled in those
principles, seem to wonder at it, and promise to consider it. And I
think it is worth considering: for a new appearance, if it cannot
be explained by our old principles, may afford us new ones, of use
perhaps in explaining some other obscure parts of natural knowledge.
I am, &c.
B. FRANKLIN.
FOOTNOTE:
[27] Dr. Pringle. _Editor._
_Of the Stilling of Waves by Means of Oil. Extracted from Sundry
Letters between Benjamin Franklin, L. L. D. F. R. S. William
Brownrigg, M. D. F. R. S. and the Rev. Mr. Farish._
Read at the Royal Society, June 2, 1774.
_Extract of a Letter from Dr. Brownrigg to Dr. Franklin, dated
Ormathwait, January 27, 1773._
By the enclosed from an old friend, a worthy clergyman at Carlisle,
whose great learning and extensive knowledge in most sciences would
have more distinguished him, had he been placed in a more conspicuous
point of view, you will find, that he had heard of your experiment on
Derwent Lake, and has thrown together what he could collect on that
subject; to which I have subjoined one experiment from the relation
of another gentleman.
_Extract of a Letter from the Rev. Mr. Farish, to Dr. Brownrigg._
I some time ago met with Mr. Dun, who surprised me with an account of
an experiment you had tried upon the Derwent Water, in company with
Sir John Pringle and Dr. Franklin. According to his representation,
the water, which had been in great agitation before, was instantly
calmed upon pouring in only a very small quantity of oil, and that
to so great a distance round the boat as seemed incredible. I have
since had the same accounts from others, but I suspect all of a
little exaggeration. Pliny mentions this property of oil as known
particularly to the divers, who made use of it in his days, in order
to have a more steady light at the bottom.[28] The sailors, I have
been told, have observed something of the same kind in our days, that
the water is always remarkably smoother, in the wake of a ship that
has been newly tallowed, than it is in one that is foul. Mr. Pennant
also mentions an observation of the like nature made by the seal
catchers in Scotland. _Brit. Zool._ Vol. IV. _Article_ Seal. When
these animals are devouring a very oily fish, which they always do
under water, the waves alone are observed to be remarkably smooth,
and by this mark the fishermen know where to look for them. Old Pliny
does not usually meet with all the credit I am inclined to think he
deserves. I shall be glad to have an authentic account of the Keswick
experiment, and if it comes up to the representations that have been
made of it, I shall not much hesitate to believe the old gentleman in
another more wonderful phenomenon he relates of stilling a tempest
only by throwing up a little vinegar into the air.
_Extract of a Letter to Dr. Brownrigg from Dr. Franklin._
_London Nov. 7, 1773._
DEAR SIR,
I thank you for the remarks of your learned friend at Carlisle. I
had, when a youth, read and smiled at Pliny's account of a practice
among the seamen of his time, to still the waves in a storm by
pouring oil into the sea; which he mentions, as well as the use
made of oil by the divers; but the stilling a tempest by throwing
vinegar into the air had escaped me. I think with your friend, that
it has been of late too much the mode to slight the learning of the
ancients. The learned, too, are apt to slight too much the knowledge
of the vulgar. The cooling by evaporation was long an instance of the
latter. This art of smoothing the waves by oil is an instance of both.
Perhaps you may not dislike to have an account of all I have heard,
and learnt, and done in this way. Take it if you please as follows.
In 1757, being at sea in a fleet of 96 sail bound against Louisbourg,
I observed the wakes of two of the ships to be remarkably smooth,
while all the others were ruffled by the wind, which blew fresh.
Being puzzled with the differing appearance, I at last pointed it out
to our captain, and asked him the meaning of it. "The cooks," says
he, "have, I suppose, been just emptying their greasy water through
the scuppers, which has greased the sides of those ships a little;"
and this answer he gave me with an air of some little contempt, as
to a person ignorant of what every body else knew. In my own mind
I at first slighted his solution, though I was not able to think
of another, but recollecting what I had formerly read in Pliny, I
resolved to make some experiment of the effect of oil on water, when
I should have opportunity.
Afterwards being again at sea in 1762, I first observed the wonderful
quietness of oil on agitated water, in the swinging glass lamp I made
to hang up in the cabin, as described in my printed papers[29]. This
I was continually looking at and considering, as an appearance to me
inexplicable. An old sea captain, then a passenger with me, thought
little of it, supposing it an effect of the same kind with that of
oil put on water to smooth it, which he said was a practice of the
Bermudians when they would strike fish, which they could not see, if
the surface of the water was ruffled by the wind. This practice I had
never before heard of, and was obliged to him for the information;
though I thought him mistaken as to the sameness of the experiment,
the operations being different as well as the effects. In one
case, the water is smooth till the oil is put on, and then becomes
agitated. In the other it is agitated before the oil is applied, and
then becomes smooth. The same gentleman told me, he had heard it was
a practice with the fisherman of Lisbon when about to return into the
river (if they saw before them too great a surf upon the bar, which
they apprehended might fill their boats in passing) to empty a bottle
or two of oil into the sea, which would suppress the breakers, and
allow them to pass safely. A confirmation of this I have not since
had an opportunity of obtaining: but discoursing of it with another
person, who had often been in the Mediterranean, I was informed,
that the divers there, who, when under water in their business, need
light, which the curling of the surface interrupts by the refractions
of so many little waves, let a small quantity of oil now and then out
of their mouths, which rising to the surface smooths it, and permits
the light to come down to them. All these informations I at times
revolved in my mind, and wondered to find no mention of them in our
books of experimental philosophy.