the greatest possible number of fixed points should be determined by
regular triangulation; secondly, that the greatest possible number of
plane-table fixings should be made use of within each triangle; and
lastly, that eye-sketching should be reduced to a minimum. If these
rules are well attended to, the surveyor can always rely on the value
of the work performed by his subordinates. But all these conditions
cannot be secured in many parts of the ground allotted to the several
topographical parties owing to the quantity of forest land and the
extremely rugged nature of the country. Hence arises the necessity
for test-lines to verify the details, or for some vigorous system of
check; and this is more especially the case where native assistants are
employed.
So soon as the country has been accurately planned, the configuration
of the ground has to be sketched up. This process is the end and aim of
all the preceding work.
The first point attended to is the arterial system, or water drainage,
constituting the outfall of the country; whence are deduced the lines
of greatest depression of the ground. Next the watersheds or ridges of
hills are traced in, giving the highest level. Lastly, the minor or
subordinate features are drawn in with the utmost precision attainable.
‘The outlines of table-land should be well defined,’ says Sir A. Waugh,
‘and ranges of hills portrayed with fidelity, carefully representing
the watersheds or _divortia aquarum_, the spurs, peaks, depressions or
saddles, isthmuses or connecting-links of separate ranges, and other
ramifications. The depressed points and isthmuses are particularly
valuable, as being either the sites of ordinary passes or points which
new roads should conform to.’
And here we must draw a distinction between survey and reconnaissance.
It is absolutely necessary in making a survey that the outlines of
ground as defined by ridges, water-courses, and feet of hills should
be rigorously fixed by actual observation and careful measurement. In
reconnoitring, more is trusted to the eye.
The scale of the Indian topographical survey is that of one inch per
mile; the scale of half an inch per mile being only resorted to in
very densely wooded or jungly country, containing a few inhabitants
and little cultivated, or where the climate is so dangerous that it is
desirable to accelerate the progress of the survey.
On the scale of one inch per mile the practised draughtsman can survey
about five square miles of average country per day. In intricate
ground, intersected by ravines or covered by hills of irregular
formation, the work proceeds much more slowly; on the other hand, in
open and nearly level country, or where the hills have simple outlines,
the work will cost less and proceed more rapidly. On the scale of one
inch per mile all natural features (such as ravines or watercourses)
more than a quarter of a mile in length can be clearly represented.
Villages, towns, and cities can be shown, with their principal streets
and roads, and the outlines of fortifications. The general figure and
extent of cultivated, waste, and forest lands can be delineated with
more or less precision, according to their extent. Irrigated rice-lands
should be distinctly indicated, since they generally exhibit the
contour of the ground.
The relative heights of hills and depths of valleys should be
determined during the course of a topographical survey. These vertical
elements of a survey can be ascertained by trigonometrical or by
barometrical observations, or by a combination of both methods.
‘The barometer,’ says Sir A. Waugh, ‘is more especially useful
for determining the level of low spots from which the principal
trigonometrical stations are invisible. In using this instrument,
however, in combination with the other operations, the relative
differences of heights are to be considered the quantities sought, so
that all the results may be referable to the original trigonometrical
station. The height above the sea-level of all points coming under any
of the following heads is especially to be determined, for the purpose
of illustrating the physical relief of the country:—
‘1st. The peaks and highest points of ranges.
‘2nd. All obligatory points required for engineering works, such as
roads, drainage, and irrigation, viz.:—the highest points or necks of
valleys; the lowest depressions or passes in ranges; the junctions
of rivers, and _débouchements_ of rivers from ranges; the height of
inundation-level, at moderate intervals of about three miles apart.
‘3rd. Principal towns or places of note.’
Of the various methods employed to indicate the steepness of slope,
that of eye-contouring seems alone to merit special comment. In true
contouring, regular horizontal lines, at fixed vertical intervals,
are traced over a country, and plotted on to the maps. This is an
expensive and tedious process, whereas eye-contouring is easy,
light, and effective. On this system all that is necessary is that
the surveyor should consider what routes persons moving horizontally
would pursue. He draws lines on his chart approximating as closely as
possible to these imaginary lines. It is evident that when lines are
thus drawn for different vertical elevations, the resulting shading
will be dark or light, according as the slope is steep or gentle. This
method of shading affords scope as well for surveying skill as for
draughtsmanship.
(From _Once a Week_, May 1, 1869.)
_A SHIP ATTACKED BY A SWORD-FISH._
I have always been puzzled to imagine how the ‘nine-and-twenty knights
of fame,’ described in the ‘Lay of the Last Minstrel,’ managed to
‘drink the red wine through the helmet barr’d.’ But in nature we meet
with animals which seem almost as inconveniently armed as those chosen
knights, who
... quitted not their armour bright,
Neither by day nor yet by night.
Amongst such animals the sword-fish must be recognised as one of the
most uncomfortably-armed creatures in existence. The shark has to
turn on his back before he can eat, and the attitude scarcely seems
suggestive of a comfortable meal. But the sword-fish can hardly even
by that arrangement get his awkwardly projecting snout out of the way.
Yet doubtless this feature, which seems so inconvenient, is of great
value to Xiphias. In some way as yet unknown it enables him to get his
living. Whether he first kills some one of his neighbours with this
instrument, and then eats him at his leisure, or whether he plunges it
deep into the larger sort of fish, and attaching himself to them in
this way, sucks nutriment from them while they are yet alive, is not
known to naturalists. Certainly, he is fond of attacking whales, but
this may result not so much from gastronomic tastes as from a natural
antipathy—envy, perhaps, at their superior bulk. Unfortunately for
himself, Xiphias, though cold-blooded, seems a somewhat warm-tempered
animal; and, when he is angered, he makes a bull-like rush upon his
foe, without always examining with due care whether he is likely to
take anything by his motion. And when he happens to select for attack
a stalwart ship, and to plunge his horny beak through thirteen or
fourteen inches of planking, with perhaps a stout copper sheathing
outside it, he is apt to find some little difficulty in retreating. The
affair usually ends by his leaving his sword embedded in the side of
the ship. In fact, no instance has ever been recorded of a sword-fish
recovering his weapon (if I may use the expression) after making a
lunge of this sort. Last Wednesday the Court of Common Pleas—rather
a strange place, by-the-bye, for inquiring into the natural history
of fishes—was engaged for several hours in trying to determine under
what circumstances a sword-fish might be able to escape scot-free
after thrusting his snout into the side of a ship, The gallant ship
‘Dreadnought,’ thoroughly repaired, and classed A 1 at Lloyd’s, had
been insured for 3,000_l._ against all the risks of the seas. She
sailed on March 10, 1864, from Colombo, for London. Three days later,
the crew, while fishing, hooked a sword-fish. Xiphias, however,
broke the line, and a few moments after leaped half out of the water,
with the object, it would seem, of taking a look at his persecutor,
the ‘Dreadnought.’ Probably he satisfied himself that the enemy was
some abnormally large cetacean, which it was his natural duty to
attack forthwith. Be this as it may, the attack was made, and at four
o’clock the next morning the captain was awakened with the unwelcome
intelligence that the ship had sprung a leak. She was taken back to
Colombo, and thence to Cochin, where she was hove down. Near the keel
was found a round hole, an inch in diameter, running completely through
the copper sheathing and planking.
As attacks by sword-fish are included among sea risks, the insurance
company was willing to pay the damages claimed by the owners of the
ship, if only it could be proved that the hole had really been made by
a sword-fish. No instance had ever been recorded in which a sword-fish
had been able to withdraw his sword after attacking a ship. A defence
was founded on the possibility that the hole had been made in some
other way. Professor Owen and Mr. Frank Buckland gave their evidence;
but neither of them could state quite positively whether a sword-fish
which had passed its beak through three inches of stout planking could
withdraw without the loss of its sword. Mr. Buckland said that fish
have no power of ‘backing,’ and expressed his belief that he could
hold a sword-fish by the beak; but then he admitted that the fish had
considerable lateral power, and might so ‘wriggle its sword out of a
hole.’ And so the insurance company will have to pay nearly six hundred
pounds because an ill-tempered fish objected to be hooked, and took its
revenge by running full tilt against copper sheathing and oak planking.
(From the _Daily News_, December 11, 1868.)
_THE SAFETY-LAMP._
As recent colliery explosions have attracted a considerable amount
of attention to the principle of the safety-lamp, and questions have
arisen respecting the extent of the immunity which the action of this
lamp secures to the miner, it may be well for me briefly to point out
the true qualities of the lamp.
In the Davy lamp a common oil-light is surrounded by a cylinder of
wire-gauze. When the air around the lamp is pure the flame burns as
usual, and the only effect of the gauze is somewhat to diminish the
amount of light given out by the lamp. But so soon as the air becomes
loaded with the carburetted hydrogen gas generated in the coal-strata,
a change takes place. The flame grows larger and less luminous. The
reason of the change is this:—The flame is no longer fed by the
oxygen of the air, but is surrounded by an atmosphere which is partly
inflammable; and the inflammable part of the gas, so fast as it passes
within the wire cylinder, is ignited and burns within the gauze.
Thus the light now given out by the lamp is no longer that of the
comparatively brilliant oil flame, but is the light resulting from the
combustion of carburetted hydrogen, or ‘fire damp,’ as it is called;
and every student of chemistry is aware that the flame of this gas has
very little illuminating power.
So soon as the miner sees the flame thus enlarged and altered in
appearance he should retire. But it is not true that explosion would
necessarily follow if he did not do so. The danger is great because
the flame within the lamp is in direct contact with the gauze, and if
there is any defect in the wire-work, the heat may make for itself
an opening which—though small—would yet suffice to enable the flame
within the lamp to ignite the gas outside. So long, however, as the
wire-gauze continues perfect, even though it become red-hot, there will
be no explosion. No authority is required to establish this point,
which has been proved again and again by experiment; but I quote
Professor Tyndall’s words on the subject to remove some doubts which
have been entertained on the matter. ‘Although a continuous explosive
atmosphere,’ he says, ‘may extend from the air outside through the
meshes of the gauze to the flame within, ignition is not propagated
across the gauze. The lamp may be filled with an almost lightless
flame; still explosion does not occur. A defect in the gauze, the
destruction of the wire at any point by oxidation hastened by the
flame playing against it, would cause explosion;’ and so on. It need
hardly be said, however, that, imprudent as miners have often been,
no miner would remain where his lamp burned with the enlarged flame
indicative of the presence of fire-damp. The lamp should also be at
once extinguished.
But here we touch on a danger which undoubtedly exists, and—so far as
has yet been seen—cannot be guarded against by any amount of caution.
Supposing the miner sought to extinguish the lamp by blowing it out, an
explosion would almost certainly ensue, since the flame can be forced
mechanically through the meshes, though it will not pass through them
when it is burning in the ordinary way. Now of course no miner who had
been properly instructed in the use of the safety-lamp would commit
such a mistake as this. But it happens, unfortunately, that sometimes
the fire-damp itself forces the flame of the lamp through the meshes.
The gas frequently issues with great force from cavities in the coal
(in which it has been pent up), when the pick of the miner breaks an
opening for it. In these circumstances an explosion is inevitable, if
the issuing stream of gas happen to be directed full upon the lamp.
Fortunately, however, this is a contingency which does not often
arise. It is one of those risks of coal-mining which seem absolutely
unavoidable by any amount of care or caution. It would be well if it
were only such risks as these that the miner had to face.
Another peculiarity sometimes noticed when there is a discharge of
fire-damp is worth mentioning. It happens, occasionally, that the light
will be put out owing to the absolute exclusion of air from the lamp.
This, however, can only happen when the gas issues in so large a volume
that the atmosphere of the pit becomes irrespirable.
With the exception of the one risk which we have pointed out above, the
Davy lamp may be said to be absolutely safe. It is necessary, however,
that caution and intelligence should be exhibited in its use. On this
point Professor Tyndall remarks that unfortunately the requisite
intelligence is not often possessed nor the requisite caution exercised
by the miner, ‘and the consequence is that even with the safety-lamp,
explosions still occur.’ And he suggests that it would be well to
exhibit to the miner in a series of experiments the properties of the
valuable instrument which has been devised for his security. ‘Mere
advice will not enforce caution,’ he says; ‘but let the miner have the
physical image of what he is to expect clearly and vividly before his
mind, and he will find it a restraining and monitory influence long
after the effect of cautioning words has passed away.’
A few words on the history of the invention may be acceptable. Early
in the present century a series of terrible catastrophes in coal
mines had excited the sympathy of enlightened and humane persons
throughout the country. In the year 1813, a society was formed at
Sunderland to prevent accidents in coal mines or at least to diminish
their frequency, and prizes were offered for the discovery of new
methods of lighting and ventilating mines. Dr. William Reid Clanny, of
Bishopwearmouth, presented to this society a lamp which burnt without
explosion in an atmosphere heavily loaded with fire-damp; for which
invention the Society of Arts awarded him a gold medal. The Rev.
Dr. Gray called the attention of Sir Humphry Davy to the subject,
and that eminent chemist visited the coal mines in 1815 with the
object of determining what form of lamp would be best suited to meet
the requirements of the coal miners. He invented two forms of lamp
before discovering the principle on which the present safety-lamps
are constructed. This principle—the property, namely, that flame will
not pass through small apertures—had been, we believe, discovered by
Stephenson, the celebrated engineer, some time before; and a somewhat
angry controversy took place respecting Davy’s claim to the honour
of having invented the safety-lamp. It seems admitted, however, by
universal consent, that Davy’s discovery of the property above referred
to was made independently, and also that he was the first to suggest
the idea of using wire-gauze in place of perforated tin.
In comparing the present frequency of colliery explosions with what
took place before the invention of the safety-lamp, we must take
into consideration the enormous increase in the coal trade since the
introduction of steam machinery. The number of miners now engaged in
our coal mines is far in excess of the number employed at the beginning
of the present century. Thus accidents in the present day are at
once more common on account of the increased rapidity with which the
mines are worked, and when they occur there are more sufferers; so
that the frequency of colliery explosions in the opening years of the
present century and the number of deaths resulting from them, are in
reality much more significant than they seem to be at first sight. But
even independently of this consideration, the record of the colliery
accidents which took place at that time is sufficiently startling.
Seventy-two persons were killed in a colliery at North Biddick at the
commencement of the present century. Two explosions in 1805, at Hepburn
and Oxclose, left no less than forty-three widows and 151 children
unprovided for. In 1808, ninety persons were killed in a coal-pit at
Lumley. On May 24, 1812, ninety-one persons were killed by an explosion
at Felling Colliery, near Gateshead. And many more such accidents might
readily be enumerated.
(From the _Daily News_, December 4, 1868.)
_THE DUST WE HAVE TO BREATHE._
A microscopist, Mr. Dancer, F.R.A.S., has been examining the dust of
our cities. The results are not pleasing. We had always recognised
city dust as a nuisance, and had supposed that it derived the
peculiar grittiness and flintiness of its structure from the constant
macadamizing of city roads. But it now appears that the effects
produced by dust, when, as is usual, it finds its way to our eyes,
our nostrils, and our throats, are as nothing compared with the
mischief it is calculated to produce in a more subtle manner. In every
specimen examined by Mr. Dancer animal life was abundant. But the
amount of ‘molecular activity’—such is the euphuism under which what
is exceedingly disagreeable to contemplate is spoken about—is variable
according to the height at which the dust is collected. And of all
heights which these molecular wretches could select for the display of
their activity, the height of five feet is that which has been found to
be the favourite. Just at the average height of the foot-passenger’s
mouth these moving organisms are always waiting to be devoured and
to make us ill. And this is not all. As if animal abominations were
insufficient, a large proportion of vegetable matter also disports
itself in the light dust of our streets. The observations show that in
thoroughfares where there are many animals engaged in the traffic, the
greater part of the vegetable matter thus floating about ‘consists
of what has passed through the stomachs of animals,’ or has suffered
decomposition in some way or other. This unpleasing matter, like
the ‘molecular activity,’ floats about at a height of five feet, or
thereabouts.
After this, one begins to recognise the manner in which some diseases
propagate themselves. What had been mysterious in the history of
plagues and pestilences seems to receive at least a partial solution.
Take cholera, for example. It has been shown by the clearest and most
positive evidence that this disease is not propagated in any way
save one—that is, by the actual swallowing of the cholera poison. In
Professor Thudichum’s masterly paper on the subject in the ‘Monthly
Microscopical Journal,’ it is stated that doctors have inhaled a full
breathing from a person in the last stage of this terrible malady
without any evil effects. Yet the minutest atom of the cholera poison
received into the stomach will cause an attack of cholera. A small
quantity of this matter drying on the floor of the patient’s room, and
afterwards caused to float about in the form of dust, would suffice to
prostrate a houseful of people. We can understand, then, how matter
might be flung into the streets, and, after drying, its dust be wafted
through a whole district, causing the death of hundreds. One of the
lessons to be learned from these interesting researches of Mr. Dancer
is clearly this, that the watering-cart should be regarded as one
of the most important of our hygienic institutions. Supplemented by
careful scavengering, it might be effective in dispossessing many a
terrible malady which now holds sway from time to time over our towns.
(From the _Daily News_, March 6, 1869.)
_PHOTOGRAPHIC GHOSTS._
On the outskirts of the ever-widening circle lighted up by science
there is always a border-land wherein superstition holds sway. ‘The
arts and sciences may drive away the vulgar hobgoblin of darker days;
but they bring with them new sources of illusion. The ghosts of old
could only gibber; the spirits of our day can read and write, and
play on divers musical instruments, and quote Shakespeare and Milton.
It is not, therefore, altogether surprising to learn that they can
take photographs also. You go to have your photograph taken, we will
suppose, desiring only to see your own features depicted in the
_carte_; and lo! the spirits have been at work, and a photographic
phantom makes its appearance beside you. It is true this phantom is of
a hazy and dubious aspect: the ‘dull mechanic ghost’ is indistinct,
and may be taken for anyone. Still, it is not difficult for the eye
of fancy to trace in it the lineaments of some departed friend, who,
it is to be assumed, has come to be photographed along with you. In
fact, photography, according to the spiritualist, resembles what Byron
called—
The lightning of the mind,
Which out of things familiar, undesigned,
When least we deem of such, calls up to view
The spectres whom no exorcism can bind.
The phenomena of spiritual photography were first observed some
years since, and a set of carte photographs were sent from America
to Dr. Walker, of Edinburgh, in which photographic phantoms were
very obviously, however indistinctly, discernible. More recently an
English photographer noticed a yet stranger circumstance, though he
was too sensible to seek for a supernatural interpretation of it. When
he took a photograph with a particular lens, there could be seen not
only the usual portrait of the sitter, but at some little distance a
faint ‘double,’ exactly resembling the principal image. Superstitious
minds might find this result even more distressing than the phantom
photographic friend. To be visited by the departed through the medium
of a lens, is at least not more unpleasing than to hold converse with
spirits through an ordinary ‘rapping’ medium. But the appearance of a
‘double,’ or ‘fetch,’ has ever been held by the learned in ghostly lore
to signify approaching death.