'The Transactions of the Bombay Geographical Society. From December 1854 to March 1856. (New Issue.) Edited by the Secretary. Volume XII.' [‎23] (42/258)

INSTRUMENTS EMPLOYED IN GEOGRAPHICAL RESEARCH. XXU
15 lbs. on the square inch anl our body sustains a load of It of about seven tons over the wholes
surface. If I enclose a column of mercury 30 Inches in length in a glass tube closed at the
upper extremity, so that the pressure of the air is removed from it, and open at the lower to its
influence, I find that it will exactly counterpoise or sustain it; and thus a Barometer is formed.
A scale is now attached to the other extremity, and for the sake of strength, or ornament, or
both the tube is enclosed in a brass or wooden sheath, and thus stands complete. When the air
becomes lighter, which it does before storms or falls of rain, especially in the Tropics, the baro.
meter falls, that is, the atmophere is no longer able to support a column of mercury so long
or sustain a load so heavy as before. But the same thing happens irrespetclve of the weather.
If I ascend an elevation : the column of air becomes shortened, and diminishes in weight at the
rate of half a pound on the square inch every 1,000 feet, and so the mercury falls by one-
thirtieth of its whole length, or by one inch ; while, on the other hand, as I descend, it rises
a similar fraction. On subdividing the scale into tenths and hundredths of inches, for every
hundredth it falls, I as sume that I have descended ten feet, for every tenth a hundred feet, and
for every inch a thousand, the converse holding good in the case of ascents.
The Rain Gaugk, and its sister the Evapobatino Dish are vessels with apertures of very
accurately established area ; the former receives the rain that falls, and conveys it into a
receiver, from whence it can be measured ; the latter is filled with water, and the quantity carried
off in a given time determined. Both represent spaces on the surface of the earth, of their
own area, and it is quite clear that if made in all points to assimilate as closely to the surface,
desired to be examned, there can be no reason why they should not correctly represent them
or why, by knowing how much rain falls, or vapour is carried off from a square foot of space, we
should not infer with tolerable exactitude how much ought to be carried off by an acre, a square
mile, or square degree, or any given surface, whatever its extent. The thermometer slightly
modified comes to assist us in one of these divisions of investigation. The air in its natural
state is never perfectly dry, and it requires much care and labour to free it from its moisture ;
unless when rain actually falling it is never so thoroughly we tthat it will not take up an additional
amount of moisture—the avidity with which it drinks this in is the test of its thirst, a counterpart
to this being the leluctance or readiness with which it parts with its moisture. If X take a couple
of thermometers,and cover the bulb of the one with wetted muslin, on being exposed to a breeze
it will sink below the other in proportion to the celerity with which its moisture is carried off,
this is called the wet and dry bulb thermometer or hygrometer. From this the point may
be inferred when dew must be precipitated or rain fall—what is the hygrometric condition of
the climate, and what moisture the air holds in suspension. If, again, I insert a thermometer in a
vessel artificially cooled, it will be seen that there is a point at which its outer surface, if polish
ed, becomes covered with moisture. The temperature at which this occurs is called the
dewing point, as being that at which dew is precipitated. From this, by reference to figure
tables, may be ascertained how many grains of moisture are contained in a cubic foot of air—■
vast oceans of water that are suspended in the form of invisible gas or of visible clouds in the
air, with everything that relates to vapour in situ.
Though these descriptions are all theoretically correct, and sufficient for enabling the subject
to be understood, to go into the minutise and refinements that are resorted to to secure ac
curacy and exactitude would require an hour at least for the description of each instrument.
The instruments for measuring angles, of which the chronometer, is a constant adjunct, the
telescope an indispensable part, enable us to penetrate into the depths of space, and to measure
the size and distance of every object where a base of sufficient length can be found to give the
lines proceeding from its extremities a suitable convergence to the point to be determined.
With these we track the planets in their paths, and determine the times of their appearance,
and lay down with the utmost nicety the courses they pursue through space—with these we
follow the comets deep into the recesses of immensity and if we can say but little of the fixed
stars by reason of their enormous remoteness, we can assign them a distance the imagination
can hardly comtemplate, which the reason assents to, but fails to follow. By these we can tell
our position on the surface of the globe, and trace and lay down the continents, islands, and
oceans which diversify its surface. With the thermometer I obtain a register of the temper
ature, and with the hygrometer a record of the moisture of the atmosphere in all its changes,
and the barometer gives me not only the pressure of the air, at any given hour at a fixed spot,
but enables me to tell when 1 have ascended above, or descended benenth the level of the sea or
of any established position, and how much has been my ascent or descent. With this little mer
curial divining rod, scarcely larger or more cumbersome than a walking cane, I can fathom
through the depths of the atmosphere, and decide to half an ounce on its surface the amouu t