'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎7] (26/458)

DR. FORD’S ATMOMETER.
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After the equilibrium has b een obtained, either the entire apparatus may be raised by
the gear referred to, or the cylinder or the equipoise weight, or both, may be detached
from the beam. The whole should be placed under a large cage made of fine bamboo
sticks with large interspaces, otherwise birds and insects will give an erroneous value to the
evaporation. This circumstance is overlooked when the ordinary evaporating dishes are
employed.
I was induced to devise, and construct, the above described instrument, in the hope that
by its use the solution of one of the most difficult problems of Meteorology might be facili
tated, if not overcome. By it all theoretical views as to the force of vapour at various
temperatures are avoided—*the units here, without a risk of extraneous errors, being the
drain weight, the cubic inch, the degree of Fahrenheit’s Thermometer, and a given interval
of time. The Barometer will of course be a most valuable adjunct. As weighing, however, is
a far more accurate process than measuring, it should invariably be had recourse to in pre
ference, and the obtained weight subsequently be reduced to cubic inches if required.
Even in graduated narrow tubes it is difficult to fill up so that the measure shall at once cor
respond with a given weight. Let any one fill a wineglass to a certain mark, and note
its weight ■ fill it, to all appearance, as before, and then remark the disparity when it
has been weighed. Success in these attempts amounts to a sheer impossibility when a ves
sel of large area is employed. It is this circumstance that renders the common evapo
rating dish so untrustworthy. It is, likewise, most essential that the evaporating surface
should be maintained atone mean height, otherwise, when it is on the gradual decrease,
and falls much beneath the margin of the vessel containing the water, the evaporation I <
retarded. This desideratum is fulfilled in the present instrument, since the fountain will
act only as required.
At present every formula (be it Dalton’s, Ure’s, Robinson’s, Galbraith’s, Joory’s, or of
others and they all differ from each other) employed to ascertain the amount of evapora
tion, is more or less empirical and arbitrary. In this Atmometer, authorized units are
only used, and so long as the accuracy of the balance remains unimpaired, and any degree
of attention be paid to the registrations, the results must approach in character and degree
those of a natural process.
As no strictly true value can be allowed to the ever varying power and dryness of the
wind, it will be more advisable to place the instrument in a shaded open space, cut off
equally from irregular currents of air and refracted heat.
C. G. E. Ford.
Hoshungabad, July 16 th, 1850.
Two years ago I had the pleasure of directing the attention of the Society to the ex
treme difficulty of carrying out hygrometrical experiments,—pointing out the extraordinary
degree of neglect and inattention with which the subject of evaporation had been treated.
The common wet and dry bulb, the hygrometer of Daniell, and others, merely indicate the
quantity of moisture in the air—evaporation is not only dependent on this, but on the velo
city of the wind, I then laid before you a model of an atmometer on the steel-yard
principle, consisting of a balance, with unequal arms,—the shorter of these having an
arched segment at the extremity. From this an evaporating dish, like that of Dr., Ford,
was suspended; this, when filled with water, was so adjusted and counterpoised as to be
in equilibrio with the steel-yard when this stood in a horizontal position, and of course a s
the water was drawn off by evaporation, the long arm descended, the amount of descent