'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎10] (29/458)

pin not quite touching the surface. The two ought to be as far apart as to preclude the
second from being affected by the little mass of water raised by the capillarity of the
fl rst above the general level. The arrangemnet may be very conveniently reversed by placing
a stick athwart the dish, and pointing the pins upwards—or a combination of the two may
be resorted to. An evaporating dish of this sort is carefully filled at sta rting . the next
day as much water is poured into it as brings it up to the original position, from a
measure, previously accurately divided into so many grains and so many inches
and decimals, for both ought to be indicated on the measure,—the latter
only to be read : the water required for this will indicate the amount carried ofl
in vapour. These observations can only be made when the air is perfectly still, and as the
evaporating dish ought to be exposed to the breeze, so a moveable screen must be provided to
secure shelter while observing, and permit the breeze to exercise its full force when the dish
is in use.
« j have found the following contrivance very convenient indeed for the purpose of deter
mining evaporation. Procure a dish edged and turned the same as the rain-gauge funnel al
ready described (page 28.) In this case the mou h of
the funnel must be provided with a stop-cock—or
where this is not procurable, with a plug or cork.
The funnel ought to be made to hold a quart
exactly, quite up to the brim, and this it will do if made two inches deep,—the conical
portion one, and the cylindrical portion one ; and
six and a half inches wide—the extra half inch
being allowed for the insertion of the turned rim,
which will bring it down to six inches. The
funnel being put in its place, a quart bottle,
carefully filled with water exactly to the neck, is
now emptied into it, and the bottle itself placed
under it, on a stand like that represented in the
accompanying diagram. On first filling the evapo
rating dish, the bottle full should be poured in and
emptied out several times to see how much exactly
is kept back on the sides of the funnel after some
minutes have been allowed for it to drip—the
plug being thoroughly soaked at the same time.
All these precautions having been duly attended to,
the dish is filled—the deficiency next day in the
bottle will indicate the amount of evaporation.
“ The marks of the measuring tubes are made to read both ways by figures both above
and below the lines. The best way is to fill the tube up to the top of the scale exactly :
then open the stop-cock and let the water flow into the measure, taking great care that
none is allowed to run over or spill. Note the number of grains run out, and divide this by
the number of grains corresponding to the area of the evaporating dish as found in the
table, adding ciphers-—the quotient will give the amount of evaporation.
Example. The area of the evaporating dish is 6 inches:—a stratum of water of this area
one inch in thickness weighs 7158-5974152 grains—the decimal may be dismissed. Sup-