166 Scientific Proceedings, Royal Dublin Society. 
5 p.m. and 8 p.m. During the hot and dry hours about mid-day, 
whatever evaporation due to the absolute or relative humidity 
there may be, the columns of air in the pipes are in stable equi- 
librium, with warm air above and colder (though moister) air 
below, so that whatever is evaporated from the water-surfaces 
can only be carried off by the comparatively slow process of 
diffusion. After sunset, however, although the evaporation actually 
due to the relative and absolute humidities alone is less, a condi- 
tion of unstable equilibrium is set up inside the pipes, and hence 
the aqueous vapour is the more readily removed to a distance, as 
it is formed by convective action. In the case of the open cup, 
which is practically always cooler than the surrounding air, the 
maximum convection effect will necessarily be the opposite to that 
in the pipes, namely, when the temperature of the air exceeds that 
of the water by the greatest amount—say about noon—for at that 
time the cooler vapour-laden air overlying the water will the 
more readily flow off and be displaced by the warmer air surround- 
ing it. In the case of the tub, which is warmer than the air for a 
great part of the night, but colder at other times, we should expect 
a maximum about noon on account of the overflow of cool vapour- 
laden air, and a tendency to asecond maximum between sunset 
and midnight on account of a mild upceast of relatively warm, moist 
air. With the object of testing this point, I have made a short 
series of observations upon the rate of evaporation in the metal 
pipes when these have been slightly raised to permit some commu- 
nication between the air at the bottom of the pipes and that 
outside. With the pipes raised ;3, and ;°, inch by means of pieces 
of wooden lath placed under the base flanges, the following were the 
average results :— 
Taste 20. 13 days. 
One-foot Pipe Two-foot Pipe 
Time Intervals SS 
<2; inch “2; inch <3; inch -2; inch 
8a.m.to dp.m. . 35 03 33 27 
5p.m.to 8 p.m. , 20 27 21 31 
8 p.m. to 1l p.m. . 14 17 15 21 
ll p.m. to 8am. . 29 27 24 32 
