148 Scientific Proceedings, Royal Dublin Society. 
four terms of which give Gs & BF 
b=-71 
ce = +706 
d = — 920 
This would give a relative evaporation in a five-foot pipe of 36, 
which is perhaps not far wrong. For longer tubes, however, say 
of upwards of 10 feet, more terms would be necessary. Of course 
such a formula is not in any sense the expression of a “law.” Nor 
is it applicable from hour to hour, although it answers very well 
for the whole-day totals. 
An important point arises in connexion with the quantities of 
water evaporated in the iron pipes at different hours of the day: 
suppose the water and the air above it to be at the same tempera- 
ture, and evaporation to be proceeding at a certain uniform rate ; 
if the overlying air have its temperature raised without raising 
the temperature of the water, will the rate of evaporation be 
increased ? If Fitzgerald’s formula be true, it will not; for, 
according to that, the rate depends only upon the differences of 
vapour-tension. A moment’s consideration, however, will show 
that d priori the evaporation should probably increase. Yor if the 
air were cooled sufficiently, it would become saturated at its tem- 
perature, and any more vapour than would be required to 
saturate the space must obviously be condensed, and evaporation 
would cease. Wherefore, since again raising the temperature of 
the air to its original degree would restore the evaporation to its 
original rate, it follows that a raising of the temperature of the air 
from the dew-point must increase the rate of evaporation. ‘That 
is to say, there should be a factor in any evaporation formula 
dependent on the relative humidity of the air over and above the 
factor dependent on the difference p’ — p”. 
Clerk Maxwell gave expression to an idea’ which in essentials 
is much the same as Fitzgerald’s :—“ Consider two gases in the 
same vessel, the proportion of the gases being different in different 
parts of the vessel, but the pressure being everywhere the same. 
The agitation of the molecules will still cause more molecules otf 
the first gas to pass from places where that gas is dense to places 
where it is rare than in the opposite direction.” Such a statement 
1 Encyclopedia Britannica, 9th. ed., 1877, vol. vii., p. 216. 
