30 Scientific Proceedings, Royal Dublin Society. 
gives the fall of temperature in four hours for different initial 
temperatures, according to the relative humidity at 2 a.m., for the 
two years 1896 and 1897 :— 
RELATIVE Houmiprry. 
Initial Under 80 °/, 
3 50-59 °/.. | 60-69 °/.. | 70-79 °/_. 9 Average. 
Temperatures.| 90 low ‘ Io lo | and over. 5 
Fatt or TEMPERATURE. 
Under 35° ee 51 3°2 3°8 1°9 3°6 
35°-39°°9 51 4-9 3°7 3-2 3-2 3-7 
40°-4.4°:9 6-7 5°6 4°5 4-4 3°0 4°6 
45°-49°-9 - Beil 5:6 4-7 3°38 3°5 4°7 
50°-54°°9 o:5 - 65 5:2 5:0 3°0 5-1 
55°-59°°9 6°4 6°5 5'8 4-0 2°5 5-2, 
60 and over. el 6°2 5:5 4-2 2°9 6-2 
Thus for a given relative humidity the fall will be the more 
rapid as the initial temperature is higher. As might be expected, 
there is a good deal of irregularity in the individual items, but the 
general tendency is evident enough. And the important fact 
remains that for any assigned initial temperature the fall becomes 
less as the relative humidity increases. Both facts refute the 
assertion that the quantity of water-vapour determines the rate 
of cooling of the air. To particularise the first deduction, it 
need only be pointed out that the vapour-tension for an initial 
temperature of 388°, and a relative humidity of sixty per cent., 
is scarcely one-third of the vapour-tension for an initial tempera- 
ture of 63° and a relative humidity of sixty per cent. There- 
fore, if it be true that the cooling will be the more rapid as the 
quantity of water-vapour decreases, we should expect a greater fall 
for the lower initial temperature ; whereas the table shows very 
decidedly that the contrary is the case. It is to be observed that, 
especially for the lower relative humidities of the table, the satura- 
tion-point has little, if any, influence upon the falls of temperature 
shown, For these falls are not more than one-third of the 
