Surron—ZInfluence of Water-Vapour upon Nocturnal Radiation. 15 
This criticism applies at once to “a representative series of 
observations made between the 4th and 25th of March, 1850 [by 
General R. Strachey, R.E.]; during which period the sky re- 
mained remarkably clear, while great variations in the quantity of 
vapour took place. The first column of numbers gives the tension 
of aqueous vapour ; the second the fall of the thermometer from 
6.40 p.m. to 9.40 a.m. 
Tension of Fall of 
Vapour. Thermometer. 
888 inch. aN Ea 6°:0 
849 ,, E a 8°-3 
805 ,, ie - 82-3 
‘749 ,, ne Ai 10°3 
‘708, Pr Hy 10°3 
699) ©; A Be 12°°6 
605 ,, - ux 12°1 
554 ,, we A ieee 
435 ,, ve “'¢ 16°°5 
“The general result is here unmistakable. In clear nights the 
fall of the thermometer, which expresses the energy of the radia- 
tion, is determined by the amount of transparent aqueous vapours 
in the air. The presence of the vapour checks the loss, while its 
removal favours radiation and promotes the nocturnal chill.”! 
But it is to be observed that because the period mentioned is only 
three weeks, the temperature of the air at 6.40 p.m. may not have 
varied to any great extent during the whole time; and hence the 
humidity and tension may have varied together. If this is so, 
then the effect which Tyndall ascribed to the absolute humidity 
may, as before, have been just as probably due to the relative 
humidity. 
Commenting on the agreement which Tyndall deduces between 
the observed and computed absorption, Professor Very remarks 
compressed to 7 the depth at 34 times the pressure. But, on the other hand, doubling 
the actual quantity in a given depth did not double the absorption, 
1J. Tyndall, ‘‘ Heat a Mode of Motion,” 1880, p. 383. 
