32 Scientific Proceedings, Royal Dublin Society. 
A previous set of observations made in the same ground, 
during the winters of 1897 and 1898, gave the following falls 
of Earth-temperature just under the surface between 8 p.m. 
and 8 a.m. on clear nights :— 
Dew- Point of Tak N ube Relative of Po N we 
8 ae a ea Clear Nights pet . pee Glos Nianeet 
8 p.m.-8 a 8p.m.-8a 
Under 30° 116 57 Under 40°/, 14-5 29 
30°-34°-9 12:5 70 | ADO 12-8 23 
35°-39°-9 13-2 34 | 45 ,,49 ,, 12:4 31 
40°-44°-9 11:9 13 | SO 494 55 11-8 39 
45° and over 10°9 5 55 ,,—-09 ,, 11°3 24 
60 ,, & over iliteil 33 
Considering the falls of Harth-temperature as compared with 
the relative humidity, between the hours 8 p.m. and 11 p.m., we 
see that a simple division by three gives practically the same 
value as those for the cooling of the air, so long as the air is not 
more than half saturated. For humid states in excess of this, the 
air cools faster than the ground. 
The falls of Harth-temperature as compared with the dew-point 
are of exactly the same nature in the two series as those of the 
cooling of the air, namely, that they increase at first to a dew-point 
of about 40-45°, after which they decrease again. The explana- 
tion is that the humidity of the air chances to have on the whole 
its minimum values when the dew-point is about 40°—49°. 
We see, then, that the lower air takes its temperature chiefly 
from the ground, but that the great radiative power of its contained 
water-vapour, when the relative humidity is high, lowers its 
temperature somewhat faster than it can absorb radiation from the 
ground. Only in one class of observations have I been able to 
detect any regulating effect upon the temperature due to the actual 
quantity of water-vapour present. ‘The observations in question 
are, however, differential, and the recognised effect probably only 
indirect. A summary is given below. It shows for clear nights 
