202 
BIRDS OF PASSAGE. 
Remarks on 
the tempera- 
ture of deep 
^caverns it 
does riot 
vary. 
Hence the 
lethargic ani- 
mals do not 
retire to great 
depths. 
facts respecting the temperature of places situated at great 
depths below the surface of the land and water. 
Every place on the globe has an invariable temperature pecu- 
liar to itself, which cannot be found at less than SO feet below 
the externa] soil. Mr. Boyle kept a thermometer for a year, 
in a cave which was situate under a roof of earth 80 feet in 
thickness j and found that the liquor in the instrument re- 
mained stationary all the time. In compliance with my re- 
quest, the late Dr. Withering made a similar experiment on a 
well 84 feet deep, at Edgbaston, near Birmingham, the tem- 
perature of which was found to be 4Q° in every month of the 
year 1/98. Pits or weils of a less depth give more or less 
annual variation of temperature, according to the distance to 
which they penetrate the superficial strata of the earth. A 
remarkable singularity, however, is observeable in experiments 
made on pits of a moderate depth. I kept a monthly account 
of the temperature of a well, for the. year 17g5 and 1/98, 
the perpendicular depth of which was twenty feet ; and the 
annual variation of its temperature fell a little short of 4°. 
But the following circumstance deserves to be carefully re- 
marked on the present occasion. The temperature of the 
ground, at the distance of twenty feet from the surface, is at 
the highest in October, when a thermometer exposed to the 
atmosphere makes the monthly mean coincide with that of the 
year : on the contrary, the subterranean temperature does 
not arrive at a minimun before the end of March ; which is 
three months later than the coldest weather above ground. 
The facts just stated throw much light on the subject of the 
present essay, by pointing out the reason which determines 
animals of known lethargic habits to form their winter retreats 
near the surface of the ground. This choice exposes them, 
according to the experiments of Dr, Hales, to a variable tem- 
perature, which sinks slowly at first, and keeps them benumbed 
by a sleepy torpor 3 but after the rigours of winter are past, 
the hiding places of these slumbeiers are gradually warmed 
by the returning sun, which reanimates their torpid limbs, 
and recalls them from their secret dens, at the proper moment 
for their appearance above ground. Had the hedgehog, the 
field mofise, &c. made a contrary choice, and retired to ca- 
verns 80 feet deep, all the benefit they could have derived 
from 
