Summer Birds of Passage. 459 
found at less than 80 feet below the external 
soil. Mr. Boyle kept a thermometer for a 
year, in a cave which was situate under a 
roof of earth 80 feet in thickness; and found, 
that the liquor in the instrument remained 
stationary all the time. In compliance with 
my, request, the late Dr. Withering made a 
similar experiment on a well 84 feet deep, 
at Edgbaston near Birmingham, the tempe- 
rature of which was found to be 49° in every 
month of the year 1798. Pits or wells of 
a less depth give more or less annual varia- 
tion of temperature, according to the distance 
to which they penetrate the superficial strata 
of the earth, A remarkable singularity, 
however, is observable in experiments made 
on pits of a moderate depth. I kept a 
monthly account of the temperature of a 
well, for the Years 1795 and 1798, the 
perpendicular depth of which was 20 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 tem- 
perature of the ground, at the distance of 
20 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 
3 M2 
