Summer Birds of Passage. 457 
covered by a thin stratum of earth, ora sheet 
of water of a moderate depth; in conse- 
quence of which, they are warmed in due 
season by the rays of the sun, after he has 
entered the northern half of the ecliptic. 
The preceding assertion, is not a plausible 
conjecture built upon probabilities; but a 
fact, which has been determined by. experi- 
ment; for the Rev. Dr. Hales, in the course 
of his experimental enquiries into the process 
of vegetation, discovered that a thermometer, 
the bulb of which was buried 16 inches below 
the earth’s surface, stood at 25° of his scale 
in September, at 16° in October, and at 10° 
in November during a severe frost; from 
which point it ascended again slowly, and 
reached 23° in the beginning of April (old 
style). Now the latter part of September 
and the whole of October is the season in 
which the bat, the hedgehog, the shrew, the 
toad, and the frog are seen but seldom, and 
finally disappear. ‘The same animals all leave 
their retreats and are observed abroad again 
in the time betwixt the vernal equinox and 
the middle of April; which circumstance 
makes the preceding theory agree very well 
with the variations of temperature, that take 
place in the winter habitations of those ani- 
3M 
