BIRDS OF PASSAGE. 
20i 
vices of walls or rocks j and a few, such as frogs, female toads, 
and wa'er newts, bury themselves in the mud of shal- Their retreats 
low ponds. These retreats are of all them but slightly are snallow * 
covered by a thin stratum of earth, or a sheet of water of a 
moderate depth ; in consequence 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 experiment $ for the Rev. Dr, Facts stated. 
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 ]6° 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 
animals, which are actually known to pass the cold season in a 
torpid condition. 
After making the foregoing remarks on torpidity, I come to Birds though 
certain facts, which are far from favouring the supposed analogy numerous- are 
r , , . , f , , , . . . not found in 
of those animals which are known to be lethargic in winter, the torpid 
and our summer visitors of the feathered tribe, Birds of this state, 
description are very numerous in this part of the world at the 
time of their disappearance $ from which circumstance it is 
reasonable to conclude, that if they take up their winter abode 
near the surface of the earth, they would be frequently found 
in the cold season ; which is the case with bats, field-mice, 
and hedgehogs. Though discoveries of this kind are mentioned 
by various authors, the uncommonness of the circumstance 
obliges the advocates of torpidity to dispose of the periodical 
brrds during winter, in places which are inaccessible to men, 
such as the vaults of profound caverns, or the bottoms of deep 
lakes. My objections to this opinion, are derived from certain 
facts 
