possibility of doubt, and they must, therefore, be admitted as such, however inexplicable, or however at variance with our theories or preconceived opinions.
In the memoir from which some of the foregoing details are extracted, I have further proved that by far the greater part of the superior temperature of mines is derived from the rocky mass of the earth, and does not arise from any extraneous or artificial causes. This, however, is a matter of little consequence to the subject of our present enquiries, since the degree of temperature will act similarly on the animal bodies exposed to it, whether arising from natural or artificial causes. The degree of temperature varies considerably in different mines at the same depth; but this variation seems fairly attributable to the state of ventilation and other extraneous causes, and not to any difference of the natural heat. The increase of temperature does not begin immediately below the surface; a fact which is sufficiently proved by the copious permanent springs in every country, which remain invariably of the mean temperature of the climate. This fact holds equally true in Cornwall, not only in the vicinity of mines, but in the shallower levels of mines themselves. At what precise point below the surface the augmentation of temperature commences, I am unable to say with any degree of confidence; but from a consideration of the influence of extraneous causes in modifying the temperature of the superior galleries of mines, and from some particular observations made by myself, I am disposed to place this point at about 200 feet below the surface. From this point
