The same reasoning is applicable to move ments of the temperature of bodies under the influence of that of the air. Heat will be propagated from without inwards, and will raise the general temperature of the body, un le: s it lessens in the same proportion as it re ceives external temperature, that which it pro duces of itself.
The consideration of the changes in the in tensity of the internal focus, in other words, in the faculty of producing heat, and of the con ditions which determine these, is the most im portant point of all in the study of animal heat, on account of the multitude of practical applications which result from it.
It is obvious from what has already been said that there is an essential difference between inert or inanimate and animate bodies subjected to the influence of external temperature. The temperature of the former depends solely on the general laws which regulate the propaga tion of heat, whilst the temperature of the others is subjected to the influence of another element, namely, the heat which they them selves produce. Did this element continue fixed and invariable, it would be possible to determine, by the application of the known data of physics, what must be the temperature of a living body under the influence of a given external temperature. But if this element varies, and the laws according to which it varies are unknown, it becomes impossible to predict in what manner the temperature of an animal will be affected by that of the medium in which it lives. It is only very lately, therefore, that the temperature of man arid warm-blooded animals, with the exception of those that hy bernate, has been believed to continue unaf fected in the midst of extensive variations in the temperature of the surrounding atmosphere.
Variations in the temperature of animal bodies in a state of' health, independently of external temperature.—Duly to appreciate the inquiries that have been instituted in this direc tion, the first question to be asked is, whether or not the temperature of the body presents variations, although external conditions continue the same, or nearly the same ? The answer here must be in the affirmative : the body varies in temperature at different times, exter nal circumstances as to temperature continuing nearly the same. This is apparent in the ob servations of Dr. John Davy instituted with another view, but quite available here. We perceive that the individual designated No. 1, having a temperature of 8 (100° F.), when the air was at 26°, 4 F.), had a tempera ture of only 37°, 5 75 F.) when the air was at 7 (80° F.), that is to say, the same person showed a third of a degree c. less of
temperature, when the air, instead of becoming colder, had actually become warmer in the same proportion. The temperature of No. 3 was 37°, 2 F.), when the air marked 25°, 5 (78° F.), and on another occasion it was only 36°, 9 (98° F.), when the air was 26° (79° F.) ; in other words, the temperature of the body, instead of rising, had actually fallen by 0°, 7 cent., when that of the external air had risen 0°, 9 cent.
Ilylucnce of the natural temperature of the air on that of the body.—It must be obvious from the facts of the last paragraph that the influence of external temperature cannot be appreciated without having recourse to means of observation calculated to make variations dependent on foreign causes to disappear.
To render the comparison of the mean sums obtained more certain, we shall confine our selves to the observations of the inquirer just quoted, made upon the same individuals at different temperatures. The following series is after the data supplied by Dr. Davy :— Temperature of the air. Mean temperature of fire men.
5 (60° F) 85 F.) 5 (78° F.) 16 F.) 4 (79° F.). 32 5, F.) 7 F.) 58 F.) 8 F.) 70 5, F.) It is apparent that these differences, even the extremes, do not surpass the limits of the variations which the same individual exhibits, or may experience spontaneously under the same temperature of the air. But when it is con sidered that these differences are mean results, forming a series increasing with the rise of the external temperature, it is impossible to doubt of their standing in the relation to one another of cause and effect. If this dependence and connection actually exists, we must allow that it is very little obvious at the temperatures within the limits of which these observations were made ; for whilst the temperature of the air varied to the extent of 3 c., the changes in the mean temperature of the body did not exceed 9. Such slight differences being apt to leave uncertainty in the mind as to the cause producing them, we shall confirm the impres sion they are nevertheless calculated to make by citing others, for which we are indebted to Dr. Ileynaud, surgeon of the corvette La Chevrette, in a voyage of discovery in the Asiatic seas undertaken in the year 1827. The instruments used were furnished by the best makers of Paris, and were compared by M. Arago with those of the Observatory ; and the observations were made conjointly with M. Blosville, lieutenant of the vessel, charged by the Academy of Sciences of Paris with various researches in natural philosophy.