thus vary in any substance, merely because it has transmitted or in-
tercepted it for a greater or less length of time. Hence he thinks it
essential to have recourse to some permanent rule from which the
results may in all cases be accurately derived, and which, when the
phaenomena do not correspond, may lead us to the investigation of
some other cause. Such a law has been deduced from direct expe-
riments, and implies that a body placed in a medium of a constant
temperature, becomes heated or cooled in such a manner, that the
(inferences of its heat from that of the medium are in a geometrical
progression, while the times of heating or of cooling are arithmeti-
cally proportionate. It will readily be perceived in what manner it
is practicable to deduce from the two progressions mentioned in this
law, a third progression, which will apply to the intermediate steps
of any series of observations.
This law, when adapted both to Dr. Herschel's experiments and to some new ones here described, is found to apply with singular ac- curacy through the three or four first minutes of increasing heat; but after this period the series manifestly varies, the increase of heat by computation according to the law falling progressively short of that indicated by the thermometers. The author is at considerable pains to explain this anomaly, and at length ascribes it to the heat accu- mulated in the intercepting body, which renders it in a manner a new source of heat, the emanation from which, it must be admitted, cannot but cooperate with the transmitted rays, to raise the ther- mometers near it.
If the progress of this accumulation of heat be perfectly regular, its efi'ect will be confounded with that of the transmitted rays, as was actually found to be the case when a thin plate of talc was used as an intercepting medium. The cause of this difference is ascribed chiefly to the thickness of that medium, and in some measure also to the weakness of the source of heat. It will scarcely be necessary to explain the operation of these concurrent causes, it being obvious that the greater the bulk of a body, the greater will be the accumu- lation it admits of, and the greater the source of heat, the more rapid will be this accumulation.
The next object of inquiry is how long an experiment should last for the thermometer to acquire the maximum of heating, that is, the tperature of the source of heat, or medium in which it is immersed. Here the experiments can be made only on direct heat, since the in. termediate body containing accumulated heat, might, and probably does in most cases, continue to emit this heat after the thermometer has arrived at the maximum, that is, the temperature of the source of heat. In the direct heat of the sun this maximum was obtained in little more than 12'.
The author hereupon examines a number of Dr. Herschel’s experiments, in which he mentions only the initial and final degrees of the thermometer. After showing what the mean ratio is between the degrees computed for the progression of the differences, and those determined by observation, which he finds is as 13 to 10, he deter.
