[41] The reader will find this subject further developed in a paper lately communicated to the Geological Society.
[45] A very curious instance of the pursuit of a law completely empirical into an extreme case is to be found in Newton’s rule for the dilatation of his coloured rings seen between glasses at great obliquities. Optics, book ii. part i. obs. 7.
[47] “When we are told that Saturn moves in his orbit more than 22,000 miles an hour, we fancy the motion to be swift; but when we find that he is more than three hours moving his own diameter, we must then think it, as it really is, slow.” Thirty Letters on various Subjects, by William Jackson, 1795.
[48] Thomson’s First Principles of Chemistry.
[49] There seems no doubt, however, that an achromatic telescope had been constructed by a private amateur, a Mr. Hall, some time before either Euler or Dollond ever thought of it.
[50] We allude to the recently invented achromatic combinations of Messrs. Barlow and Rogers, and the dense glasses of which Mr. Faraday has recently explained the manufacture in a memoir full of the most beautiful examples of delicate and successful chemical manipulation, and which promise to give rise to a new era in optical practice, by which the next generation at least may benefit. See Phil. Trans. 1830.
[52] Jackson, Letters on Various Subjects, &c.
[53] Thomson’s First Principles of Chemistry, Introduction.
[54] The progress of astronomical discovery has since shown that this law cannot be relied on (1851).
[55] Novum Organum, part ii. table 2. (24), (30), &c. on the form or nature of heat.
[56] We will mention one which we do not remember to have seen noticed elsewhere in the case of a disturbance of the equilibrium of heat produced by means purely mechanical, and by a process depending entirely on a certain order and sequence of events, and the operation of known causes. Suppose a quantity of air enclosed in a metallic reservoir, of some good conductor of heat, and suddenly compressed by a piston. After giving time for the heat developed by the condensation to be communicated from the air to the metal which will be thereby more or less raised in temperature _above_ the surrounding atmosphere, let the piston be suddenly retracted and the air restored to its original volume in an instant. The whole apparatus is now precisely in its initial situation, as to the disposition of its material parts, and the whole quantity of heat it contains remains unchanged. But it is evident that the distribution of this heat within it is now very different from what it was before; for the air in its sudden expansion cannot re-absorb in an instant of time all the heat it had parted with to the metal: it will, therefore, have a temperature _below_ that of the general atmosphere, while the metal yet retains one above it. Thus, a subversion of the equilibrium of temperature has been _bonâ fide_ effected. Heat has been driven from the air into the metal, while every thing else remains unchanged.