repaired. On his return from this labor in 1866, Prof. Thomson along with others of his distinguished coadjutors, received the honor of knighthood. Subsequently he invented a recording receiver for long cables, called the siphon recorder. We have seen that in 1860 Thomson and Tait entered upon the preparation of their treatise on natural philosophy, which was planned to extend to four volumes, but of which the first and last appeared in 1867. In this interval of years Thomson was likewise engaged on the Atlantic Cable, and in writing several cosmological papers, which have ever since been famous subjects for discussion: they were on the age of the Sun, the physical state of the interior of the Earth, and the age of the Earth as an abode for life.
The last mentioned subject was treated of in a paper "On the secular cooling of the Earth," read before the Royal Society of Edinburgh in 1862. He introduced the subject as follows: "For eighteen years it has pressed on my mind, that essential principles of thermodynamics have been overlooked by those geologists who uncompromisingly oppose all paroxysmal hypotheses, and maintain not only that we have examples now before us on the Earth, of all the different actions by which its crust has been modified in geological history, but that these actions have never, or have not on the whole, been more violent in past time than they are at present. It is quite certain the solar system cannot have gone on, even as at present, for a few hundred thousand, or a few million years, without the irrevocable loss (by dissipation, not by annihilation) of a very considerable proportion of the entire energy initially in store for Sun heat, and for Plutonic action. It is quite certain that the whole store of energy in the solar system has been greater in all past time than at present; but it is conceivable that the rate at which it has been drawn upon and dissipated, whether by solar radiation, or by volcanic in the Earth or other dark bodies of the system, may have been nearly equable, or may even have been less rapid, in certain periods of the past. But it is far more probable that the secular rate of dissipation has been in some direct proportion to the total amount of energy
