came more attractive as it held out the hopes of giving a means of definitely measuring vast periods of time. It was calculated that the concentration from a widely-diffused nebula to its present size would produce as much heat as the sun would lose in 20,000,000 years, according to the present rate of radiation. This period was accordingly fixed as the age of the sun and the duration of solar light. Another step was soon taken in this direction by fixing a limit to the age of worlds. It was concluded, with much confidence, that less than 20,000,000 years have elapsed since the earth became a planet, and that previously it must have formed a part of the solar atmosphere. In Prof. Tait's "Recent Advances in Science," the estimate obtained in this way for the age of our globe is placed between 15,000,000 and 20,000,000 years; and geologists are given to understand that they must recognize the infallibility of mathematical authority and abstain from their usual extravagance in making exceedingly large drafts on the limited fund of time.
But the conditions on which this surrender of geological belief has been demanded are far more liberal than any which the eminent mathematician is legitimately authorized to offer. The estimate on which he relies has been made for an homogeneous nebula supposed to be equally dense at its borders and in its central regions, whereas there must be a preponderating density near the centre, according to the necessary inferences from the doctrines of Laplace. This early central condensation must be adopted to account for the great mass of the sun compared with that of the planets. In an able investigation on the subject, in this point of view, published in Silliman's Journal in 1864, Prof. Trowbridge concludes that, even in the earliest stage of planetary development, there must have been a very great concentration of matter around the central nucleus of our solar nebula. If we adopt the law which he deduces for the rapid increase of density toward the centre, it may be found that the amount of heat due to contraction since the supposed birth of our world would not be enough to compensate for the calorific waste which the sun sustains in 1,000,000 years.
To obtain information of the age of the earth's crust from the increase of subterranean temperature with the depth, according to the method devised by Fourier, is an object to which much labor has been devoted, and from which valuable fruits may be expected. On this principle, the time since the permanent solidification of the surface of our globe has been estimated by Sir William Thomson at about 100,000,000 years. But this estimate, which is obtained by taking 7,000° Fahr. as the highest limit of internal temperature, will appear too low when we consider the vast amount of heat arising from the primitive concentration of terrestrial materials, and the obstacles which central density or igneous fusion may present to its escape. Instead, however, of controverting the peculiar views of the eminent scientist on physical geology, I will only trace the consequences to which they lead. He maintains
