576 THE POPULAR SCIENCE MONTHLY. 



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, ac- 

 cording 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 mathe- 

 matician 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 Sillimari'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 mat- 

 ter 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 in- 

 crease of subterranean temperature with the dej)th, 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 concentra- 

 tion of terrestrial materials, and the obstacles which central density or 

 igneous fusion may present to its escape. Instead, however, of contro- 

 verting the peculiar views of the eminent scientist on physical geology, 

 I will only trace the consequences to which they lead. He maintains 



