654 THE POPULAR SCIENCE MONTHLY. 



hundreds of millions of years ago that the figures convey no practi- 

 cal idea to the mind, and amount in effect almost to what a distin- 

 guished geologist calls " eternity a parte ante.'''' 



The principal grounds upon which scientific opinion has recently 

 declared itself in favor of limited periods for the duration of the solar 

 system ai-e based, first, on the belief that the earth is cooling if not 

 rapidly at such a rate as to make it impossible that it should have 

 existed, for very many millions of years ; secondly, because there is 

 reason to believe that the earth is not now rotating on her axis with 

 the same rapidity as in former ages, and that, as her shape would 

 have been different if, at the time she was in a molten state, she had 

 been rotating more rapidly than now, she has not been rotating so 

 long as has been supposed ; thirdly, because the sun is parting with 

 caloric at such a rate as to make it certain that he could not have 

 continued to radiate heat at the same rate for more than a few millions 

 of years ; and lastly, because the changes in the earth's crust, stupen- 

 dous and varied as they are, could have been, and probably were, 

 accomplished in the course of much shorter periods than popular 

 geology has hitherto considered possible. 



It will, of course, be understood that any inquiry as to the date 

 of creation must necessarily have relation only to the solar system 

 the sun, that is, and the planets which accompany the earth in its 

 orbit round the central luminary. 



The investigation is of necessity thus narrowed, because we have 

 not, and cannot expect to have, any definite information as to the age 

 of the rest of the visible universe. The stars are forever beyond our 

 ken. If the spectroscope can bring intelligence of their component 

 elements, it is as much as we can hope to attain ; for their immeasu- 

 rable distance effectually removes them from investigation. No action 

 of gravity emanating from those distant luminaries affects the inter- 

 nal economy of the solar system. In the vast eternity of space the 

 sun and his attendant satellites are altoo;ether alone. 



It is difficult to gaze upon the thousands of stars that brighten 

 the night with their radiance, and yet realize our entire isolation. 

 The solar system, with the radius of its orbit stretching from the sun 

 to farthest Neptune, is but a point in a vast solitude. No star is 

 nearer to us than 200 millions of millions of miles. 



It is difficult, in dealing with such enormous numbers, to retain a 

 definite impression on the mind. Our powers of conception are fitted 

 rather to the wants of common life than to a complete survey of the 

 universe. 



Perhaps an intelligent may be substituted for a merely formal 

 assent to these numbers, if they are considered on a greatly-dimin- 

 ished scale. Consider the figures on the scale of one mile to 100,- 

 000,000. On that scale the sun's distance from the earth will be repre- 

 sented by nearly one mile. Let the sun be represented by a globe 



