6 7 o THE POPULAR SCIENCE MONTHLY. 



same part of her surface. Then the liquid protuberance directed 

 toward the rnoou will no longer be a cause of delay, and the retarda- 

 tion will cease. This cessation of effect, owing to the cause having 

 ceased, appears to have actually happened with regard to the moon 

 herself. At some time the moon's crust, and, indeed, her whole sub- 

 stance, was in a molten state. Enormous tides must have been j>ro- 

 duced by the attraction of the earth in this viscous mass of molten 

 rock, and the time of the moon's rotation must have been quickly 

 compelled, by the friction, to become identical with the time of its 

 revolution round the earth, and now, as is well known, the moon 

 always presents to the earth the same side of her sphere. 



It being thus established that there is retardation of the earth's 

 motion, and the amount of retardation being calculated, it remains 

 only to inquire how the fact affects the question of the world's age. 

 We know that the flattening at the poles and bulging at the equator 

 is the result of rotation ; from the amount of retardation it can be 

 calculated how fast the earth was rotating in by-gone ages. Two 

 thousand millions of years ago she would, according to such calculation, 

 have been revolving twice as fast as at present, and the amount of 

 centrifugal force at the equator would have been four times as great 

 as now. If the earth, subjected to such strong centrifugal force, had 

 been liquid or even pasty, when it began to rotate, the equatorial 

 protuberance would have been much greater than it is. It therefore 

 follows that she was rotating at about the same rapidity as now, 

 when she became solid, and as the rate of rotation is certainly dimin- 

 ishing, the epoch of solidification cannot be more than ten or twelve 

 millions of years ago. 



A third argument for restricted periods is founded on an examina- 

 tion of the question, How long can the sun be supposed to have kept 

 the earth, by its radiation, in a state fit to support animal and vege- 

 table life ? Here, as might be expected, a wider range of opinion 

 exists. 



It will be conceded at once that the age of organic life upon the 

 earth must, of necessity, be more recent than the age of the sun. 

 The several theories as to the way in which the sun may have derived 

 his heat may be put aside in favor of that of Helmholtz, viz., that 

 the sun has been condensed from a nebulous mass, filling at least the 

 entire space at present occupied by the whole solar system. The 

 gravitation theory of Helmholtz is now generally admitted to be the 

 only conceivable source of the sun's heat. The opinion that it can 

 be obtained from combustion is not tenable for a moment. The 

 amount of heat radiated is so enormous that, if the sun were a mass 

 of burning coal, it would all be consumed bodily in 5,000 years ! ' 

 On the other hand, a pound of coal falling on the surface of the sun 



1 To maintain the present rate of radiation it would require the combustion of 1,500 

 pounds of coal on every square foot of the sun's surface per hour. Croll, 346. 



