368 Mr. J. Croll on Geological Time, and the probable 



must be felt to be conclusive. It would be superfluous to state 

 them here ; I shall, however, for reasons which will presently 

 appear, refer briefly to one of them, and the one which seems to 

 be the most conclusive of all, viz. the argument derived from the 

 limit to the age of the sun's heat. 



It is found that 83*4 foot-pounds of heat per second is inci- 

 dent upon a square foot of the earth's surface exposed to the 

 perpendicular rays of the sun. The amount radiated from a 

 square foot of the sun's surface to that incident on a square foot 

 of the earth's surface is as the square of the sun's distance to 

 the square of his radius, or as 46,400 to 1. Consequently 

 3,869,000 foot-pounds of heat is radiated off every square foot 

 of the sun's surface per second — an amount equal to about 7000 

 horse-power. The total amount radiated from the whole surface 

 of the sun per annum amounts to 8340 x 10 30 foot-pounds. To 

 maintain the present rate of radiation, it would require the com- 

 bustion of about 1500 lbs. of coal per hour on every square foot 

 of the sun's surface ; and were the sun composed of that mate- 

 rial, it would be all consumed in less than 5000 years. The 

 opinion that the sun's heat is maintained by combustion cannot 

 be entertained for a single moment. A pound of coal falling 

 into the sun from an infinite distance would produce by its con- 

 cussion more than 6000 times the amount of heat that would be 

 generated by its combustion. 



It is well known that thevelocity with which a body falling from 

 an infinite distance would reach the sun would be equal to that 

 which would be generated by a constant force equal to the weight 

 of the body at the sun's surface operating through a space equal 

 to the sun's radius. One pound would at the sun's surface 

 weigh about 28 pounds. Take the sun's radius at 441,000 

 miles*. The energy of a pound of matter falling into the sun 

 from infinite space would equal that of a 28-pound weight de- 

 scending upon the earth from an elevation of 441,000 miles, 

 supposing the force of gravity to be as great at that eleva- 

 tion as it is at the earth's surface. It amounts to upwards of 

 65,000,000,000 foot-pounds. A better idea of this enormous 

 amount of energy exerted by a one-pound weight falling into 

 the sun will be conveyed by stating that it would be sufficient 

 to raise 1000 tons to a height of 5J miles. It would project 

 the ' Warrior,' fully equipped with guns, stores, and ammunition, 

 over the top of Ben Nevis. 



Gravitation is now generally admitted to be the only con- 

 ceivable source of the sun's heat. But if we attribute the energy 

 of the sun to gravitation as a source, we attribute it to a cause 



* I have taken for the volume and mass of the sun the values given in 

 Professor Sir William Thomson's paper, Phil. Mag. vol. viii. (1854). 



