2 6 THE POPULAR SCIENCE MONTHLY. 



times theirs. Forty years later Langley,* in an excellently worked out 

 consideration of the whole question of absorption by our atmosphere, 

 of radiant heat of all wave-lengths, accepts and confirms Forbes's rea- 

 soning, and by fresh observations in very favorable circumstances on 

 Mount Whitney, 15,000 feet above the sea-level, finds a number a little 

 greater still than Forbes (1*7, instead of Forbes's 1*6, times Pouillet's 

 number). Thus Langley's number expressing the quantity of heat 

 radiated per second of time from each square centimetre of the sun's 

 surface corresponds to 133,000 horse-power per square metre, instead 

 of the 78,000 horse-power which we have taken, and diminishes each 

 of our times in the ratio of 1 to 1"7. Thus, instead of Helraholtz's 

 twenty million years, which was founded ou Pouillet's estimate, we 

 have only twelve millions, and similarly with all our other time-reck- 

 onings based on Pouillet's results. In the circumstances, and taking 

 fully into account all possibilities of greater density in the sun's inte- 

 rior, and of greater or less activity of radiation in past ages, it would, 

 I think, be exceedingly rash to assume as probable anything more than 

 twenty million years of the sun's light in the past history of the earth, 

 or to reckon on more than five or six million years of sunlight for time 

 to come. 



But now we come to the most interesting part of our subject the 

 early history of the sun. Five or ten million years ago he may have 

 been about double his present diameter and an eighth of his present 

 mean density, or *175 of the density of water ; but we can not, with 

 any probability of argument or speculation, go on continuously much 

 beyond that. We can not, however, help asking the question, What 

 was the condition of the sun's matter before it came together and be- 

 came hot? It may have been two cool solid masses, which collided 

 with the velocity due to their mutual gravitation ; or, but with enor- 

 mously less of probability, it may have been two masses colliding with 

 velocities considerably greater than the velocities due to mutual gravi- 

 tation. This last supposition implies that, calling the two bodies A 

 and B for brevity, the motion of the center of inertia of B relatively 

 to A must, when the distances between them was great, have been 

 directed with great exactness to pass through the center of inertia of 

 A ; such great exactness that the rotational momentum after collision 

 was of proper amount to let the sun have his present rotational period 

 when shrunk to his present dimensions. This exceedingly exact aim- 

 ing of the one body at the other, so to speak, is, on the dry theory of 

 probability, exceedingly improbable. On the other hand, there is cer- 

 tainty that the two bodies A and B at rest in space if left to them- 

 selves, undisturbed by other bodies and only influenced by their mutual 

 gravitation, shall collide with direct impact, and therefore with no 

 motion of their center of inertia, and no rotational momentum of the 



* " On the Selective Absorption of Solar Energy," " American Journal of Science," 

 vol. xxv, March, 18S3. 



