Professor TyndaVs Lecture on Force. 247 



omers as the zodiacal light, is probably a cloud of meteors ; and 

 moving, as they do, in a resisting medium they must continually 

 approach the sun. Falling into it, they would be competent to 

 produce the heat observed, and this would constitute a source 

 from which the annual loss of heat would be made good. The 

 sun, according to this hypothesis, would be continually growing 

 larger ; but how much larger ? Were our moon to fall into the 

 sun it would deveiope an amount of heat sufficient to cover one 

 or two years' loss ; and were our earth to fall into the sun a cen- 

 tury's loss would be made good. Still, our moon and our earth, 

 if distributed over the surface of the sun, would utterly vanish 

 from perception. Indeed, the quantity of matter competent to 

 produce the necessary effect would, during the rang« of history, 

 produce no appreciable augmentation in the sun's magnitude. 

 The augmentation of the sun's attractive force would be more 

 appreciable. However this hypothesis may fare as a represen- 

 tant of what is going on in nature, it certainly shows how a sun 

 might be formed and maintained by the application of known 

 thermo-dynamic principles. 



Our earth moves in its orbit with a velocity of 68,040 miles 

 an hour. Were this motion stopped, an amount of heat would 

 be developed sufficient to raise the temperature of a globe of lead 

 of the same size as the earth, 384,000*^ of the centigrade ther- 

 mometer. It has been prophesied that " the elements shall melt 

 with fervent heat." The earth's own motion embraces the con- 

 ditions of fulfilment ; stop that motion, and the greater part, if 

 not the whole of her mass, would be reduced to vapour. If the 

 earth fell into the sun, the amount of heat developed by the shock 

 would be equal to that developed by the combustion of 6435 

 earths of solid coal. 



There is one other consideration connected with the perma- 

 nence of our present terrestrial conditions, which is well worthy 

 of our attention. Standing upon one of the London bridges, we 

 observe the current of the Thames reversed, and the water poured 

 upward twice a-day. The water thus moved, rubs against the 

 river's bed and sides, and heat is the consequence of this friction. 

 The heat thus generated is in part radiated into space, and then 

 lost, as far as the earth is concerned. What is it that supplies 

 this incessant loss ? The earth's rotation. Let us look a little 

 more closely at the matter. Imagine the moon fixed, and the 

 earth turning like a wheel from west to east in its diurnal rota- 



