532 Professor Tyndall [June 6, 



ones. That lenticular envelope which surrounds the sun, and which 

 is known to astronomers as the Zodiacal light, is probably a crowd 

 of meteors ; and moving as they do in a resisting medium they must 

 continually approach the sun. Falling into it, they would be com- 

 petent 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 develope an amount of heat sufficient \o cover one or two years* 

 loss; and were our earth to fall into the sun a century's loss would be 

 made good. Still, our moon and our earth, if distributed over the sur- 

 face of the sun, would utterly vanish from perception. Indeed, the 

 quantity of matter competent to produce the necessary effect would, 

 during the range 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 

 representant 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 de- 

 veloped sufficient to raise the temperature of a globe of lead of the 

 same size as the earth 384,000 degrees of the centigrade thermometer. 

 It has been prophesied that " the elements shall melt with fervent 

 heat." The earth's own motion embraces the conditions of fulfilment ; 

 stop that motion, and the greater part, if not the whole, of her mass 

 would be reduced to vapour. Tf 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 permanence 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 radi- 

 ated 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 rotation. 

 Suppose a high mountain on the earth's surface ; on approaching the 

 moon's meridian, that mountain is, as it were, laid hold of by the moon, 

 and forms a kind of handle by which the earth is pulled more quickly 

 round. But when the meridian is passed the pull of the moon on the 

 mountain would be in the opposite direction, it now tends to diminish 

 the velocity of rotation as much as it previously augmented it ; and thus 

 the action of all fixed bodies on the earth's surface is neutralized. 

 But suppose the mountain to lie always to the east of the moon's 

 meridian, the pull then would be always exerted against the earth's 



