ON FORCE 397 



no appreciable augmentation in the sun^s magnitude. The 

 augmentation of tlie sun's attractive force would be more 

 sensible. However this hypothesis may fare as a repre- 

 sentant of what is going on in Nature, it certainly shows 

 how a sun might be formed and maintained on 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 tempera- 

 ture of a globe of lead of the same size as the earth 

 884,000 degrees of the centigrade thermometer. It has 

 been prophesied that **the elements shall melt with fer- 

 vent heat.'* The earth's own motion embraces the con- 

 ditions of fulfilment; stop that motion, and the greater 

 part, if not the whole, of our planet would be reduced 

 to vapor. If the earth fell into the sun, the amount of 

 heat developed by the shock would be equal to that de- 

 veloped by the combustion of a mass of solid coal 6,435 

 times the earth in size. 



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 re- 

 versed, and the water poured upward twice a day. The 

 water thus moved rubs against the river's bed, and heat 

 is the consequence of this friction. The heat thus gen- 

 erated is in part radiated into space and lost, as far as 

 the earth is concerned. What 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 turn- 

 ing like a wheel from west to east in its diurnal rotation. 

 Suppose a high mountain on the earth's surface approach- 



