234 INTERACTION OF NATURAL FORCES. 



equilibrium of a fluid mass, is indicative of a former state of 

 fluidity. If I thus permit an immense quantity of heat 

 to disappear without compensation from our system, the 

 principle of the conservation of force is not thereby invaded. 

 Certainly for our planet it is lost, but not for the universe. 

 It has proceeded outwards, and daily proceeds outwards into 

 infinite space ; and we know not whether the medium which 

 transmits the undulations of light and heat possesses an end 

 where the rays must return, or whether they eternally pursue 

 their way through infinitude. 



The store of force at present possessed by our system, is 

 also equivalent to immense quantities of heat. If our earth 

 were by a sudden shock brought to rest on her orbit which 

 is not to be feared in the existing arrangements of our system 

 by such a shock a quantity of heat would be generated 

 equal to that produced by the combustion of fourteen such 

 earths of solid coal. Making the most unfavourable assump- 

 tion as to its capacity for heat, that is, placing it equal to that 

 of water, the mass of the earth would thereby be heated 11,200 

 degrees ; it would therefore be quite fused and for the most 

 part reduced to vapour. If, then, the earth, after having been 

 thus brought to rest, should fall into the sun, which of course 

 would be the case, the quantity of heat developed by the shock 

 would be four hundred times greater. 



Even now, from time to time, such a process is repeated 

 on a small scale. There can hardly be a doubt that meteors, 

 fire-balls, and meteoric stones, are masses which belong to 

 the universe, and before coming into the domain of our earth, 

 moved like the planets round the sun. Only when they enter 

 our atmosphere do they become visible and fall sometimes to 

 the earth. In order to explain the emission of light by 

 these bodies, and the fact that for some time after their 

 descent they are very hot, the friction was long ago thought 

 of which they experience in passing through the air. We 

 can now calculate that a velocity of 3000 feet a second, 



