206 H. Helmholtz on the Interaction of Natural Forces. 



the universe. It lias proceeded outwards, and daily proeeeds 

 outwards into infinite space ; and we know not whether the me- 

 dium which transmits the undulations of light and heat, pos- 

 sesses an end where the rays must return, or whether they eter- 

 nally 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 in her orbit, — which is not to be 

 feared in the existing arrangement of our system — by such a 

 shock a quantity of heat would be generated equal to that pro- 

 duced b}^ the combustion of fourteen such earths of solid coal. 

 Making the most unfavorable assumption as to its capacity for 

 heat, that is, placing it equal to that of water, the muss of the 

 earth would thereby be heated 11200 degrees: it would there- 

 fore be quite fused and for the most part reduced to vapor. If 

 then the earth, after having been thus brought to rest, should 

 fall into the sun, which of course would be the case, the quan- 

 tity 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, fireballs, 

 and meteoric stones are masses which belong to the universe, 

 and before coming into the domain of our earth, moved like the 



Slanets round the sun. Only when they enter our atmosphere 

 o 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 fric- 

 tion was long ago thought of which they experience in passing 

 through the air. We can now calculate that a velocity of SOOO 

 feet a second, supposing the whole of the friction to be expended 

 in heating the soUd mass, would raise a piece of meteoric iron 

 1000"^ C. in temperature, or^ in other words, to a vivid red heat. 

 Now the average velocity of the meteors seems to be thirty or 

 forty times the above amount. To compensate this, however, 

 the greater portion of the heat is doubtless carried away by the 

 condensed mass of air which the meteor drives before it. It is 

 known that bright meteors generally leave a luminous trail be- 

 hind them, which probably consists of severed portions of the 

 red-hot surfaces. Meteoric masses which fall to the earth of^en 

 burst with a violent explosion, which may be regarded as a re- 

 sult of the quick heating. The newly-fallen pieces have been 

 for the most part found hot, but not red-hot, which is easily ex- 

 plainable by the circumstance, that during the short time occu- 

 pied by the meteor in passing through the atmosphere, only a 

 thin superficial layer is heated to redness, while but a small 

 quantity of heat has been able to penetrate to the interior of the 

 mass. For this reason the red heat can speedily disappear. 



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