Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 521 



being taken as yielding 6000 thermal units. Hence it follows 

 that the velocity of the motion of shooting-stars and fire-balls, 

 which, as is well known, attains, according to astronomical 

 observations, to from 4 to 8 miles, is a cause fully sufficient to 

 produce the most violent evolution of heat, and an insight into 

 the nature of these remarkable phenomena is thereby afforded 

 to us*. 



The following is a problem of a similar kind : if two cosmical 

 masses, moving in space about their common centre of gravity, 

 were by any cause whatever, for example by the resistance of 

 the surrounding medium, caused to fall together, the question 

 again arises, How great is the thermal effect corresponding to 

 this process of mechanical combination ? 



Even though the elements of the orbits (that is, their excen- 

 tricity) may be unknown, we can nevertheless calculate from 

 the given weight and volume of the masses in question the 

 maximum and the minimum of the required effect. Thus let it 

 be supposed, for the sake of an example, that our earth had been 

 divided into two equal globes which had united in the manner 

 described : calculation teaches us that the amount of heat 

 which would have been evolved in such a case would consider- 

 ably exceed that which an equal weight of matter could furnish 

 by the most intense process of chemical action. 



It is more than probable that the earth has come into existence 

 in some such way, and that in consequence our sun, as seen from 

 the distance of the fixed stars, exhibited at that epoch a transient 

 burst of light. But what took place in our solar system perhaps 

 millions of years ago, still goes on at the present time here and 

 there among the fixed stars ; and the transient appearance of 

 stars, which in some cases, like the celebrated star of Tycho 

 Brahe, have at first an extraordinay degree of brilliance, may be 

 satisfactorily explained by assuming the falling together of pre- 

 viously invisible double stars. 



Contrasting with such explosive bursts of light is the steady 

 radiation, shown continuously through enormous periods, by the 

 greater number of fixed stars, and among them by our sun. Do 

 these appearances, which in so special a manner tempt to higher 

 speculations, constitute a real exception to the exhaustion of a 

 cause in producing its effect, which, in accordance with the fore- 

 going considerations, we have regarded as an established law of 

 Nature ? or does the small sum of human knowledge authorize 

 us in supposing that here also there is an equivalence between 



* The idea that the meteors here referred to owe their light to a mecha- 

 nical process — whether friction, or the compression of the air— is not new ; 

 but without a knowledge of the mechanical equivalent of heat it could 

 have no scientific foundation. 



