354 THE MECHANICAL EQUIVALENT OF HEAT. 



The following is one problem of this kind. It is assumed 

 that a cosmical body enters the atmosphere of our earth with 

 a velocity of four geographical miles per second, and that, in 

 consequence of the resistance which it here encounters, it 

 loses so much of its vis viva of motion that its remaining ve- 

 locity when it again quits the atmosphere amounts to three 

 miles : the question now arises, How great is the thermal 

 effect which accompanies this process ? 



A simple calculation, based upon the mechanical equiva- 

 lent of heat, shows that the quantity of heat required is about 

 eight times as great as the heat of combustion of a mass of 

 coal of equal weight with the body in question, one kilo- 

 gramme of coal being taken as yielding 6,000 thermal units. 

 Hence it follows that the velocity of the motion of shooting- 

 stars and fire-balls, which, as is well-known, attains, accord- 

 ing to astronomical observations, to from four to eight 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 cos- 

 mical masses, moving in space about their common centre of 

 gravity, were by any cause whatever, for example by the re- 

 sistance of the surrounding medium, caused to fall together, 

 the question again arises, Ho'w great is the thermal effect cor- 

 responding to this process of mechanical combination ? 



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

 centricity) 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 idea that the meteors here referred to owe their light to a me- 

 chanical 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. 



