4 W. J. M. Rankine on the Means of 



by one pound of such kinds of coal as are commonly used for 

 engines in Britain may be taken on an average as equal to 

 that of the heat required to raise seven pounds of water from 

 the temperature of 50° to 212° Fahr., and to evaporate it at 

 the latter temperature ; that is to say, in round numbers, 

 6,000,000 foot-pounds. 



The total heat produced by the combustion of the coal is 

 considerably greater ; but a portion necessarily escapes with 

 the gases which ascend the chimney, and the above may be 

 considered as a fair average estimate of the mechanical equi- 

 valent of that which is practically available. 



4. Mechanical Hypothesis respecting Heat. — Heat, being 

 convertible with mechanical power, is convertible also with 

 the vis-viva of a body in motion. The British unit of heat, 

 one degree of Fahrenheit in a pound of liquid water, is equi- 

 valent to the vis-viva of a mass weighing one pound, moving 

 with the velocity of 223 feet per second, being the velocity 

 acquired in falling through a height of 772 feet. A mass of 

 water, of which each particle is in motion with this velocity, 

 has its temperature elevated by one degree of Fahrenheit, 

 upon the extinction of the motion, by the mutual friction of 

 the particles. 



It is natural to suppose that the motion, during this phe- 

 nomenon, has not been really destroyed, but has been con- 

 verted into revolutions of the particles in vortices or eddies 

 too small to be perceptible by any of our modes of observation ; 

 and that the centrifugal force of such eddies is the cause of 

 the tendency of hot bodies to expand, melt, and evaporate. 



A hypothesis of this kind has long been entertained, and 

 within the last few years it has been used so as to deduce the 

 laws of the mechanical action of heat from the principles of 

 ordinary mechanics, to a certain extent in anticipation of the 

 results of experiment.* As those laws, however, have now 

 been exactly ascertained by experiment, it must be borne in 

 mind, that their certainty is in no way dependent on the truth 

 of the hypothesis in question ; the probability of the hypo- 

 thesis being, on the contrary, dependent on the truth of the 

 laws. 



* See Transactions of the Royal Society of Edinburgh, vol. xx. 



