262 CELESTIAL DYNAMICS. 



not on the conditions tinder which the burning takes place. 

 The same amount of heat is given out whether the combus- 

 tion proceeds slowly or quickly, in atmospheric air or in pure 

 oxygen gas. If in one case a metal be burnt in air and the 

 amount of heat directly measured, and in another instance 

 the same quantity of metal be oxidized in a galvanic battery, 

 the heat being developed in some other place say, the wire 

 which conducts the current, in both of these experiments 

 the same quantity of heat will be observed. 



The same law also holds good for the production of heat 

 by mechanical means. The amount of heat obtained is only 

 dependent on the quantity of power consumed, and is quite 

 independent of the manner in which this power has been ex- 

 pended. If, therefore, the amount of heat which is produced 

 by certain mechanical work is known, the quantity which 

 will be obtained by any other amount of mechanical work 

 can easily be found by calculation. It is of no consequence 

 whether this work consists in the compression, percussion, or 

 friction of bodies. 



The amount of mechanical work done by a force may be 

 expressed by a weight, and the height to which this weight 

 would be raised by the same force. The mathematical ex- 

 pression for " work done," that is to say, a measure for this 

 work, is obtained by multiplying the height expressed in feet 

 or other units by the number of pounds or kilogrammes lifted 

 to this height. 



We shall take one kilogramme as the unit of weight, and 

 one metre as the unit of height, and we thus obtain the 

 weight of one kilogramme raised to the height of one metre 

 as a unit measure of mechanical work performed. This 

 measure we shall call a kilogrammetre, and adopt for it the 

 symbol Km. 



Mechanical work may likewise be measured by the velo 

 city obtained by a given weight in passing from a state of rest 

 into that of motion. The work done is then expressed by 



