Dr. J. R. Mayer on Celestial Dynamics. 243 



would heat 1° C. The quantity of heat necessary to raise one 

 kilogramme of water one degree is called a unit of heat *. 



It has been established by numerous experiments that the 

 combustion of one kilogramme of dry charcoal in oxygen, so as 

 to form carbonic acid, yields 7200 units of heat, which fact may 

 be briefly expressed by saying that charcoal furnishes 7200° of 

 heat. 



Superior coal yields 6000°, perfectly dry wood from 3300° to 

 3900°, sulphur 2700°, and hydrogen 34,600° of heat. 



According to experience, the number of units of heat only 

 depends on the quantity of matter which is consumed, and not 

 on the conditions under which the burning takes place. The 

 same amount of heat is given out whether the combustion pro- 

 ceeds 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 quan- 

 tity 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 de- 

 pendent on the quantity of power consumed, and is quite inde- 

 pendent of the manner in which this power has been expended. 

 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 expression 

 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 Kmf. 



Mechanical work may likewise be measured by the velocity 



* The heat requisite to raise 1 kilogramme of water 1° C. will heat 1 lb. 

 av. of water 3*968 1° F. 



[f If one metre=3-2808 English feet, and one kilogramme=2'2045 lbs. 

 av., it follows that one Km=7'2325 foot-pounds. — Tr.] 



R2 



