PHYSICS NINETEENTH CENT. THERMO-DYNAMICS. 529 



the scientific sense may require to be mentioned. The labouring 

 power of a man or of a horse has long been for practical purposes ex- 

 pressed by engineers and others in terms of the loads that the muscular 

 effort of either could raise to some specified height in a given time. 

 The invention of the steam-engine was the occasion of bringing into 

 more notice this conception of forces doing work. Watt estimated the 

 efficiency of his engines by their "horse-power" Not that the engine 

 was directly compared with the actual labouring power of horses, but 

 the "horse-power" was arbitrarily defined as that which would in one 

 second raise 550 Ibs. one foot high. In the scientific sense, work 

 is done when anything is moved against a resistance, and the work is 

 measured by the space through which the thing is moved, and by the 

 amount of the resistance. Time does not enter at all into the measure 

 of work. The space may be measured in feet, and the resistance may 

 be compared with that which is overcome when the lump of metal 

 called a pound weight is raised vertically. Hence the compound unit 

 of work (in England) is the "foot-lb" The expression " 100 units of 

 work " would therefore imply 100 Ibs. weight raised i foot ; i Ib. raised 

 100 feet; 10 Ibs. raised 10 feet; 20 Ibs. raised 5 feet; 5 Ibs. raised 

 20 feet ; or, in fine, a weight raised to such a height that the product 

 of the weight (expressed in Ibs.) into the height (expressed in feet) is 

 100 



Joule's experiments proved that by the expenditure of 772 units of 

 work one unit of heat can be produced, and with greater generality 

 they also proved that chemical action, electricity, magnetism, heat, and 

 mechanical energy are convertible into each other in definite and in- 

 variable equivalents. Meanwhile the relations of heat and motive 

 power were being studied from the theoretical side by HELMHOLTZ 

 and CLAUSIUS in Germany, and by WATERSTON, RANKINE, W. THOMP- 

 SON, MAXWELL, and others in England. The laws of these relations 

 are now formulated in a branch of science denominated as Thermo- 

 Dynamics, but their discovery and statement cannot here be discussed 

 at length. One important law of thermo-dynamics was enunciated as 

 early as 1824 by CARNOT, a son of Napoleon's famous general of that 

 name, in a work entitled " Reflexions sur la Puissance Motrice du Feu" 

 This law holds good now, although the reasonings on which it is 

 founded were based on the then prevailing view of the material nature 

 of caloric. Carnot compared the mechanical power of heat to the 

 mechanical power of a head of water ; for, just as work is done by the 

 latter only in its descent from a higher to a lower level, so, he declared, 

 work done by heat is necessarily attended by a fall of some body from 

 a higher to a lower temperature. Whatever might be the nature of the 

 substance, whether steam, air, vapour, gas, or liquid, this fall of tem- 

 perature Carnot showed to be essential to the production of work, 

 and the amount of work done by the heat to be independent of the 

 nature of the substance employed. 



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