PHYSICS-NINE TEENTH CENT.THERMO-D YNAMICS. 527 



case the air in leaving the receiver performed work, for by expanding 

 against the pressure of the superincumbent atmosphere, it acted pre- 

 cisely as if it had raised a weight exerting a pressure upon it of 1 5 Ibs. 

 to the square inch. The mechanical equivalent of the unit of heat, 

 as calculated from this experiment, agreed well with the former 

 determinations. Mr. Joule, in concluding the paper in which these 

 experiments are detailed, very clearly points out the application of 

 his principles to the theory of the steam-engine. " From them," he 

 says, " we may infer that the steam, while expanding in the cylinder, 

 loses heat in quantity exactly proportional to the mechanical force 



FIG. 244. JOULE'S APPARATUS. 



which it communicates by means of the piston ; and that, in the con- 

 densation of the steam, the heat thus converted into power is not 

 given back. Supposing no loss of heat by radiation, etc., the theory 

 here advanced demands that the heat given out in the condenser shall 

 be less than that communicated to the boiler from the furnace, in exact 

 proportion to the equivalent of mechanical power developed." 



Another method which Mr. Joule adopted for the determining of 

 the mechanical equivalent will be understood by the diagram Fig. 244, 

 in which the apparatus is represented. A is a vessel which may be 

 filled with water, oil, mercury, or other liquid. The spindle s passes 

 through the cover of the vessel and carries four vanes at right angles 

 to each other. These vanes are cut out or shown by the dotted lines, 

 and they pass through corresponding apertures in eight fixed radial 

 partitions. By this means the liquid is prevented from whirling round 

 with the rotating paddles. The spindle s carries in its upper part a 

 cylinder <:, which turns with it or without it, according as the peg p is 

 inserted or withdrawn. The cords, 1 1', attached to the drums, d d\ 

 are wound upon the cylinder, and the descent of the weights, w w', 



