SIR WILLIAM SIEMENS, F.R.S. 57 



loss of pressure. When the effective stroke of the working trunk 

 was nearly completed, the regenerative piston commenced to re- 

 ivdr, and the steam below the hollow piston expanded into the 

 regenerative cylinder, depositing on its regress through the respi- 

 rator the heat it had received on its egress through the same, less 

 only the quantity that had been lost in its expansion below the 

 working piston, which was converted into dynamic effect or 

 engine-power, and had to be supplied by the fire. The expansion 

 ami simultaneous reduction of temperature of the steam caused 

 a diminution of its pressure from four to nearly one atmosphere ; 

 and the working trunk could now effect its return stroke with- 

 out opposing pressure, and while the second working trunk made 

 its effective or outward stroke impelled by a pressure of four 

 atmospheres. 



The respirator, which was invented by the Rev. Mr. Stirling, of 

 Dundee, in 18 l(i, fulfilled its office with surprising rapidity and 

 perfection, if it were made of suitable proportions. Its action 

 was proved at the end of the lecture by a working model. It had 

 been applied without success to hot-air engines by Stirling and 

 Ericsson, but failed for want of proper application ; for it had 

 been assumed (in accordance with the material theory of heat) that 

 it was capable of recovering all heat imparted to the air, and, in 

 consequence, no sufficient provision of heating apparatus had been 

 made. It having been found impossible to produce, what in effect 

 would have been a perpetual motion, the respirator had been 

 discarded entirely, and was even now looked upon with great 

 suspicion by engineers and men of science. Mr. Siemens had, 

 however, no doubt that its real merits to recover heat that could 

 not practically be converted by one single operation into mechani- 

 cal effect, would be better appreciated. The rapidity with which 

 the temperature of a volume of steam was raised from 250 to 

 650 Fahr. by means of a respirator, was indicated by the fact 

 that he had obtained with his engines a velocity of 150 revolutions 

 per minute. The single action of heating the steam occupied 

 only a quarter the time of the entire revolution of the engine, and 

 it followed that it was accomplished in one-tenth part of a second. 

 But, in explanation of this phenomenon, it was contended, that 

 the transmitting of a given amount of heat from a hotter to a 

 cooler body, was proportionate to the heating surface multiplied 



