1856.] degenerative Steam- Engine. 233 



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

 of Dundee, in 1816, fulfilled its office with surprising rapidity and 

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

 proved at the end of the discourse 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 mechanical effect, would be better appre- 

 ciated. The rapidity with which the temperature of a volume of 

 steam was raised from 250" to 650° Fah. 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 by the time occupied, and that the latter factor might be 

 reduced ad libitum, by increasing the former proportionately. The 

 air-engines of Stirling and Ericsson had failed also, because their 

 heated cylinders had been rapidly destroyed by the fire ; but the 

 cause for this was, that an insufficient extent of heating surface 

 had been provided, and it was well known that even a steam- 

 boiler would be rapidly destroyed under such circumstances. Mr. 

 Siemens was led by his own experience to believe that his heating 

 vessels would last certainly from three to five years ; and being only 

 a piece of rough casting, that could be replaced in a few hours, and 

 at a cost below that of a slight boiler repair, he considered that he 

 had practically solved the difficulty arising from high temperature. 

 It was however important to add, that all the working parts of his 

 engine were at the temperature of saturated steam, and therefore 

 in the same condition of ordinary steam-engines ; whereas in Erics- 

 son's engine, the hot air had entered the working cylinder. In 

 surrounding the heating vessel with the boiler, an excessive accu- 

 mulation of heat was prevented from taking place, and the pressure 

 of the steam in the boiler became the true index to the engine-driver 

 of the temperature of the heating vessel. Another essential pro- 

 perty of the heating vessel was, that all its parts should be free to 

 expand by heat without straining other parts, which was accom- 

 plished by a free suspension, and by undulating its surface. Lastly, 

 it should be massive, to withstand the fire with impunity, for iron 



