58 THE SCIENTIFIC PAPERS OF 



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 of 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 con- 

 sidered 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 condition of ordinary steam-engines ; 

 whereas in Ericsson's engine, the hot air had entered the working 

 cylinder. In surrounding the heating vessel with the boiler, an 

 excessive accumulation 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 property of the heating vessel was, that all its 

 parts should be free to expand by heat without straining other 

 parts, which was accomplished by a free suspension, and by 

 undulating its surface. Lastly, it should be massive, to withstand 

 the fire with impunity, for iron was, strictly speaking, a com- 

 bustible material. The pyropherus, or finely-divided metallic iron, 

 took fire spontaneously on exposure to the atmosphere, a chip of 

 iron was ignited in flying through the flame of a candle ; an iron 

 tea-kettle was destroyed by exposing it (unfilled with water) to a 

 kitchen fire ; whereas, in forging a crank shaft, the solid mass of 

 iron withstands the white heat of the forge fire for several weeks 

 without deteriorating. A heating vessel, properly constructed and 

 protected, might be heated with safety to 700 Fahr., at which 

 temperature it would be almost as able to resist pressure, as at the 

 ordinary temperature of the atmosphere, the point of maximum 

 strength of iron being at 550 Fahr., as had been proved by 

 experiments made for the Franklin Institution. The construction 

 of a heating vessel combining these desiderata was of paramount 

 importance for the success of Mr. Siemens's engine, and had not 



