512 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



balance of, say, from twenty-six to thirty-two strokes or explosions 

 per minute left to be applied to -useful work under the regulation of 

 the governor. As this engine can be started and stopped at a 

 moment's notice, giving full power at once, and is free from the risks 

 of a boiler explosion, it is peculiarly suited for use as a motor in a 

 laboratory. The consumption of gas is seldom over 2s. 6d. worth per 

 per week for a one horse- power engine. 



From a theoretical point of view, gas, and hot-air engines (they 

 are both founded on the same principle), should as we said at the 

 commencement, have this advantage over steam-engines, that the 

 temperature of the gas may attain to a much higher value, without 

 giving more than a comparatively feeble pressure. Since the 

 'mechanical work depends only on the difference between the extreme 

 temperatures, it follows that a larger part of it may be used for work 

 without fearing accidents from explosion. For the same amount of 

 power the sides of the different parts may be thinner ; but on the 

 other hand, we have seen also that a too high temperature in the gas 

 has a destructive effect on the fittings and metallic parts. The ad- 

 vantages therefore are in great part counterbalanced by this serious 

 drawback. 



But gas-engines have an" incontestable superiority] over steam- 

 engines so far as regards security ; they are almost entirely free from 

 any possibility of explosion, or fear of fire. It is easy to put them 

 in action and requires but little time : they may be set going or 

 stopped by the simple opening of a tap. Having neither grate nor 

 boiler, they are less cumbersome and require much less personal 

 attention for working and overlooking. 



Economically speaking they are, on the contrary, inferior to steam- 

 engines. It follows indeed from experiments made by M. Tresca on the 

 Lenoir engine that the consumption of gas is 2,500 to 3,000 litres for 

 each horse-power per hour, which is five or six times the expense in fuel 

 of the steam-engine. It requires also a great expense in water for cool- 

 ing the driving cylinder and piston. Otto and Langen's gas-engine is 

 much more heating than Lenoir' s, which has however the same fault 

 to a certain degree, and the sudden motions of the piston must be a 

 quickly acting cause of deterioration. All gas-engines have also this 

 inconvenience, that they can only be used where gas is to be had ; 

 and it is in gas houses that the disadvantages of steam-engines 



