364 THE STEAM-ENGINE. 



formed and the loss of energy due to atmospheric pressure 

 is avoided. Such an engine is known as a condensing, or 

 low-pressure engine. 



(a.) Low-pressure engines are always condensing engines. A low- 

 pressure engine will do more work with a given amount of fuel 

 than a high-pressure non-condensing engine will, is less liable to 

 explosion, and causes less wear and tear to the machinery. But it 

 must be larger, more complicated, more costly, and less portable. 



578. Heat and Work of Steam-Engines. 



More heat is carried to the cylinder of a steam-engine than 

 is carried from it. The piston does work at every stroke, 

 and this work comes from the heat that disappears. Every 

 stroke of the piston annihilates heat. Careful experiments 

 show that the heat destroyed and the work performed are 

 in strict agreement with Joule's equivalent. With a given 

 supply of fuel, the engine will give out less heat when it 

 is made to work hard than when it runs without doing 

 much work. 



EXERCISES. 



1. The mechanical equivalent of heat is 1390 foot-pounds. What 

 is it in kilogrammeters ? 



2. Find the weight of water that may be warmed 15 C. by burn- 

 ing 1 ounce of sulphur in oxygen. 



3. What weight of water would be heated from C. to 1 C. by 

 the combustion of one gram of phosphorus ? 



4. One gram of hydrogen is burned in oxygen. To what tempera- 

 ture would a kilogram of water at C. be raised by the combustion ? 



5. Prom what height must a block of ice at C. fall that the heat 

 generated by its collision with the earth shall be just able to melt it? 



6. From what height must it fall that the heat generated may be 

 sufficient to vaporize it ? 



7. To what height could a ton weight be raised by utilizing all the 

 heat produced by burning 5 Ibs. of pure carbon ? 



8. Find the height to which it could be raised if the coal had the 

 following percentage composition : 



= 88.42; H= 5.61 ; = 5.97. 



