MECHANICS. 89 



expanded to about 5, 10, 20, 30, and 40 times its 

 original bulk. 



One atmosphere is equal to a pressure of 15 

 pounds on the square inch ; 2 of 30 ; 3 of 44 ; 4 of 

 58; 5 of 73 ; 6 of 87; 7 of 102 ; and 8 atmospheres 

 of 1 1 7 pounds on the square inch. 



Power and Velocity of Steam Boats. 



The immense increase of power requisite to obtain 

 a small increase of velocity, ought to have its influence 

 in determining the speed of a steam boat during a 

 long voyage, and its proportions ought to be adapted 

 to that speed, with a small excess of power for emer- 

 gencies. The powers required to give a boat different 

 velocities in still water are as follows : 



Miles per hour. Horses' power. 



3 5J 



4 13 



5 25 



6 43 



7 69 



8 102 



9 - - 7 146 

 10 - .''>. - - 200 



The mechanical power, or power of a steam engine 

 to impel a boat in still water, must be as the cube of 

 the velocity. Therefore, if an engine of 12 horse 

 power will impel a boat 7 miles per hour, it will re- 

 quire one of 35 horse power to impel the same boat 

 at the rate of 10 miles per hour. 



3 3 10 3 xl2 



7 . 10 :: 12 : =35 



73 



The action of what is called a 25 horse power engine, 

 is just equal to the impulse given by 1000 cubic feet 

 of water falling in a minute through the height of 

 10 feet. 



