412 



THE STEAM-ENGINE. 



Fig. 4. 



such a machine would depend entirely on the magnitude of the piston ; and 

 being independent of highly elastic steam, would not expose the materials to 

 the destructive heat which was necessary for working Savery's engine. Sup- 

 posing a perfect vacuum to be produced under the piston in the cylinder, an 

 effective downward pressure would be obtained, amounting to fifteen times as 

 many pounds as there are square inches in the section of the piston.* Thus, 

 if the base of the piston were 100 square inches, a pressure equal to 1,500 

 pounds would be obtained. 



In order to accomplish this, two things were necessary: 1. To make a 

 speedy and effectual vacuum below the piston in the descent ; and, 2. To con- 

 trive a counterpoise for the atmosphere in the ascent. 



The condensation of steam immediately presented itself as the most effectual 

 means of accomplishing the former ; and the elastic force of the same steam 

 previous to condensation an obvious method of affecting the latter. Nothing 

 now remained to carry the design into execution, but the contrivance of means 

 for the alternate introduction and condensation of the steam ; and Newcomen 

 and Cawley were accordingly granted a patent in 1707, in which Savery was 



* As the calculation of the power of an engine depends on the number of square inches in the 

 section of the piston, it may be useful to give a rule for computing the number of square inches in 

 a circle. The following rule will always give the dimensions with sufficient accuracy : Multiply 

 the number of inches in the diameter by itself; divide the product by 14, and multiply the quotient 

 thus obtained by 11, and the result will be the number of square inches in tlie circle. Thus, if there 

 be 12 inches in the diameter, this multiplied by itself gives 144, which divided by 14 gives lOy 

 which multiplied by 11 gives 113, neglecting fractions. There are, therefore, 113 square inches 

 a circle whose diameter is 12 inches. 



