18 



POWER AND THE PLOW 



the weight of the tractor itself over various grades and surfaces. 

 In order to compare these various losses a number of tests have 

 been adopted to determine the efficiency of the engine in various 

 particulars. 



The work done in the engine cylinder is known as the in- 

 dicated horsepower (i. h. p.). It is always less than the energy 

 which is supplied to the engine in the shape of fuel. In the 

 steam engine the first losses occur in unburned gases and bits 

 of carbon that pass out of the chimney; in unconsumed fuel in 

 the ashes; and in radiation from the boiler, steam pipes, and 

 the cylinder itself. The indicated power is calculated by the 

 aid of an apparatus known as the indicator. This consists of a 

 small cylinder containing a piston and spring; a larger cylinder 

 or drum for holding a sheet upon which a record may be traced; 

 and mechanisms for controlling the movements of the recording 

 pencil and the drum. The piston of the indicator, which is 

 usually one inch in area, is exposed directly to the pressure with- 

 in the cylinder by means of a pipe connection. The pressure 

 on the piston compresses a spring, the resistance of which has 

 been calibrated by determining the number of pounds neces- 



Indicator cards from actual charts of steam and gas engines 



sary to compress it one inch in length. A reducing movement 

 connected to some reciprocating part of the engine, such as 



