THE LOCOMOTIVE. 



ing-rods being attached to the intermediate pair, and through 

 them acting also on the hinder pair. 



15. It has been shown ^hat to give a revolution to the driving- 

 wheels, each of the pistons must move once backwards and for- 

 wards in the cylinder, and consequently the boiler must supply to 

 the cylinders four measures of steam. In this way, the consump- 

 tion of steam necessary for a given progressive speed of the car- 

 riage may be calculated. Thus, if the circumference of the 

 driving-wheels be thirty feet, four cylinders full of steam will be 

 consumed for each thirty feet through which the carriage 

 advances. It is apparent, therefore, that the ability of the engine 

 to move the load with any requisite speed is resolved into the 

 power of the boiler to produce steam of the requisite pressure at 

 this required rate. 



Let it be supposed that it is desired to transport a certain load 

 at the rate of thirty miles an hour, which is at the rate of half a 

 mile, or 2640 feet per minute. Let the circumference of the 

 driving-wheels be twenty-six feet and four-tenths. These wheels 

 will revolve one hundred times in moving over 2640 feet, or half 

 a mile, that is to say, one hundred times per minute. But since 

 each revolution requires the boiler to supply four cylinders full of 

 steam, the consumption of steam per minute will be four hundred 

 times the contents of the cylinder. 



16. The pressure of the steam will depend upon the resistance 

 of the load. By the common principles of mechanics, the power 

 acting upon the pistons necessary to balance a given resistance at 

 the circumference of the wheel can be easily calculated, and thus 

 the necessary pressure of the steam ascertained. In this manner 

 it can always be determined how much steam, of a given pressure, 

 the boiler must produce, in order to enable the engine to carry a 

 given load with any required speed. 



The mechanism being properly constructed, it follows, there- 

 fore, that the efficacy of the engine must depend ultimately on the 

 evaporating power of the boiler. 



In the case of the locomotive engine there are particular condi- 

 tions which limit the magnitude and weight of the machinery, 

 and create impediments and difficulties in the construction of the 

 machine, which are not encountered in stationary engines. As 

 the engine itself is transported, and travels with its load, it must 

 necessarily be subject to narrow limits as to weight and bulk. 

 It has to pass under bridges, and through tunnels, which circum- 

 stance not only limits its general magnitude, but almost deprives 

 it of the appendage of a chimney, so indispensable to the efficiency 

 of stationary steam-engines. 



It follows that this limitation of weight and bulk can only be 

 120 



