468 STEPHENSON'S PATENT 



the brakes are of many different constructions, but this one is very simple and con- 

 venient and has great power. A step and a handle are fixed on to the tender 

 on each side for the convenience of getting upon it or upon the engine. 



The tender weighs, when empty, 3^ tons, and about 7 tons when filled with 

 water and coke. The tank holds about 700 gallons of water, and the quantity of 

 coke that is carried is about 8 cwt. ; these are sufficient to supply the engine for run- 

 ning from thirty to forty miles, according to the load taken ; but the tender is seldom 

 run farther than twenty miles without being refilled. 



The power of a locomotive engine cannot readily be estimated in the same manner 

 as that of other engines, by taking the actual force upon the piston, and the velocity 

 of its motion ; for it is very difficult to ascertain the effective pressure of the steam 

 upon the piston, in consequence of its differing often very considerably from that of 

 the steam in the boiler, and because of the large amount of the resistance of the 

 waste steam, owing to the great velocity with which the piston moves. The 

 power is also different at different velocities, as these circumstances vary with the 

 velocity. The only correct means therefore, of ascertaining the power of a loco- 

 motive, is by deducing it from the work that it is capable of performing. 



This engine has drawn a load up an inclined plane that was equivalent to 220 

 tons gross weight upon a level, (including engine and tender,) at a velocity of 14 miles 

 an hour ; which appeared to be about the extent of the power of the engine with 

 the steam at the usual pressure of 50 Ibs. on the square inch, in the boiler. The force 

 required to perform this, is about 2050 Ibs. moving at that velocity ; which is equal 

 to 77 horse power. The effective pressure on the piston, or the actual force with 

 which it was propelled, must therefore have been 47^ Ibs. per square inch, instead of 

 50 Ibs., which was the pressure of the steam in the boiler ; the difference being the 

 power that was lost by the resistance of the waste steam, and by the diminution of 

 the pressure of the steam, in consequence of the throttling or wire drawing that takes 

 place in passing through the ports of the cylinders, and which was in this instance 

 very inconsiderable. 



The horse power of an engine is less when drawing a lighter load at a greater ve- 

 locity, as the loss of power from the throttling and the waste steam is then increased ; 

 and it would cease altogether at a certain speed, varying according to the proportions of 

 the engine, when the velocity of the piston became as great as that with which the steam 

 can enter into the cylinder, or the waste steam escape. This engine has drawn 40 tons 

 at 35 miles an hour, which is equivalent to 40 horse power ; in which case the ef- 



