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 lbs. on the square inch, in the boiler. The force 
required to perform this, is about 2050 lbs. 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 47i lbs. per square inch, instead of 
50 lbs., 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-