458 
STEPHENSON’S PATENT 
rail, and the driving wheels could not be case-hardened, as the others were, from its / 
diminishing the adhesion upon the rails. Wheels with wooden spokes and rims and 
wrought iron tires, were tried on the Liverpool and Manchester Railway, and found 
to wear better, being more elastic; and flat wrought iron spokes were then tried. 
The wheels with tubular spokes, and cast iron rims with wrought tires, are now 
very generally used, and wear very well, lasting two or three years ; the driving 
wheels being subject to the most wear, in consequence of the slipping to wdiich they 
are liable. The tires squeeze out at the sides as they wear, and when worn out are 
replaced by new ones; they are now made wider, the flanch tires being six inches, 
and those of the driving wheels seven inches, in order to prevent squeezing out at the 
sides, which is the greatest cause of their wearing out. The cast iron rims are rather 
objectionable from their brittleness, as they have to run with so great a velocity; and to 
obviate this, some engines have wheels with wrought iron rims, to which the spokes 
are fixed by rivets, having the tires shrunk upon them ; this construction is con 
siderably more expensive, though very durable. 
All the earlier locomotives on the Liverpool Railway, and many of the present 
ones, have been made with only four wheels, D' L'; the third pair of wheels, M', 
placed behind the fire box, has been added but lately; but six-wheeled engines are now 
coming into more general use, and on several railways none others are used. In the 
earlier engines the fire-box was considerably smaller than the present size, and that 
end of the engine behind the crank axle was but little heavier than the other end 
before the front axle, so that the engine was nearly balanced upon the axles and ran 
steadily along. But the weight of the hind end of the engine has been so much in 
creased, by increasing the fire-box, that it has a considerable preponderance, and the 
present engines are far from balanced; in the engine shown in the engravings, the 
weights at the wheels, D' L', supposing the hind wheels, M', removed, are six tons at 
the large wheels, D', and only four tons at the front wheels, L', including the weights 
of the wheels. This excess of weight behind the wheels causes in the four-wheeled 
engines a pitching motion, which makes them rise on the springs of the front axle, 
and is considered dangerous when running very fast. The pitching of the engine 
causes also great injury to the rails, as the wheels are made continually to strike upon 
them with very great force. 
The hind wheels in the six-wheeled engines support the fire-box, and prevent this 
action ; the springs over their axle are hung very light, so that in the ordinary state of 
the engine they only just bear against the frame, and take scarcely any weight away 
from the driving wheels; but they serve to catch the weight in the oscillations of 
the engine, and prevent that overbalancing which causes the pitching motion. The 
weight on the rails at these wheels is therefore only that of the wheels and axle, or