LOCOMOTIVE ENGINE. 
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reason an engine, which is not loaded so much as the full adhesion of the 
wheels upon the rails, will often slip, and let the wheels turn round quicker 
than the engine is running, on passing a station, or any part of the line where 
the rails are liable to be dirtied by the traffic of persons across them. 
The adhesion of the wheels is found to be about one fifth of the weight upon them 
when the rails are in a good state, and it varies between that and one tenth or twelfth. 
The weight upon the driving wheels as they are termed, is six tons, and the adhesion 
is therefore sufficient for drawing a load of 280 tons besides the engine upon a level. 
When the first locomotives were made, it was thought that the adhesion of the wheels 
upon the rails could not be sufficient to draw any load besides the engine, if it were 
enough for that; and various contrivances were resorted to, in order to obtain the 
necessary fulcrum from which to move the engine. Levers were first tried which 
resembled a horse’s legs, and were thrust against the ground by the piston rods; a chain 
was also tried lying on the ground between the rails and taken hold of by a wheel 
in the engine; also a rack was fixed inside the rails and a toothed wheel turned 
by the engine worked in it. Locomotives which are intended for conveying heavy 
goods have their adhesion upon the rails generally increased by coupling four wheels 
together so as to make them all turn together, and thus obtaining the adhesion of all the 
four to assist in drawing the load. The power of the engine can then be increased 
as the increased adhesion will enable it to be exerted; for the power of engines 
with only two driving wheels cannot exceed a certain limit, or it will be greater than 
the adhesion of the wheels and the excess will be useless. The wheels that are 
coupled together are of the same diameter, and have connecting rods attached to 
cranks, which are fixed on the axles outside of the wheels. Some of the old engines 
had their wheels coupled by a pair of cog wheels; and also by an endless chain 
passed round a pulley on each axle. 
The plan of driving the wheels of a locomotive by means of cranks upon the axle, 
is attended by the disadvantages that the axle is weakened very much by the cranks 
in it, and the power is applied at some distance from the wheels where it is wanted. 
The action of the pistons upon the cranks, alternately pulling and pushing them, and 
the great weight that the cranked axle has to carry, make it necessary that it should 
be made very strong in order to stand its work ; they are therefore very heavy and 
expensive, costing about £50 each. They are very seldom broken, though they some 
times get bent by the engine running off the line; but the older locomotives had their 
cranked axles broken more frequently, as they were not made so strong at first. 
Several plans have been tried for obviating the necessity for a cranked axle, but they 
do not appear to be any of them so good upon the whole. The Rocket, and some of 
the first locomotives upon the Liverpool and Manchester Railway, had their cylinders 
3 k