LOCOMOTIVE ENGINE. 441 



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 locomot ives upon the Liverpool and Manchester Railway, had their cylinders 



3 K 



