RAILWAYS. 



791 



evident that if the horse draws the car to which he 

 is attached, the others fastened to it must follow, 

 it being no objection that either the wheels of the 

 carriage to which the horse is harnessed, or of 

 those of the train following, do not take hold of 

 the rails, but, on the contrary, the less hold they 

 take, the more easy it will be to move the train. 

 But where one carriage is impelled forward by the 

 action of the engine in turning the wheels, and the 

 following train of wagons is drawn by the engine 

 car, if the resistance by gravity and friction is 

 greater than the force with which the wheels 

 adhere to the rails, the engine will only revolve 

 the whet'ls to which it is geared, which would turn 

 upon tlie rails, and the car and whole train remain 

 stationary. To prevent this, different contrivances 

 were heretofore resorted to, one of which was to 

 let teeth project from the sides of the wheels to 

 interlock with rack-work on the side of the rail. 

 It has, however, been found, in practice, that, for 

 the ordinary inclinations of railroads, to the extent 

 of about thirty feet per mile, the wheels may be so 

 constructed as to move a train of wagons by their 

 mere adhesion to the rails. The inclination which 

 can be so overcome must evidently depend on the 

 kind of surfaces of the rim of the wheel and the 

 rail, the weight bearing upon the wheels, the 

 weight to be moved, and the resistance from the 

 friction of the train of wagons ; so that no precise 

 rule can be given that shall be applicable to roads 

 and wheels of different materials and construction. 

 One of the first expedients for increasing the adhe- 

 sion of the wheels to the rails, without incurring 

 any considerable loss by additional weight or fric- 

 tion, was to gear the four wheels of the engine car 

 together, so as to have the advantage of the friction 

 of all of them upon the rails ; for, if the piston of 

 the engine is connected by gearing only with the 

 wheels of one axle, a resistance in the other 

 wheels of the engine, and by the whole train, only 

 equal to the friction of those two wheels, can be 

 overcome. By gearing the piston of the engine 

 with the four wheels, by means of an endless chain 

 passing round the two axles upon two cog-wheels, 

 or by otherwise gearing the four wheels together 

 or to the piston, the hold of the wheels on the rails 

 is doubled. For the same purpose, an additional 

 set of wheels, making six in the whole, for the 

 engine car, is sometimes added ; but such an addi- 

 tion to the number of sets of wheels is evidently 

 attended with disadvantages on the score of ex- 

 pense, complication of structure, weight to be 

 moved, and friction of parts to be overcome. The 

 advantage proposed by adding another set of wheels 

 is, that a greater weight may be carried by the 

 engine car, thus making a greater adhesion to the 

 rails by the wheels geared together, without throw- 

 ing so great a weight upon any of the wheels as to 

 injure the road. But resort is rarely had to this 

 expedient. An improvement, having the same 

 object, and attended by no loss from addition of 

 weight or friction, is a contrivance for securing the 

 adhesion of all the wheels to the rails ; for it will 

 be obvious that, if the two axles of the two sets of 

 wheels are fastened to a strong unyielding car 

 frame, the car will rest upon three wheels, when- 

 ever the surface of the road does not precisely cor- 

 respond in relative altitude to the lower points in 

 the rims of the wheels; that is, if the surfaces of 

 the rails are precisely in the same plane, and the 

 bearing surfaces of the rims of the wheels are also 

 precisely in the same plane, all the wheels will rest 

 upon and take hold of the rails, whether the axles 

 are fastened to an unyielding frame or not. But 

 uo road or carriage can be so perfectly constructed, 



that the surfaces of the rails and bearings of the 

 wheels am always exactly correspond. Mr Knight, 

 the chief engineer of the Baltimore and Ohio rail- 

 road, says, in his report of October, 1831, that the 

 whole weight of a wagon, with an unyielding 

 frame, will frequently be supported on two only of 

 the four wheels, thus making a load bear twice as 

 much upon one part of the rail, as it would do if its 

 weight were equally supported by the four wheels. 

 To remedy this difficulty, the whole weight carried 

 upon the axles is supported by springs, or some 

 interposed elastic power, that of the condensed 

 steam being taken advantage of for the purpose in 

 some cars, whereby each wheel is pressed upon the 

 rail, though the relative surfaces on which the 

 wheels may bear, on different places in the road, 

 may vary. Mr Knight, in the same report, makes 

 a suggestion worthy of consideration in the con- 

 struction of wagons, as well as engine cars. He 

 proposes that in all cases the weight should be sup- 

 ported on springs, not only for the purpose of dis 

 tributing the weight equally, but also to prevent 

 shocks and jars, whereby both the road and car- 

 riages are injured. Another expedient to secure a 

 sufficient adhesion of the wheels to the surfaces of 

 the rails, is to use wheels for the engine car that 

 are not case-hardened. 



The experiments stated by Mr Tredgold and Mr 

 Wood show a very great advantage in the use of 

 large wheels. Mr Wood states that the motive 

 power required to overcome the same friction of 

 rubbing parts of the car and engine, in case of 

 wheels four feet in diameter, is less by one fourth 

 than in case of those three feet in diameter. But 

 there is some limit to the extent of this advantage ; 

 for an increase of the diameter of the wheel adds 

 to the weight, and the expense of construction, so 

 that wheels of not more than four or five feet 

 in diameter are ordinarily used, and a great part 

 of those in use are not above two and a half feet. 

 Some of the locomotives used on the Liverpool 

 and Manchester railroad have sets of wheels of 

 different sizes, the diameter of one being nearly 

 double that of the other. The state of the rail 

 will have some effect upon the adhesion of the 

 wheels, which is least when the rails are slightly 

 wet. The experiments of Mr Booth, on the Liver- 

 pool and Manchester railroad, prove that in the 

 most unfavourable state of the rails, the adhesion 

 of wheels of malleable iron upon rails of the same 

 material, is equal to one twentieth of the weight 

 upon them. The locomotives vary in weight, from 

 three or four to ten or eleven tons. A locomotive, 

 with its apparatus and appendages, weighing four 

 and a half tons, will adhere to the rails with suffi- 

 cient force to draw thirty tons weight on a level 

 road, at the rate of fifteen miles per hour, and 

 seven tons up an ascent of one in ninety-six, or 

 fifty-five feet in a mile; at a slower rate, it will 

 draw a greater weight. The slower the rate of 

 travelling is, the greater is the weight that may 

 be supported by the same wheel, without injury 

 to the road from shocks, though the weight must 

 of course be limited by the size and strength of 

 the rails, whether the rate of motion be quick or 

 slow. 



Curvatures in the Road. The curvatures of the 

 railroad present some obstructions, since, the axles 

 of the car and wagons being usually fixed firmly to 

 the frames, every bend of the tracks must evidently 

 cause some lateral rubbing, or pressure of the 

 wheels upon the rails, which will occasion an 

 increased friction. If the wheels are fixed to the 

 axles, so that both must revolve together, according 

 to the mode of construction hitherto most usually 



