KACK KAIL WAYS. 85 



]f the resultant between the direction of the force of gravity, 

 and that of the traction force of a locomotive, developed at the 

 circumference of the driving wheel, forms a less angle with the 

 rail surface, longitudinally, than the angle of friction between 

 steel and steel, evidently, when the driving wheel is impelled^ 

 there will be insufficient resistance, and it will slip, causing 

 no motion to the vehicle in the contrary direction. There is no 

 purchase to work from. Extra weight on the driving wheel 

 increases the resultant angle referred to, and, by distributing the 

 weight over as many driving or coupled wheels as possible, the 

 purchase is increased, but there is a limit to this, and when in 

 surmounting heavy gradients, the resistance due to the gravity is 

 added to that due to the friction of the load to be drawn, a point 

 is reached in the amount of the load, when the traction is so great 

 in proportion to the greatest practicable weight on the drivers 

 that the resultant angle referred to cannot be kept greater than 

 the angle of friction, and adhesion ceases unless either the angle 

 of friction — which varies with the weather — is decreased by sand- 

 ing the rails, or some special contrivance is adopted. Such a 

 contrivance is the rack and pinion, which is the subject of this 

 paper. 



The apparatus, in its simplest form, consists of a rack, laid 

 centrally between the ordinary rails, with which one or more 

 steam driven pinions, which can be coupled in sets under the 

 engine, engages. At first the rack took the form of a ladder, the 

 rungs of which were acted upon by the cogs of the central engine 

 wheel, but this was soon abandoned for the ordinary rack and 

 pinion. It is clear that such a contrivance must be absolutely 

 free from all danger of breakage, for if either rack or pinion were 

 to break, the train would have nothing to hold it but the brakes, 

 and as the incline to which the system is applied is necessarily 

 severe, a great strain would be put upon these, and a great risk 

 of a dangerous runaway incurred. 



This led to the introduction by Mr. Roman Abt, whose rack 

 system has been more generally adopted than any other, to devise 



