26 Mr Adamson on Rail-Roads. 



be depended on, we should be able to tell exactly either the 

 steam power and weight of the engine necessary to carry a given 

 load, or determine correctly the load which any given steam 

 power could overcome. We are perhaps most in doubt respect- 

 ing the relation between the weight of an engine and its power, 

 or between the size of the boiler and the force of the steam which 

 it can be made to afford. As there is a certain velocity of the 

 piston which produces a maximum of effect, it is clear that 

 this velocity alone should be preserved as much as possible, and 

 the velocity of the load should be determined by the machinery, 

 independent of that of the piston. Each engine ought, in fact, 

 to be constructed for one determined velocity ; and as the dimi- 

 nution of the engine's power by its friction, increases as its weight 

 increases, it will be less expensive to have light engines and high 

 velocities. None of those, as yet in use have been intended to 

 travel faster than 6 miles per hour. The highest velocity which 

 I have witnessed was about twice this ; but then the force of the 

 steam was lost on account of the excessive velocity of the piston, 

 — ^there was no load to be overcome except the friction of the 

 engine ; and even this was diminished by the engine-man assist- 

 ing to open and shut the valves. The experiments by Mr 

 Wood, from which an estimate has been drawn of the travel- 

 ling engine''s work, cannot by any means give too favourable a 

 measure of it : for the progressive effort of the engine, or that 

 part of its power exerted on the load, must, on account of the 

 undulation of the road, have varied in the ratio of 1 : 8, and there 

 must have been a corresponding variation in the rate of the pis- 

 ton. Such inequalities in the load, and in the velocity of the 

 machinery, are a disadvantage attending the apphcation of steam 

 power to rail-roads in every form, except when a dead level can 

 be secured. The greatest irregularities would be found, when 

 a fixed engine was made to work over a considerable extent of 

 country, if such a thing were possible. But the apphcabihty of 

 this method of using the steam-engine must be reduced far with- 

 in the limits which Mr Tredgold assigns to it. The risk of in- 

 terruption, in the traffic of a whole line, by the failure of one en- 

 gine, is almost decisive against the system ; and, besides this oc- 

 casional inconvenience, there would be the constant one of being 



