STEAM-CARRIAGES on common roads have for many years occu pied the attention of engineers and mechanicians, on account of the evident economy which would result from the use of a road surface adapted to. and previously constructed for, ordinary traffic ; but, up to the present time, none of the systems which have been proposed for the construction of these enginee can be considered to have entirely overcome the practical difficulties attending the transport of the weights required in the machinery and boilers, or the variable resist ances encountered. Some of the more recently introduced traction engines seem, however, to contain, in germ, tho elements of the success ful solution of the problem of steam locomotion, by engines of small power, running on roads possessing an inferior degree of theoretical perfection than well maintained railways; and the introduction of the street railways certainly justifies the belief that, ere long, steam engines may be used upon them between points which would not require, or justify, the construction of regular railroads. A short notice of the most celebrated attempts to adapt steam-carriages to common roads may, therefore, be desirable, accompanied by a few remarks on the characteristic distinctions between the resistances to be overcome in the respective cases of rail, or of common, roads.
When steam-carriages work upon railroads, they traverse a surface of uniform hardness and rigidity ; and it is easy to distribute the load over a number of carriages drawn by the engine, because the rails maintain the various parts of the train in the desired positions with respect to ohe another. No difficulty either occurs, in this case, on the score of the guidance of the engine or of the load ; nor is it neces sary to provide for avoiding obstacles in the road, or on account of sharp curves or bends. On common roads, however, the yielding nature of the surface imposes a limit to the weight brought upon the driving-wheels ; and the state of the surface as affected by wet, by the addition of fresh metal, or by other accidental causes, must materially increase the irregularity of the resistance. The rates of inclination
adopted on common roads, and the small radii of curvature frequently met with upon them, are far more unfavourable than those met with in railways; and it is found in practice that there are serious objections to the use of more than two carriages in immediate connection with one another on common roads ; so that forcedly the space devoted to the reception of the machinery, boiler, combustible, .ke., is confined within very narrow limits, in order to leave any space available for the conveyance of passengers or of goods. The irregularity of resistance of the road surface renders it necessary also to adopt great precautions in the construction of the parts of the engine designed to transmit motion from the piston-head ; and the constant changes of inclination require that there should be provided, in steam-carriages designed to run on common roads, means for varying at will the amount of power exercised. In all locomotives it is desirable that the machinery should be of the simplest construction, and that it should be susceptible of easy examination and repair ; but this is essentially the case with steam-carriages, on account of the variety of action required of them. The consequences of these peculiar conditions are that steam-carriages must be made to combine great power within small compass, and with small (lead weights; they must be constructed so solidly as not to be deranged by the shocks, or the bad state of the roadway ; and lastly, they must be able to work without producing any nuisance, or auy interference with the ordinary horse traffic of the public thoroughfares. Hitherto it has not been found possible to secure these conditions, and to secure economy of traction at the same time ; and the substitution of steam for animal power Is still confined to railways, notwithstanding the enormous amount of capital they require to be expended on their construction.