But a perfect calm in the atmosphere is very rare, and vehicles. intended for daily and constant use must be prepared to contend with the strangest winds. The power must therefore be increased to such an extent as; to enable the vehicle to travel at its wonted pace in all weathers. Now, according to Mr. Smeaton, a " hard gale " is found to sweep along the surface • of the earth at the rate of from forty to fifty miles an hour. This velocity, which would be increased to sixty or seventy by that of the steam-coach when travelling at twenty miles an hour, would produce a resistance-of six. hundred• pounds upon the thirty feet of front of the steam coach, or three hundred pounds upon the front of the steam waggon. With a speed of eight miles an hour, the coach and waggon would encounter a resistance about one-half less. The vehicles, should not be constructed entirely with a view to extreme cases ; and, for r the conveyance of mails and other similar purposes, an average velocity of twenty miles an hour for vehicles of the weight and description mentioned would be secured by a power varying from 200 to 500 pounds; that is, from one fifth to one tenth of the power required to produce the same effect on water. We see, however, that the resistance of the air, which, in vulgar apprehension, passes for nothing, comes to be the greatest impediment to the motion of the vehicles, and may, in some oases, absorb five parts in six of the whole power. Let it be remembered, at the same time, that this aerial resistance rises into consequence solely because the high perfection of the machinery (the vehicle and the road) almost annihilates every other. The atmosphere equally opposes the progress of the stage-coach, the traok.boat, and the steamboat; but the motion of these vehicles is comparatively so slow, and the power of impulsion required to overcome the other impediments so great, that the resistance of the air is disregarded.
There have been various propositions to construct the periphery and tire of carriage wheels, so that they may roll not only upon the common road, but also on iron railways : but if this were allowed, they would be kept in order but a very short time, owing to the injurious effects that would be produced by the gritty mud taken off' the ravelled road and ground upon the rails. The temptation to use a railway in this manner is great ; for the load which required a horse on the common road might be drawn by a man on the railway ; thus enabling them to go with a greater speed, and yet with 'less injury to the horses. Mr. Wood justly observes, that the object of all railroads being to present to the wheels of the carriages a smooth, straight, and level surface, all depressions or displacement of the rails therefore defeat the object for which such a road is formed; and, consequently, their formation must be on the principle of forming and preserving such a level and uninterrupted surface. The nature of the foundation upon which we have generally to form a railway renders this a task of no ordinary difficulty. Perhaps it is almost impossible to form an absolutely perfect railway according to the above principles ; we must therefore endeavour to approximate as nearly as possible towards such a per fection. Two modes of effecting this suggest themselves ; either to form the joinings of the rails to the chairs in such a manner that the stone supports can adapt themselves to the yielding of the foundation, without disturbing the parallelism of the rail; or, that the stone supports be made of that size, and be so embedded upon the foundation, that the weight of the carriages shall not be capable of disturbing them; in which latter case the joinings of the rails to the chairs must be such that the action of the carriages has not the power of deranging the continuity of the rail. To carry the former of these modes into
practice, and to preserve the continuity of the rail with ease and freedom, the stone should be capable of moving round, or assuming any degree of inClina tion to the line of the road that might occur in practice, without either straining the pin or distorting the ends of the rails. To effect this, if the pin be made the centre of motion, the under side of the rail should be a portion of the circumference of a circle, formed from the pin as a centre ; the base of the chair could then be either the apex of a curve, or a circular cavity correspond ing with the exterior semicircular surface of the rail. The stone might then be depressed on either side, without straining the pin or deranging the joints : or we might otherwise make the bearance of the rail upon the chair or pedestal the centre of motion ; in such case the pin-hole should be a circular slit or opening formed from the bearing upon the chair as a centre ; the pin being made exactly to fit this cavity in a perpendicular direction, would prevent the rails from starting upwards out of their proper position, and the semicircular slit would allow it to turn longitudinally ; when the stone then became depressed towards one Bide, the chair could move round without injuring the pin, or deranging the joints of the rails. Innumerable forms of joinings might be devised, every one of which might, in some degree, effect the purpose intended; the essential consideration being to secure a continued and permanent paral lelism in the rails, under every derangement that may take place in the supports on which they rest. " It is not enough (adds Mr. Wood) that the bearing be such that the rails are all in the same plane, when the stones on which they rest are in good order, or in their proper position, parallel with the line of road: the parallelism of the rails should be preserved, when, by the yielding of the ground, or from any other cause, the stones are displaced from their proper position, and are made to form a considerable angle with the line of road. It would not have been necessary to have been thus diffuse on this point, had I not found that several, even of the most modern forms of chair, were evidently formed contrary to this principle ; many with a view of causing the mode of joining to keep the support or stone in its proper position, rather than allowing it to adapt itself to the unavoidable yielding of the ground on which it rests ; but the least consideration will evince the futility of this, especially when the yielding of the ground causes the stone to rest entirely on one side; it will at once be seen, that when the carriages come upon the rails, something must yield and give way, by the great strain thrown upon the fastening from the oblique action of the weight.' Mr. Stephenson has, in forming the greatest part of the Liverpool and cheater railway, adopted the latter mode, cal has endeavoured to obviate those difficulties and imperfections, by making the blocks very large, and embedding them firmly upon the surface of the road ; in the hopes that the weight of the carriages will have no effect in displacing them. NI here stone is readily obtained, though expensive in the first formation, this mode will, no doubt, be found ultimately to be the most beneficial, especially if proper care is taken to keep the surface on which the stones rest dry, and free from water. Upon public lines of road, where the traffic is considerable, it is highly advisable to avoid the necessity of any interruption, by having displaced blocks to set right again ; and therefore it becomes the more necessary to secure their permanent stability in the first formation.