It will be seen from this description, that Messrs. Dodd and Co.'s improve ment consisted chiefly (like that of all others who succeeded) of a renovation of Trevithick's plan of propulsion by the mere friction produced by the contact of the wheel and rail. The only material difference between the two plans being in the using of two,working cylinders, instead of one with a fly wheel; and in a method of connecting the axles, so as to cause the cranks to continue working at right angles to each other ; the object of this being, that when the one crank is passing the centre, the other shall be at its greatest power, and consequently aid the former in its revolution, when, for want of the momentum imparted to a fly wheel, it would have to stop in that situation. Upon refer ence to our engraving of Trevithick's railway engine, at page 388, our mecha nical readers will not fail to observe that the improved mode proposed by Messrs. Dodd and Stevenson, of applying the power direct to the running wheels, is not to be compared to it in efficiency or durability.
In the subsequent year 1816, a patent was granted to Mr. Stephenson, in con junction with Mr.W. Losh, of Newcastle, for "a method or methods of facilita ting the conveyance of carriages and all manner of goods and materials along railways and tramways, by certain inventions and improvements in the con struction of the machine, carriages, carriage-wheels, railways, and tramways employed for that purpose." The specification of this patent is more ably written than such documents usually were; and as it contains much valuable practical intelligence, we shall make some extracts from it, and accompany them by the necessary illustrations. The patentees commence their specifica tion by explaining the distinction between edge-rails and the tram or plate-rails, as introductory to their improvements, which they thus explain. "In the con struction of our edge-railways, our objects are, first, to fix both the ends of the rails, or separate pieces of which the rails are formed, immovable, in or upon the chairs or props by which they are supported ; secondly, to place them in such a manner that the end of any one rail shall not project above or fall below the correspondent end of that with which it is in contact, or with which it is joined ; thirdly, to form the joinings of the rails with 'the pedestals or props which support them, in such a manner, that if these props should vary from their perpendicular position in the line of the way, (which in other railways is often the case,) the joining. of the rails with each other would remain as before such variation, and so that the rails shall bear upon the props as firmly as before.
" In the formation of our wheels, it is our object to construct them in such a manner, and to form them of such materials, as shall make them more durable and less expensive in the repairs than those hitherto in use. This invention we accomplish by forming our wheels either with spokes of malleable iron, and with cast-iron rime, or by making the wheels and spokes of cast-iron, with hoops, tires, or trods, of malleable iron ; and in some instances, particularly for wheels of very small diameters, instead of spokes of malleable iron, we employ plates of malleable iron to form the junction between the naves and the cast iron rims of the wheels.
Fig. 1 is a side view of the wheels, with wrought-iron arms. a a a show the arms cast in the nave, and dropped into mortice holes made in the rim, which are dovetailed, to suit the dovetailed ends of the arms u a a. The arms are heated red hot previous to dropping them into the holes, in order to cause them to extend sufficiently for that purpose, for when cold they are too short, owing to the property which iron possesses, of expanding on the application of heat, and of contracting again to its former dimensions on cooling down to the same temperature from which it was raised ; the arms, therefore, on cooling, are drawn with a force sufficient to produce a degree of combination between their dovetailed ends and the mortices of the rim, which prevents the possibility of their working loose; they are afterwards keyed up ; the mortice holes are also dovetailed, from the tail side of the wheel, (a a, Fig. 2.) to the crease side (b in the same figure.) Fig. 2 is a cross section through the centre of the wheel, with wrought-iron arms.
Fig. 3 is an end view of Fig. 2.
Fig. 4 represents an elevation of the edge railway, showing a rail a con nected with the two adjoining rails, the ends of which are shown by b b, and resting in the props or pedestals, the bases of which are the metal chairs that are bolted to the stone supports c c. The joints e e are made by the ends of the rails being applied to each other by what is termed. a half lap; and the pin or boltg which fixes them to each other, and to the chair in which they are inserted, is made to fit exactly a bole which is drilled through the chair and both ends of the rails, at such a height as to allow both ends of the rails to bear on the chair, and the bearance being the apex of a curve, they both bear at the same point. Thus the end of one rail cannot rise above that of the adjoining one; for although the chair may move on the pin in the direction of the line of the road, yet the rails will still rest upon the curved surface of their bearance without moving.