The rapid motion of railway carriages, coupled with their great weight, so greatly increases the efieet of such concussions as must occur on even the smoothest road, that wooden wheels have been found utterly unsuitable for them. Cast-iron wheels have been much used en colliery railways, and in some cases where rapid motion is required; but while they may be made abundantly strong, as far as direct pres sure is concerned, their brittleness renders them very unsuitable for passenger carriages. Matiy ingenious plans for the combination of wrought-iron and cast-Iron In the same wheel have been devised ; but while soine of these have been brought into operation, wheels entirely composed of wrought-Iron have been by far the most generally adopted. The facility with which that material may to worked into form has led to endless variety of plans, 801110 of which are highly ingenious, for combining the requisite degrees of strength and elasticity. In some wheels the annular space between the central boss or nave and the rim is filled up by a series of elliptical loops, formed of thin bars of iron, abutting against each other ; in others there are spokes, but instead of consisting of single rigid bars, each consists of two halves, having a slight degree of curvature. By these and similar contrivances elasticity is insured without dishing the wheels, which would, for railway narriages, be inconvenient. In some cases a portion of the annular apace above described is filled with segmental blocks of wood, resem bling the felloes of a common wheel ; but while this arrangement claims some advantages, its appearance is very inferior to that of the light and often elegant wheels formed entirely of wrought-iron. One kind of iron wheel which claims notice is that patented by Mr. Theodore Jones. These wheels may be compared to double- dished wheels in general appearance, but their principle is very different. They consist of an iron rim pierced at intervals with conical holes, the largest apertures of which are on the outside ; two sets of round rods or spokes, with pyramidal heads to fit in the conical holes of the rim, the two sets radiating or inclining alternately inwards and outwards, like the spokes of a double-dished wheel ; and a cast-iron nave, which is formed hollow, with boles to receive the inner ends of the spokes, which are secured by nuts screwed on to them within the nave. The
peculiarity of this construction is, that instead of the weight resting almost entirely, as in a common wheel, upon those spokes which happen to be below the nave, it is, as it were, suspended by means of the rods or spokes which are above the nave, from the top of the wheel, the rim of which is considered as an inflexible arch. On this account the wheels are called suspension-wheels ; and as the strength of wrought iron to resist tension is far greater than its strength to resist compres sion, a wheel on this principle may be made to bear a much greater load in proportion to its bulk and weight than any other.
As an example of railway-wheels, we may advert to that of Messrs. Hollis it Lee. It is built together in four parts, of which each com prises two spokes, a quadrant of rim, and one fourth part of the nave. The nave thus formed is square. The rim-pieces are a continuation of the spokes, bent round to a curve, and fastened at the spoke ends by tenon and mortice joints.
Patent noiseless wheels have been made with tires of india-rubber, and with various contrivances for ensuring durability while obtaining elasticity and noiselessness; but they have not come extensively into mac.
The wheels used by cutlers, lapidaries, scal-engravers, and glass engravers, under the names of brush-wheels, buff-wheels, cloth-wheels, composition-wheels, crocus-wheels, emery-wheels, lap-wheels, ke., are mostly made of metal or wood, coated with some other substance at the edge. Their manufacture needs no description.