The letters a and b indicate hoops of dif ferent sizes. To fix this hoop to the wheel, it is heated in a ibrnace of a circular form, so that the fire may act uniformly on every In a large wheel, this process of heating the hoop causes it to expand about one inch in circumference, and it is thereby med. large enough to slip over the u•ooden wheel, previously prepared, of greater dimensions than the interior circle of the iron hoop, in the cold state. Whilst the iron hoop is being heated, the wooden wheel is clamped to a fiat circular plate, which is feed to a central axis, to enable it to revolve; and this axis is fixed upon an horizontal bar, by which the wheel and iron plate to which it is clamped may be turned in either a ver tical or horizontal direction. Underneath the circular plate is a semicircular well or cistern, containing cold water, into which the wheel is immersed and turned round as soon as the hoop is put on it. This application of cold to the hot expanded hoop causes it to contract with irresistible force, pressing the spokes into the fairies and nave, and binding all the parts together. The annexed little figure exhibits a section of the sew patent tire, as ap plied to the felloe d; e representing the end of a spoke. A curved tire like the foregoing was made many years prior to Mr. Meaden's patent, but by simpler and cheaper means,—it being rolled di y from the bloom into curved bars; and we think we remember seeing them on the mail coaches more than twenty years ago. The curve on the interior side of the tire is of unquestionable advantage, in causing it to hold more securely on the felloes,—as must be evident from the preceding figure; but the external curvature of the tire is, in our opinion, of doubtful utility. The rounding of the extreme edges of a fiat-bearing wheel may prevent dirt being hitched up and carried round with the wheel; but even that much being removed, reduces to the same extent the resistance of the ground to the wheel sinking into it; and if the whole bearing surface be rounded, it Indot evidently penetrate deeper into the ground, and in so drag force the materials of the road sideways. Nevertheless, tire of this construction is, we believe, still employed in our mail coaches. But however injurious to the roads may be tire of this kind, the practice of giving a conical form to the rims of carriage wheels is infinitely more destructive. This form has an evident tendency to move in a different direction to the line of draught ; and the power which is required to keep it in a straight line is so much power wasted in twist ing the materials of the road out of settings, and grinding them to powder.
The cylinder (as Mr. Cumming justly observes) having all its parts of equal diameter, will, in rolling on its run, have an equal velocity at every part of its circumference, and necessarily advance in a straight line. And as all the parts of the rim have an equal velocity, none can have a tendency to drag forward or impede theof the others; • they all advance with one consent, without the part on the surface on which they roll. As there is no rubbing there can be no friction, and consequently a cylinder perfectly round, hard, and smooth, forms the least possible resistance, however great its we weight or the pressure on its rim. It therefore follows, that all the power that is era eyed in drawing forward a cylindrical body in a straight line on a comp e sub stance, is ultimately applied in compressing smooth and levelling the substance on which it rolls. The rolling of a cylindrical body, therefore, can have no tendency to alter the relative situation or parts of materials on which it nor any how to derange them, but by a progressive deadpressure to con 'date, level, and smoothe them. If a cylinder be cut transversely into several lengths, each part will possess all the above properties • and if the rim of a carriage wheel be made exactly of the same shape, it must necessarily have the same tendencies. When wheels with cylindrical rims are connected by an axis, the tendency of each being to advance in a direct line, they proceed in this connected state with the same harmony and unity of consent that exist in the parts of the same cylinder; but, as conical rims have been universally preferred for a series of years, it is natural to suppose that there were obvious reasons for such preference. The cone diminishing gradually from its base to its paint, the velocity of every part of its circumference in rolling on an even plane, will be diminished as the diameter; and at the very point where there is no visible diameter, it will have no perceptible motion ; but if the cone be made to advance in a straight line, the natural velocity of its several parts will not be as the spaces, therefore a rubbing and friction will take place at its circumference, from the different velocities of Its parts, which must render the draught heavier. In rolling on paved streets nothing can be conceived more calculated for their destruction than the conical rim of • broad wheel. See CARRIAGE., AXLETR1i4 For TEITHID WHEELS employed in driving machinery, see that article.