Wheel, is a simple machine, consisting of a nave, fellies, and circular brim, or pieces of wood generally surrounded -with iron ; and thus revolving on its axis.

Wheels are of various breadths, according to the size of the carriages for which they are designed; Mr. Cumming (in his elaborate " Observations on the Effect which Carriage-wheels, with Rims of different Shapes, have on Row published in the 2d vol. of " Communications to the Board of Agriculture" ), has satisfactorily proved, that wheels, with cylindrical rims, are doubtless preferable to any other. Our limits will not permit us to analyze this valuable memoir; we shall, therefore, only remark, that such wheels advance in a straight line; being subject to little or no friction at the circumference : nor do they press against the linchpin ; or tend to displace, break the texture, or retard the induration of the parts on which they revolve: and, their motion on the soil meeting with the least possible resistance, they serve to improve the roads, to relieve the cattle, and to preserve the tires. On the contrary, conical rims require a constant force to keep them in a direct line, and that force occasions additional friction on the rim : farther, they increase the rubbing on the axis ; and, in dry weather, reduce the best materials to powder; but, if the roads be soft, they break and disorder the texture of the ground ; render the labour of cattle more difficult ; and contribute to the wearing of the tires. - Mr. Cumming, therefore, justly concludes, that great advantages may be derived, by substituting cylindrical for conical rims ; and it must be ascribed to an unaccountable infatuation, that the bent axis, and conical rims, are still retained in use, particularly on turnpike-roads. Those of our readers, who wish more minutely to investigate this important subject, will consult Mr. C.'s essay before quoted; and which is illustrated by an engraving.

In the year 1793, the Society for the Encouragement of Arts, etc. rewarded Mr. Joseph Dixon with the sum of 15 guineas, for his contrivance of a Preservative Wheel, that may be affixed to a walking wheel-crane; by means of which all accidents to labourers will be obviated, in case the weight should at any time overcome the power. His implement is attached to the axis of the crane-wheel; over the latter are passed two ropes, of which the men may take hold, so as to suspend themselves with the greatest safety, during the time of danger. Pullies are likewise placed near such preservative wheels, over which the ropes turn, and are thus prevented from taking fire, in consequence of the friction. - For a mere explicit account of this useful invention, we refer to the 11th vol. of the Society's " Transactions;" where his statement is elucidated by an engraving.

A patent was granted in August, 1799, to Messrs. George Don-son, and John SkidmorE; for a method of making the naves or stocks of wheels, of cast-iron, brass, or compound metal. A description of their process is inserted in the 12th vol. of the " Repertory of Arts, " etc; together with an appropriate plate.