The proportions of wheels must be es timated according to the purposes to which they are to be applied; thus wag gons have in general large hind-wheels, while in timber carriages the four are usually of the same height, or nearly so ; the London common stage carts have large wheels, while the drays used by brewers have very low ones. The reason is obvious ; waggons and carts load be hind ; but timber carriages and drays load at the sides; therefbre, in such, large wheels, however much they might favour the draught, would be extremely incon venient ; indeed incompatible. Wheels, whatever their size, should he made of well-seasoned tough wood, perfectly free from blemish ; the naves are generally of elm, the spokes of oak, and the follies of elm or of ash such are found to answer best for all carriages attached to the ord nance department; in which the following are considered as the regular standard heights.
All the horse-artillery carriages, lim bers, and waggons ; the heavy six-pound ers, and long three-pounders, and their limbers ; the carriage of a six-pounder battalion got; of a light five and a half inch howitzer ; and the hind wheels of an ammunition waggon, five feet. The lim ber to a light six-pounder, and five and a half inch howitzer ; the carriage of a me dium twelve-pounder, four feet eight inches. The limber of the latter four feet six inches. A sling-cart, five feet eight inches. The fore-wheels of an am munition waggon, four feet. A pontoon carriage has the fore-wheels three feet, and the hind ones five feet six inches. The carriage of an eight inch howitzer, five feet ; the limber, four feet. A ball ammunition cart, five feet.
We are disposed to recommend these proportions to the consideration of read ers concerned in the construction, or in the use of wheel carriages ; they being the result of innumerable experiments, submitted to unequivocal proof under every variety oflocality and of burthen. We tnink it necessary, at the same time, to observe, that a correspondent of the Agri cultural Magazine, formerly published by Longman, Hurst, Rees, and Orme, of Paternoster-row, has, in the eleventh number of that work, given, what ap pears to be, an excellent rule for the proportions of wheels in waggons. It
would not be admissible for us to give the whole of the reasonings of that cor respondent, as contained in various num bers ; but from that which we have parti cularized, we have the pleasure to furnish the following extract ; or, at least, the sense of it.
" if the fore-wheel be four feet four inches in height, and the line of traction (draught) be drawn at an elevation of twelve degrees from the centre ail* axle, the point where that line cuts the circum ference of the wheel in its front gives that height from the plane on which the carriage stands, that will determine the radius of the hinder wheel. In this in stance,the hind-wheel would have a radius of two feet nine inches, giving Of course five feet six inches for its diameter." A view of the plate given in that work, not only will illustrate the above explana tion, but will satisfy a person respecting the justness of the proportions above de tailed ; when tempered by the following cautions, we consider the instruction given to be admirable. " The fore-wheel . ought to be as nearly level with the point of draught, that is, where the shaft is suspended by the gear, as may be conve nient ; observing, that an angle of twelve degrees is to be given, on account of the difference between the horse's height as he stands at rest, and the real altitude of the point of draught from the ground, when he is in a state of exertion. During great efforts, horses lose very considera bly of their standard, and thus bring the shaft to nearly a parallel with the plane on which they move. Attention must be paid to keeping the wheel within such limits as may not trespass on other mat ters, often of more consequence even than ease of draught ; loading, turning, weight, expense, &c. must always form a part of the calculation."