ground; confequently the power is not entirely employed in drawing the carriage forward.

Four-wheeled carriages are almoft always made with the two fore wheels fmaller than the hind wheels. The fore wheels are made fmaller than the others for the conveniency of turning, as they require less room for that purpose. The finalinefs: of their fize does alfo prevent their rubbing against the traces: but, thofe objects excepted, small wheels are by no means fo advantageous as thofe of a larger diameter, as has been already mentioned, and as will be confirmed by the following illuftration.

In fig. 2. Plate IX. let the hollows BGC, and DFO be equally large, and equally deep in the ground. It is evident that the large wheel A will not go fo far into the hollowing, as the fmall wheel R. Befides, even fuppofing that they defcend equally deep into thofe hollowings, the large wheel, by the power acting far above the impediment, may be easily drawn out of it; whereas the fmall wheel can hardly be drawn out by means of an horizontal draught, unless indeed when the ground gives way before it, which is not always to be expected.

The idea of the two large wheels helping to drive the fore small ones, is a vulgar error, which has not the leaft foundation in truth. The abfurdity of this idea might be proved various ways, but by none more fatisfactorily than by the following experiment.

Take

Take a real carriage, or the model of a large one, haying two large and two small wheels. Faften a rope at each of its ends, but equally high from the ground;, then extending one of those ropes horizontally, let it go over a pulley, which must be placed at fome distance from the carriage, and tie as much weight to the defcending extremity of the rope, as may be juft fufficient to move the carriage, This done, difcharge this rope; turn the carriage with its other end towards the pulley, and, in fhort, repeat the experiment with the other end of the carriage foremost. It will be found that precifely the fame weight will be required to draw the carriage, and to draw it with equal velocity, whether the large or the small wheels be placed foremoft.

The figure, or rather the breadth of the rims of the wheels, influences confiderably the motion of the carriage. Upon a ímooth and hard road no advantage is derived from the, ufe of broad wheels; but upon a soft road the broad wheels are much more advantageous than narrow ones; the latter cutting and finking into the ground; on which account they must be confidered as always going up hill, befides their fuffering a great deal of friction against the fides of the ruts that are made by themselves; whereas the broad wheels produce nearly the fame effect as a garden-roller; that is, they smooth and harden the road, befides their moving with great freedom. It must however be obferved, that upon fand, as alfo upon stiff clayey

roads,

roads, lefs force is required to draw a cart with narrow, than one with broad, wheels. Upon fand the broad wheels form their own obftacles, by driving and accumulating the fand before them. Upon clavey roads they gather up the clay upon their furfaces, and become in a great measure clogged by it.

Some perfons imagine that the broad wheels, by touching the ground in a great many more points than narrow wheels, muft meet with proportionably greater obftruction. But it fhould be confidered, that though the broad wheels touch the ground with a larger furface, yet they prefs upon it no more than narrow wheels do. Let, for inftance, two carts be equal in every respect and equally loaded, excepting that the wheels of one of them be 3 inches broad, whilst those of the other be 12 inches in breadth. It is evident that the latter wheels reft upon the ground with a furface which is equal to four times the furface upon which the former wheels reft. But fince an equal weight is supported by the wheels of both carts, every three inches breadth on the surface of the broad wheels fuftains a quarter of that weight; whereas the three inches breadth of the narrow wheels sustain the whole weight; fo that the broad wheels touch the ground as much lighter as they are broader than the narrow wheels. It is for the fame reason that if a heavy body in the form of a parallelepipedon, viz. like a brick, be dragged upon

a plane

a plane furface, the fame power will be required to draw it along, whether its broad or its narrow fide be laid upon the plane.

If the wheels were always to go upon smooth "and level ground, the best way would be to "make the fpokes perpendicular to the naves;

1

that is, to ftand at right angles to the axles; "because they would then bear the weight of the "load perpendicularly, which is the strongest way "for wood. But because the ground is generally uneven, one wheel often falls into a cavity or rut "when the other does not; and it bears much more of the weight than the other does; in "which cafe, concave or dishing wheels are beft, "because when one falls into a rut, and the other keeps upon high ground, the fpokes become perpendicular in the rut, and therefore have the "greatest ftrength when the obliquity of the load "throws most of its weight upon them; whilft "thofe on the high ground have lefs weight to

bear, and therefore need not be at their full " ftrength. So that the ufual way of making the "wheels concave is by much the best."

"The axles of the wheels ought to be perfectly ftraight, that the rims of the wheels may be parallel to each other; for then they will move eafieft, because they will be at liberty to go on ftraight forwards. But in the ufual way of prac"tice, the axles are bent downward at their ends; which brings the fides of the wheels next the

ground nearer to one another, than their oppo"fite or higher fides are: and this not only makes "the wheels drag fidewife as they go along, and

gives the load a much greater power of crushing "them than when they are parallel to each other,

but also endangers the overturning of the carriage when any wheel falls into a hole or rut, or "when the carriage goes in a road which has one "fide lower than the other, as along the fide of a "hill;" for on that conftruction the carriage ftands upon a narrower base, than when the rims of the wheels are parallel to each other.

Upon level ground a carriage with four equal. wheels may be drawn by the same power with equal facility, whether the load be placed on any particular part of the carriage, or it be spread equally all over it.

Upon a two wheeled carriage the most advantageous difpofition of the load is, when the centre of gravity of the weight coincides with the middle of the axle, or with a perpendicular line which paffes through that middle.

A carriage having the two hind wheels large, and the two fore wheels fmall, when going upon an horizontal plane, thould have the principal part of the load laid towards its hind part; but when going upon uneven roads, up and down hill, and when the load cannot be easily shifted, the best

Ferguson's Lectures, lecture iv.

way