in Mechanics, a simple machine, consisting of a round piece of wood, metal, or other matter, which revolves on its axis. See Mechanics.

WHEEL-Carriages. See Mechanics for an account of the general principles.

No kind of wheel-carriages are of more importance to a commercial and manufacturing country than stage coaches; and perhaps in no kingdom of Europe has the system of travelling in public vehicles been carried to greater perfection, as to comfort and speed, than in Britain. The danger, however, of travelling by these coaches makes considerable deduction from their accommodation otherwise: it is but too well known that this mode of travelling is liable to frequent and serious accidents. Every attempt therefore that promises to be useful in diminishing such danger should have all possible publicity. With this view we are much gratified in having an opportunity of laying before our readers the following account of an invention to render stage coaches more secure from danger, obligingly transmitted to us by the inventor, the reverend William Milton of Heckfield, Hants. For this invention that gentleman has obtained a patent.

The danger of stage coaches arises sometimes from overturning, and sometimes from breaking down. The overturn is, in general, occasioned either by taking two side-wheels into too deep a hole or ditch, or over too high a bank; or, secondly, by running down more quickly than the carriage is calculated to do, from the top to the sides of a rounded road; or, lastly, by turning a sharp corner with too great velocity. In the two first cases the danger arises from the centre of gravity of the total coach and load being placed too high; and in the last instance, of turning the sharp corner, from the same centre (but which we must now consider as the centre of the vis inertiae) being also placed too high. The danger in the two first cases grows often out of the very circumstances of the road, and meets every one's comprehension: the last, which is less obvious, is generally owing to the mere will of the driver; and the better the road, the more he is tempted, without any intention, to go on to produce it: it requires therefore to be more generally understood than it is. It may be thus explained:—A carriage is going along a straight level road at the rate of nine miles an hour: then, though you imagine the horses or pulling power to be in an instant withdrawn, yet will the carriage continue its motion for ten, fifteen, or more yards, and at first with the same velocity, and in the same straight line, in consequence of the acquired motion. Supposing, now, the coach with its four horses going the nine miles an hour along a fine level road, but which has a sharp and sudden corner to turn:—the coachman knows it, and wishes to keep his velocity; the horses are aware of both—and by the animal dexterity with which they are gifted, contrive to make the turn without remitting any thing of their speed. Not so the coach which follows them; that has a tendency to persevere in its straight line; and the centre of its effort to do so is the centre of its vis inertiae, the very centre of its gravity. If this centre be low, the turn of the corner may be made with no other inconvenience than a short awkward slide of the hind wheels, onward in the original direction; whereas, if it be high, there will be no slide, but the coach will be overturned, and overturned nearly at that point where its broadside is at rectangles to the straight line of road it has been thus forced to quit: for at that point the base against such an overturn will be the most disadvantageous, and the check to the onward motion the greatest. The remedy offered against all these causes of the overturn, (whether by a ditch, bank, rounded road, or sharp corner), is to bring down this centre, by placing as much of the luggage as possible in a luggage-box, below the body of the carriage; the body not being higher than usual.

From the overturn, we pass to the consideration of the breaking-down; this we must reckon on happening as often in these patent stage coaches as in others. Wheels will come off or fail, or axles will break, in future, as they have done heretofore; but against the disastrous and fatal consequences of such accidents the remedy offered may be thus described.—On each side of the luggage-box, with their periphery below its floor, and each as near as may be requisite to its respective active wheel, there is placed a small strong idle wheel, ready in case of breaking down, on either side, to catch the falling carriage, and instantly to continue its previous velocity, till the coachman can pull up his horses, thereby preventing that sudden stop to rapid motion, which at present constantly attends the breaking-down; and which has so frequently proved fatal to the coachman and outside passengers. In case a fore-wheel comes off, each end of the fore-carriage has its idle wheel. By this provision we shall be, to all effect of safety, continually travelling with two carriages under us. The bottom of this luggage-box is meant to be about fourteen inches from the ground; and the idle wheels seven, fix, or five; but if at a still less distance, little inconvenience would result; for when either of them takes over an obstacle in the road, it instantly, and during the need, discharges its respective active wheel from the ground, and works in its stead. If these two principles of safety were applied to the description of the several stage-coach accidents we meet with, there is no doubt but a general conviction would arise, that the safety by these modes is (in vehicles of all kinds), perhaps as great as can consist with rapid loco-motion; and that, sooner or later, legislative authority, in some shape or other, may judge it necessary to interpose, for the purpose of controlling a prejudice against the form essential to this mode of safety. The trial and proof which these principles have been brought to, have not only been by public exhibition, and with preparation; but in all the suddenness, also, of actual heavy work: and the result in both cases has been so exactly the same, as to give continual assurance of the full effect of the remedy, as often as the casualties of the road shall bring it into action.

The aim in the arrangement of this coach of safety, has been to bring down the load, and consequently the centre of gravity, as low as possible: this is thought to make the coach look heavy; and this word, by the ready operation of a prejudice, has been transferred to its going; and one specific reason added withal, that, because the load is low, the draught must be heavy. This point, however, has, in the presence of 10 or 12 competent persons, been brought to the most decisive proof; and it comes out, that it is as indifferent to draught, as it is material to danger or safety, whether a ton be placed on the roof of a coach, or a ton on the floor of the patent luggage-box, about 15 or 16 inches from the ground.

It has been asked, "What would this coach do in snow?" The question has been thus answered by the result of actual work; for the patent coach, after being detained on the road with several other coaches, by a sudden fall of snow, when at last they started together, came in six or seven hours before any of them. They were bound in prudence, to go cautiously along the ground, whose unevenness was invisible; while the patent coach dashed along it with all the confidence and safety of a post-chaise. See Plate DLXXVII.

WHEEL-Animal. See ANIMALCULE, No 16—23.