wholly owing to a most absurd regulation respecting the breadth of the tires of the wheels. But although stage coaches are so built in England as to allow of travelling with safety and convenience, and at a cheap rate, the comfort of travellers might be increased, and the labour of horses diminished, if the body of a coach were made larger, the fore wheels made higher, the springs made longer and slighter, and the weight made to rest chiefly on the hind wheels. The following observations, which were drawn up for the Commissioners of Inquiry into the Post Office, will explain fully what is proper to be done on each of these points.

ON THE CONSTRUCTION OF COACHES.

In order to form right opinions on plans of coaches, we should begin by acquiring an accurate knowledge of the use of wheels. Nothing is more common than to meet with persons who have formed the most decided opinions on the construction of carriages, without having examined the properties of wheels, and who do not know that they are treated by mathematicians as mechanical powers, and that their properties, as such, are exactly settled, and admit of no dispute among men

of science.

The following extracts have been selected from Hutton's Mathematical Dictionary, Mr. Davies Gilbert's Treatise on Wheels and Springs of Carriages, Rees's Cyclopædia, and Ferguson's Lectures

on Mechanics, as sufficient to give as much information as is necessary for deciding what should be the height of the wheels of a coach, and whether the loading of a coach should be placed, for the most part, over the fore or over the hind wheels.

Extract from HUTTON'S Mathematical Dictionary.
Article "Wheel."

"The height of the wheel is of material consideration, as the spokes act as levers; the greater the length, therefore, of the spokes, or, what is the same thing, the greater the height of the wheel, the more the labour of horses is diminished in drawing a carriage on wheels.

"Large wheels are found more advantageous for rolling than small ones, both with regard to their powers as a larger lever, to the degree of friction, and to the advantage in getting over holes, ruts, and stones, &c. If we consider wheels with regard to the friction on their axles, it is evident that small wheels, by turning oftener round, and swifter about their axles than large ones, must have much more friction. Again, if we consider wheels as they sink into holes, or soft earth, the large wheels, by sinking less, must be much easier drawn out of holes, and over soft earth, as well as more easily over stones and other obstacles, from the great length of lever, or spokes. This has been brought to a mathematical calculation by Desaguiliers, in his Experimental Philosophy, vol. i., p. 171.

"Hence it appears, then, that wheels are the more advantageous as they are larger, provided they are not more than about five feet in height; for when they exceed that height, they become too heavy in order to make them sufficiently strong.

"It would be much more advantageous to make the four wheels of a carriage large, and nearly of the same height, than to make the fore wheels much lower than the hind wheels, as is usually the

case.

"It is accounted a great disadvantage in small wheels, that as the axle is below the bow of the horses' breasts, the horses not only have the loaded carriage to draw along, but also part of the weight to bear, which tires them soon, and makes them grow stiffer in their hams than they would if they drew more on a level with the fore axle.

"If the wheels were always to roll on smooth or level ground, it would be best to make the spokes perpendicular to the naves or to the axles, because they would then bear the load perpendicularly. But because the ground is commonly uneven, one wheel often falls into a cavity, or rut, when the opposite wheel does not, and then it sustains more of the weight than the other does; in which case it is best for a wheel to be dished, because the spokes become perpendicular in the cavity, and therefore have the greatest strength when the obliquity of the road throws most of the weight on them, while those on the higher ground have less of the weight to bear, and therefore need not be at their full strength.

"The axles of the wheels should be quite straight, and perpendicular to the shafts, or to the pole. When the axles are straight, the rims of the wheels will be parallel to each other, in which case they will move the easiest, because they will be at liberty to proceed straight forward; but in the usual way of practice, the ends of the axles are bent downwards, which always keeps the sides of the wheels that are next the ground nearer to each other than the upper sides are; and this not only makes the wheels drag sideways, 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 a wheel falls into a hole or rut, or when a carriage goes on a road that has one side lower than the other.

Experience shows that narrow-wheeled carriages are more easily drawn than broad-wheeled

ones."

Extract from a Treatise on Wheels and Springs for Carriages, by DAVIES GILBERT, Esq., M.P., F.R.S., &c.

66

Taking wheels completely in the abstract, they must be considered as answering two different purposes.

"First. They transfer the friction which would take place between a sliding body and the comparatively rough uneven surface over which it slides, to the smooth oiled peripheries of the axle

and box, whence the absolute quantity of the friction, as opposing resistance, is also diminished by leverage, in the proportion of the wheel to that of

the axle.

"Secondly. They procure mechanical advantage for overcoming obstacles, in proportion to the square roots of their diameter, when the obstacles are relatively small, by increasing the time in that ratio, during which the wheel ascends, and they pass over small transverse ruts, or hollows, with an absolute advantage of not sinking, proportionate to their diameters, and with a mechanical one, as before, proportionate to the square roots of their diameters; consequently, wheels thus considered cannot be too large-in practice, however, they are limited by weight, by expense, and by convenience."

Extract from REES's Cyclopædia. Article
"Wheel."

"In concave wheels, the rims are uniformly made conical, which subjects them to many disadvantages. Every cone that is put in motion upon a plane surface will revolve round its vertex; and if force is employed to confine it to a straight line, the smaller parts of the cone will be dragged along the ground, and the friction greatly increased.

"When the fore wheels are much lower than the hind ones, many disadvantages attend this construction. A considerable force is lost that would be effectual if they were large. The carriage would go on much more easily if the fore wheels were as