Another consideration is the height of the wheels. On level ground, draught is easiest when the centre of the wheel is a little lower than the point of draught, viz. the point where the traces are affixed to the collar; but this in practice would be found rather inconvenient, as very high wheels would be required, and consequently the height of the whole vehicle would have to be increased, causing great trouble and annoyance in getting in and out of the vehicle, and the driver’s seat would have to be raised to a corresponding height. Under equal circumstances a high wheel is more efficient than a low one, and requires less power to draw it; though it may be mentioned that a low wheel on a good and level road will do its work far better than a very much higher wheel on a rougher road. The sizes of the wheels of two-wheeled vehicles vary from 3 feet to 4 feet 6 inches. It would be a very good thing if four-wheeled vehicles were to have the wheels of equal size, in order that the friction and power might be equal. But with the present mode of construction this is an impossibility, as we have only one mode of making the lock or turn. Therefore the height of the fore wheels must be regulated by the height at which the body hangs, so that the wheels may pass beneath it without striking, when the springs play. In practice this height varies from 2 feet to 3 feet 8 inches, according to the kind of carriage the wheels are intended for. The hind wheels vary from 3 feet to 4 feet 8 inches. The next point is the dishing of the wheel, which is necessary for strength to take the strain off the nuts, to throw off the mud and prevent it clogging either the wheel or the body, and to give greater room for the body between the wheels without increasing the track on the ground. Whatever be the amount of dishing or coning, which varies from 1½ to 2½ inches, one rule should always be observed, viz. so to form the wheel that when running the lower spokes should maintain a true vertical position both in the fore and hind wheels. This is mainly accomplished by the dip of the axle, but if the fore and hind wheels have the same dish, they will take the same track along the ground. The dish of a wheel will be understood by referring to Fig. 10, in which it will be seen that the extremities of the spokes are not in the same plane, thus forming a dish or hollow in the surface of the wheel. [Illustration: Fig. 10.]