Leupold has applied this mechanism to move the pistons of pumps.[5] Upon the vertical axis of a horizontal hydraulic wheel is fixed another horizontal wheel, which is furnished with seven teeth in the manner of a crown wheel (263.). These teeth are shaped like inclined planes, the intervals between them being equal to the length of the planes. Projecting arms attached to the piston rods rest upon the crown of this wheel; and, as it revolves, the inclined surfaces of the teeth, being forced under the arm, raise the rod upon the principle of the wedge. To diminish the obstruction arising from friction, the projecting arms of the piston rods are provided with rollers, which run upon the teeth of the wheel. In one revolution of the wheel each piston makes as many ascents and descents as there are teeth. [5] Theatrum Machinarum, tom. ii. pl. 36. fig. 3. (317.) Wheel-work furnishes numerous examples of continued circular motion round one axis, producing continued circular motion round another. If the axles be in parallel directions, and not too distant, rotation may be transmitted from one to the other by two spur-wheels (263.); and the relative velocities may be determined by giving a corresponding proportion to the diameter of the wheels. If a rotary motion is to be communicated from one axis to another parallel to it, and at any considerable distance, it cannot in practice be accomplished by wheels alone, for their diameters would be too large. In this case a strap or chain is carried round the circumferences of both wheels. If they are intended to turn in the same direction, the strap is arranged as in _fig. 100._; but if in contrary directions it is crossed, as in _fig. 101._ In this case, as with toothed wheels, the relative velocities are determined by the proportion of the diameters of the wheels. If the axles be distant and not parallel, the cord, by which the motion is transmitted, must be passed over grooved wheels, or fixed pulleys, properly placed between the two axles. It may happen that the strain upon the wheel, to which the motion is to be transmitted, is too great to allow of a strap or cord being used. In this case, a shaft extending from the one axis to another, and carrying two bevelled wheels (263.), will accomplish the object. One of these bevelled wheels is placed upon the shaft near to, and in connection with, the wheel from which the motion is to be taken, and the other at a part of it near to, and in connection with, that wheel to which the motion is to be conveyed, _fig. 166._ The methods of transmitting rotation from one axis to another perpendicular to it, by crown and by bevelled wheels, have been explained in (263.). The endless screw (299.) is a machine by which a rotatory motion round one axis may communicate a rotatory motion round another perpendicular to it. The power revolves round an axis coinciding with the length of the screw, and the axis of the wheel driven by the screw is at right angles to this.