strain upon the bearings. The power, too, is deflected at right angles, which is another source of loss.
In order to permit of the free vertical movement of the wheels under the springs, two universal or 'Cardan' joints, c, c, are fitted within the length of the shaft d (see fig. 8). These consist of two jaws set at right angles, with their ends connected to and rocking upon the extremities of a right-angled connecting piece. This allows movement in all directions and the shaft accommodates itself to the conditions of the drive. This method is chiefly used because of convenience and some neatness.
Another plan, employed however only by one or two firms, substitutes for the bevel gearing what is known as skew or screw gearing, a very smooth and silent drive without the spreading or bursting action of the bevels, the end of the driving shaft being fitted with a screw which drives a series of teeth cut diagonally around the circumference of the driven wheel.
I have said above that the forward end of the arbor shaft is connected to the engine shaft by a clutch, and this brings me to another almost universal and very important portion of the transmission gear. In belt-driven cars a clutch is rarely used, the slip of the belt being relied on to give the necessary immunity from shock, but in cars which are driven by chains, arbor shaft, or gearing, a clutch is a necessity, otherwise the sudden application of power would strip the teeth of the gear, break the chain, or cause other damage, and something is needed to ease the shock. Clutches may be 'positive' or frictional, but friction clutches only are referred to above. These commonly take the form of a truncated cone or inclined surface so arranged upon that portion of the transmission which carries the gearing or other connection with the wheels that, by sliding it slightly forward, it enters a socket having an internally coned surface into which it exactly fits. Sometimes one of the surfaces is covered with leather, but otherwise both are metallic, and a strong spring is usually
