I use a “Bischoff’s” gas engine, shown to the right hand in Fig. 3. It consists of a cylinder, surmounted by a hollow column serving as a guide to the piston. From the extremity of this column there is a very long connecting rod which acts upon the crank of the shaft of the fly-wheel and driving-pulley. When the piston ascends it draws in during four-tenths of its course a mixture of air and gas from the pipes. A little before the piston has ascended half-way a gas jet ignites the mixture. There is a slight explosion without the least effect on the sides of the cylinder, but sufficiently strong to push the piston and cause it to complete its course. In consequence of this movement the piston is raised and acts on the connecting rod, forcing it upwards and half turning the fly-wheel. On account of the speed acquired, and of the impulse given, the fly-wheel continues its course by means of the crank and connecting rod, and sends the piston to the starting-point again. Then it ascends again, taking in a fresh charge of air and gas, motion being imparted anew to the driving shaft by every revolution of the engine. An engine of 2-man power, with an altitude from the ground to the top of the column of 4 feet, and with a fly-wheel measuring 2 feet 6 inches in diameter, is amply sufficient for every purpose. It works evenly and without noise; it is absolutely free from danger, and it is placed instantly in or out of action by merely igniting or extinguishing a single gas jet. But it is remarkable that no one seems to have suggested--even before the gas-engine era--that a “fly-wheel” with a pulley would obviate the old difficulty of the unsatisfactory rotation of the Plate machine. Going into a hair-cutter’s shop I found the apparatus, figured to the left of Fig. 4, standing in the shop to work the customary “hair brushing machinery” instead of its being, as is usual, fixed upon an upper floor. I obtained one, and fitted it in the manner shown, to a Carrè’s machine. The fly-wheel is so heavy that it can be rotated by the operator without the aid of an assistant, by his giving the handle an occasional “swing round.” Its momentum then keeps it revolving a sufficient number of times before the next “swing” is required, to enable him to leave it and attend to his patient. The manner in which the band from it is attached to a small wheel in rear of the Static machine is also shown in the Figure. [Illustration: FIG. 4. Fly-Wheel and Static Machine.]