Water may be considered as 2400 times as efficient as air, volume for volume, in condensing steam. When a condenser is made for the purpose of using water as a cooling agent, a large number of small tubes may be closely grouped together in a box, and the water pumped in at one end of the box and discharged at the other end through relatively small openings; but when air is employed, the tubes or condensing surfaces must be widely distributed, so that a very large amount of air is encountered, and air which has struck one tube and become heated must never touch a second tube (see Figs. 30 and 31, also Appendix). [Illustration: Fig. 50.--Section showing the Antoinette motor, such as used in the Farman and De la Grange machines.] Fig. 51 shows a pneumatic buffer which I have designed, in which _a_, _a_, is a steel tube highly polished on the inside; _b_, a nozzle for connecting the air-pump, which is of the bicycle variety; _c_, a nipple to which is attached a strong india-rubber bulb; _d_, a piston which is made air-tight by a leather cup; and _f_, the connection to the lever carrying the wheels on which the machine runs. While the machine is at a state of rest on the ground, the piston-rod _d_, is run out to its full extent, and supports the weight of the machine--the pressure being about 150 lbs. to the square inch. When, however, the machine comes violently down to the earth, the piston is pushed inward, compressing the air, and by the time it has travelled, say, one-half the stroke, the air pressure will have mounted to 300 lbs. to the square inch. At this point, the rubber bulb _c_, ought to burst and allow the compressed air to escape under a high pressure. Air escaping through a relatively small hole absorbs the momentum of the descent and brings the machine to a state of rest without a destructive shock. It is, of course, necessary for the navigator to select a broad and level field for descent, and then to approach it from the leeward and slow up his machine as near the ground as possible, tilting the forward end upwards in order to arrest its forward motion, and touching the ground while still moving against the wind at a fairly high velocity. If all these points are studied, and well carried out, very little danger will result; then, again, the aeroplanes _b_, _b_, and the forward rudder _d_ (Fig. 41), should be so arranged that, in case of an accident, their outward sides may be instantly turned upwards, in such a manner as to prevent the machine from plunging, and keep it on an even keel while the engines are not running. [Illustration: Fig. 51.--Pneumatic buffer--_a_, _a_, cylinder; _b_, attachment for pumping up; _c_, air outlet, covered with a rubber thimble made to burst under about 300 lbs. pressure; _d_, the piston.] STEERING BY MEANS OF A GYROSCOPE.