In 1907 a sensation was created by the exhibition of a car which ran on a single rail. The inventor of this monorail car was Mr. Louis Brennan. The public was astonished at the ease with which this car maintained its balance on the rail, leaning in as it rounded a curve to keep its equilibrium. Passengers could move about at will without the slightest danger of upsetting the car; in fact, if a heavy weight was placed on one side of the car that side would rise rather than fall. The car could run with equal ease upon a cable of a crooked pipe line. The gyroscope that maintained the balance of the car consisted of a couple of small wheels which revolved in a vacuum chamber at the rate of 7,000 revolutions per minute. Once started, little power was required to keep them going. Interesting as this car was, it did not offer sufficient advantages over the present-day double rail cars and locomotives to justify its development on a commercial scale. Although witnesses of the exhibition marveled at the strange spectacle of this mechanical tight-rope walker, they did not realize that they themselves had had gyroscopic cars in their midst for years. The gyroscopic action of the wheels of a motorcycle is very marked. It is this action which is mainly responsible for holding the machine upright. The same is true of a bicycle, although the gyroscopic effect is not quite so marked, because of the lower velocity of the wheels. However, we all know that any tendency for the machine to fall to one side or the other may be corrected by a slight turn of the front wheel in that direction which at once has the effect of bringing the bicycle back to vertical position. THE AUTOMATIC AEROPLANE PILOT Still another recent development of the gyroscope is its use as an “automatic pilot” on aeroplanes. Two sensitive gyroscopes are used to stabilize the aeroplane. If a gust of wind tends to tilt the machine, the gyroscopes immediately sense the deviation and by closing electrical circuits operate the ailerons to bring the machine back to a horizontal plane. An aviator possesses a certain sense of balance which is highly developed by experience and long practice, but at its best it does not begin to compare with sense of balance possessed by the gyroscope. Not only will it keep the machine from tipping laterally, but it will also hold it on a level keel and can be used to steer the aeroplane in any desired direction, so that the human pilot may surrender the helm to the faithful mechanical pilot with perfect confidence in the ability of this animated machine to hold the aeroplane on a true course despite the vagaries of the wind. While this is theoretically possible, in practice certain difficulties are encountered which up to the present have prevented gyroscopic control of aeroplanes from being entirely successful. CHAPTER XXII HEAT VACUUMS