and of nitrogen, 79.10; or of 10,000 parts, there are, in perfectly dry air, of nitrogen, 7,912, oxygen, 2,080, carbonic acid, 4, carburetted hydrogen, 4, with a trace of ammonia. But air never is dry; it has always in it a varying amount of watery vapor.
The density of air being fixed at the round number 1,000, it is made the standard with which the specific gravity of other substances is compared. If water be made unity, then the specific gravity of dry air is .0012759. At 62° Fahr. it is 810 times lighter than water, and 11,000 times lighter than mercury. At the surface of the sea, the mean pressure is sufficient to balance a column of mercury 30 inches, or one of water 34 feet, in height.
Air, in music, is a piece composed of a certain number of melodious phrases, united in a regular symmetrical form, and terminating in the key in which it began.
AIR-BRAKE, a mechanical device for regulating the speed of railroad trains and for stopping them. In the course of its development, the air-brake has been known in three different forms—the straight air-brake, the automatic air-brake and the quick-action automatic air-brake, each in its turn fulfilling the requirements of its day and laying the foundation for the succeeding form.
The straight air-brake was the easiest and simplest form and was introduced by Mr. Westinghouse about the year 1869.
The automatic air-brake, introduced by Mr. Westinghouse in 1873, was designed to remedy the defects of the earlier system and to meet the advancing requirements of the time. The apparatus consisted of that already employed in the straight air-brake system, with the addition upon each vehicle of a storage reservoir, of sufficient capacity to supply the brake cylinder upon that vehicle, and a valve mechanism, called a triple valve, operated by variations in the air pressure in the train pipe, to control the operation of the brake cylinder. This triple valve was placed in the branch pipe leading from the train pipe to the brake cylinder, and was also supplied with a pipe leading to the new storage reservoir. It was called a triple valve because it performed the three functions of (1) permitting air to flow from the train pipe into the storage reservoir, for the purpose of charging the latter with air pressure; (2) permitting the compressed air to flow from the reservoir into the brake cylinder, for the purpose of applying the brakes, and (3) permitting the compressed air to flow from the brake cylinder to the atmosphere to remove the pressure from the brake cylinders and thereby release the brakes. The storage reservoir upon the locomotive became thereafter known as the main reservoir, and those upon the individual cars became known as auxiliary reservoirs. The characteristic feature of the automatic air-brake is the triple valve, under the immediate control of which are all the operations of the brakes upon individual cars.
The automatic air-brake is applied by an operation of the triple valve which results from the discharge of air from the train pipe to the atmosphere. The application of the brakes need not be confined to the manipulation of the operating valve by the engineer, but will result from any cause by which the train-pipe air pressure may become sufficiently reduced. It was this feature of the apparatus which gave it the designation automatic. Should any portion of the train become detached, or should the train pipe or hose become ruptured, a reduction of air pressure in the train pipe immediately follows, and the brakes become automatically applied upon all the cars of the train. The importance of this feature of the automatic brake is very marked. Of all the operations of the air-brake apparatus, the necessity of prompt and reliable action, when the full retarding effect of the brakes is needed, stands preeminent. Of all the various manipulations of the air pressure, that of permitting the air pressure in the train pipe to be discharged to the atmosphere is the simplest and most surely attainable. In this way the prompt response of the brake apparatus, when emergency calls for its operation, is most fully assured, and the automatic air-brake has, therefore, taken a most conspicuous place in the front rank of railroad safety appliances. No accidental disorder of the apparatus can prevent the application of the brakes in emergencies. By means of the engineer's operating valve, or of a valve called the conductor's valve, connected with the train pipe in each passenger car, or by the occurrence of any disorder which dissipates the air pressure in the train pipe, the apparatus automatically causes the train to come to a stop—in the latter case calling attention to the disorder and giving opportunity for such repair as shall again insure safety before the train proceeds.
The automatic air-brake was very generally adopted for the passenger trains of all important railroads, and fully met all the requirements of its day. When, however, in the development of railroad transportation, the necessity for the use of an automatic power brake upon freight
