Ahnosfiheric railways inade their appearance almost simultaneously with the steain-railways. The motive-power in this system is the pressure of the atmosphere. The idea of moving carriages by means of atmospheric pressure appears to have originated at the close of the seventeenth century with Dr. Papin of Blois, in France, who was also the first to study the properties of high-pressure steam. The earliest experiments in this field were by Medhurst and Pinkns, but the credit of practically constructing- and operating a railway on this principle is due to Clegg and the brothers Samuda, whose experimental atmospheric railway was put in operation at Wormwood Scrubbs, in the suburbs of London, England, in 1838. Later (1844) a short stretch of road on this principle was built between Kingstown and Dalky, in Ireland (fil. 36, figs. 1, 2). This was about two miles long. Following this came the atmospheric railways from London to Croydon, from Exeter to Plymouth, in England, and lastly (1847) from _Nanterre to St. Germain, in France.
The principle of construction and mode of operation of these railways will be understood from the following: Imagine a tubular chamber, sealed air-tight, with a piston in one end fitting exactly and capable of motion to and fro. If a partial vacuum be created in advance of this piston by pump ing out the air, the piston will 1110Ve forward, because the entire pressure of the atmosphere is exerted on the back of the piston, to which the pres sure in front, having been reduced by the pump, offers but slight resist ance. Imagine, also, this piston to be connected with a car running on rails over the tube by means of an arm projecting through a slot in the tube (fig. 1), the entrance of air through the slot being- prevented by providing the latter with a series of elastic valves. Under such circum stances the car will move forward with a force depending on the diameter of the piston and the amount of vacuum maintained in the tube. To open the valves covering the slot in the tube, so as to allow of the passage of the projecting arm, various mechanical artifices may bc employed, of which one is that of a roller suitably connected to the piston and moving in advance of it. After the arm has passed, the valve is closed behind it by a similar roller attached to the car, and the tube will thus be kept air tight, only that portion of it immediately at the projecting arm being open. The vacuum is created by means of an air-pmnp actuated by steam-power. Since the year 1849, however, roads of this character have been abandoned, for the reason—apart from their inconvenience—that their operation was found to be too costly. The amount of vacuum which it is necessary to maintain in a tube of the kind described will be from seven to ten pounds, and the unavoidable leakage through the slot is so considerable that the plan is found to be hupracticable.
Pneumatic Raihotly. —In contrast with the atmospheric is the pneu matic railway, in which the ear is placed, not outside, but inside, the closed tunnel-like tube, which completely surrounds it (fi/. 37, fig. 2). In this system the car itself forms the piston, and, in consequence of the large extent of surface which it exposes to the atmosphere, a comparatively small difference of pressure (one-tenth of an atmosphere) is required to set it in motion. On account of the trifling pressure needed to actuate the car, it is not necessary to be so exact with the fitting of the car-body to the walls of the tube, and small leaks may be disreg-arded. To maintain the necessary difference of pressure on both sides, a yielding brush-like packing is found to answer the purpose satisfactorily. The system may be operated equally well by vacuum or by pressure. Hence, the car may be moved to and fro in the tube by the use of an engine at one end of the line which shall give a blast and exhaust alternately.
Pneumatic idea of utilizing a tube and a plenum, or exhaust, for the conveyance of lighter articles of freight, small packages, letters, etc., has been universally put in practice. Medhurst had a very clear conception of the utility of this system of transportation, but was half a century in advance of his time. London was the scene of the first practical trial of pneumatic transit. In 1S59 a pneumatic despatch-tube was laid down to convey parcels and light goods from the Euston Square Station of the North-western Railway to the district post-office in Evers holt street 3o, fig. 25). It proved entirely successful, and the system has since been considerably extended in London and other large cities; so that it has now come into general use not only for the transmission of mail-parcelS to and from district offices, but also in telegraph bureaus and in large business establishments for the expeditious transfer of packages and communications from one department to another.
Lona'on Pneumatic Passenger pneumatic tube for the transmission of passengrers and freiglit as well as for light parcels has been introduced in London between Holborn and Euston Square. The tube is of cast iron, each section being about 9 feet longr, 4 feet wide, and 4 feet 6 inches high. The carriag-e, which runs on rails laid on longitudinal sleep ers, weighs half a ton and can carry a load of one and one-half tons. Knight states that in ordinary working twenty-four trains, a gross weight of two hundred and forty tons, have been carried over this road in four hours. The pressure required does not usually exceed one half-pound to the square inch. The carriages may be sent through either by pressure or by exhaustion. A fan 2 2 feet in diameter is employed.