240 P N E P N E ward far enough to close the lower and open the upper sluice, but not far enough to put on the air-pressure. The vacuum main is then put in connexion with the tube by a separate stop-cock. When the carrier arrives the vacuum is shut off and the lower sluice opened to allow it to drop out. This arrangement of double sluices admits of the insertion or removal of a carrier while other carriers are travelling in the same tube, and without sensible dis turbance of their motion. But great caution requires to be exercised in allowing two or more carriers to follow one another on a single section of line, especially on lines worked by pressure, since no two carriers travel at precisely the same speed. When the same tube is used alternately for sending and receiving the upper sluice is dispensed with. On some lines there are intermediate stations, and the sections are then worked by a block system like that used on railways. The carriers are cylindrical cases of gutta-percha covered with felt, which is allowed to project loosely at the back, so that the pressure makes it expand and fit the pipe closely. In front the carrier is closed by a buffer or piston composed of disks of felt of the diameter of the pipe. The despatches are held in by an elastic band at the back. An ordinary carrier weighs 2| oz., and holds about a dozen despatches. During business hours carriers are passing through the London tubes almost incessantly. With a pressure of 10 tb per square inch, or a vacuum of 7 lb, the time of transit, if through a 2j inch tube, is 1 minute for a length of nearly 1000 yards, and 5A minutes for a length of 3000 yards. The following statistics show the growth of the pneumatic despatch in the post office during ten years (the figures for 1875 are taken from a paper by Messrs Culley and Sabine, cited below, and those for 1885 have been furnished by Mr W. II. Preece) : January 1875. Januaiy 1885. Xo. of Tubes. Total Length. Xo. of Tubes. Total Length. London . . 25 4 1 3 5 3
Miles. Yards. 17 1160 1 1237 242 940 1 266 917
82 1 5 5 5 6 4 1 Miles. Yards. 33 635 2 39 1 1142 1 954 1 294 1235 460 Liverpool Glasgow Dublin . Manchester ... Birmingham.. Newcastle Total 41 21 1242 108 40 1239 1 Including 29 short house" tubes. In Paris large areas of the city have been covered by pneumatic circuits made up of iron pipes round which omnibus trains of carriers are sent at intervals of fifteen minutes. The trains consist of several carriers much heavier than the English type, linked to one another and to a leading piston. The trains are stopped at the suc cessive stations to take up and deposit despatches. The pneumatic despatch took root in Paris in 1866, and has been developed there in a way which differs greatly in mechanical details from the English system. An arrangement like that used in Paris has been followed in Vienna and in Berlin, where the Siemens system has also been used. In New York the English system is adopted, but with brass instead of lead tubes. < Interruptions occurring in the pipes can be localized by firing a pistol at one end and registering by a chronograph the interval of time between the explosion and the arrival of the air-wave reflected from the obstacle. In addition to its use for postal and telegraphic purposes the pneumatic despatch is occasionally employed for internal com munication in offices, hotels, &c. , and also in shops for the transport of money and bills between the cashier s desk and the counters. References,. The system as now used in the United Kingdom is fully described in a paper by Messrs Culley and Sabine (if in. 1 roc. Imt. Civ. Eny., vol. xliii.). The same volume contains adescrip ion of the pneumatic telegraphs of Paris and of experiments on them by M. lioiitemps, and also a discussion of the theory of pneumatic transmission by Prof. W. C. L nwin. Reference should also be made tu a paper by C. Siemens (J/i n. I roc. Just. Civ. Eng., vol. xxxiii.) de scribing the Siemens circuit system ; und to Lei Tvlt-graphes, by M. A. L. Ternant (Paris, 1881). (j. A. E.) PNEUMATICS is that department of hydrodynamics which treats of the properties of gases as distinct from liquids. Under HYDROMECHANICS will be found a general discussion of the subject as a branch of mathematical physics ; here we shall limit our attention mainly to the experimental aspect. The gaseous fluid with which we have chiefly to do is our atmosphere. Though practically invisible, it appeals in its properties to other of our senses, so that the evidences of its presence are manifold. Thus we feel it in its motion as wind, and observe the dynamical effects of this motion in the quiver of the leaf or the momentum of the frigate under weigh. It offers resistance to the passage of bodies through it, destroying their motion and trans forming their energy as is betrayed to our hearing in the whiz of the rifle bullet, to our sight in the flash of the meteor. In its general physical properties air has much in common with other gases. It is advisable therefore first to establish these general properties, and then consider the characteristic features of the several gases. Matter is conveniently studied under the two great Solid s divisions of solids and fluids. The practically obvious fluid distinction between these may be stated in dynamical c is . ti 1 - language thus : solids can sustain a longitudinal pressure without being supported by a lateral pressure ; fluids can not. Hence any region of space enclosed by a rigid boundary can be easily filled with a fluid, which then takes the form of the bounding surface at every point of it. But here we distinguish between fluids according as they are gases or liquids. The gas will always completely fill the region, however small the quantity put in. Kemove any portion and the remainder will expand so as to fill the whole space again. On the other hand it requires a de finite quantity of liquid to fill the region. Kemove any portion and a part of the space will be left unoccupied by liquid. Part of the liquid surface is then otherwise conditioned than by the form of the wall or bounding surface of the region ; and if the portion of the wall not in contact with the liquid is removed the form and quantity of the liquid are in no way affected. Hence a liquid can be kept in an open vessel ; a gas cannot so be. The mutual action between any two portions of matter Stress. is called the stress between them. This stress has two aspects, according as its effect or tendency is considered with reference to the one or the other body. Thus between the earth and moon there is a stress which is an attraction. The one aspect is the force which attracts the moon to the earth ; the other is the force which attracts the earth to the moon. According to Newton s third law of motion these are equal and opposite. Similarly the repulsive stress between the like poles of two magnets has its two aspects, which are equal but oppositely directed forces. In the case of a mass hanging by a cord, the stress is a tension at every point of the cord. At any given point this tension has two equal and opposite aspects, one of which is the weight of the mass and the portion of the cord below the given point. Finally, the stress between any body and the horizontal table on which it rests is a two-faced pressure, being downwards as regards the table, upwards as regards the body. The total pressure upon the table over the whole surface of contact is clearly the weight of the body. If the total pressure is supposed to be uniformly distributed, the measure of the pressure on unit surface is the quotient of this weight by the area of the surface. When we speak of pressure at a point, it is this pressure on unit surface that is meant. When the pressure varies from point to point over a surface, the pressure at any point is defined to be the limit of the ratio of the total pressure over any small element of surface around that point to the area of the element as the element is diminished indefinitely. The stress which exists between the contiguous portions Me: of a fluid is of the nature of a pressure. The ideal or ^j perfect fluid is a substance in which this stress between contiguous portions is always perpendicular to the common interface. In other words there is no stress tangential to the interface at any point. Hence if the contiguous portions are at relative rest, or have a relative motion parallel to the interface, neither state can be affected by the mutual stress. This condition is perfectly fulfilled in the case of any known fluid in equilibrium ; but for a fluid
in motion it is not even approximately fulfilled. For, any