pallets that may be attached to the controlling key; the circular pallet in the second chamber, dd, at the same time closes, and prevents any escape of wind. When the finger is withdrawn from the key, the position of the backfall, and consequently of the circular pallets, is reversed, as shown in the figure. The supply of air from the wind-chamber is now cut off by the descent of the pallet; at the same time the second pallet in the chamber d d is raised, allowing the wind to descend through the pallet-hole, and to escape through the opening z into the atmosphere. The contents of the lever being thus exhausted, it returns to its state of rest, as shown at B, Fig. 431, the rapidity of the change being accelerated by a spring.

"In consequence of the width of the pneumatic lever-about 3 in. onlyevery fifth lever is placed in the same row; hence the pneumatic action always appears in five rows, as shown in the general section. The pneumatic action which effects such remarkable results as those already detailed is not entirely unattended with disadvantage: in many of the specimens made by the best builders, English and Continental, the working of the levers is as audible as the motion of the old rattling key movements of old organs. This arises partly from the nature of the action itself, which to be effectual must also be very energetic. Nevertheless, the defect alluded to will, no doubt, be speedily ameliorated, if not entirely removed, under the exercise of the ingenuity possessed by so many English builders."

Nearly all the experiments which have been explained serve to prove the mechanical power of the atmosphere; and the question how it could be converted into a motive power available for the conveniences of society has been

a problem of great interest to engineers. Even two centuries ago the notion was entertained of producing motion economically, for the purpose of transit, by means of the pressure of the atmosphere. The original thought may at least be traced back with certainty to the celebrated Dr. Papin; in succession long afterwards came Lewis, Vallance, Medhurst, and Pinkus, whose speculations excited in their day some attention. Towards the close of the last century Murdoch was devoting his attention to this subject. The means of propulsion he proposed to employ was watery vapour working an air-pump; his plan, however, consisted simply of an exhausted tube, through which might be propelled a hollow sphere containing letters and packages. In the year 1810 a proposal was made by Medhurst, the Danish engineer, to put letters and goods in a canal, 6 ft. high and 5 ft. wide, and containing a road of stone and iron, and project them by means of atmospheric rarefaction and condensation. In 1824 an Englishman-Mr. Vallance-made a similar suggestion. His daring plan was to connect Brighton and London by means of an enormous tube, through which, by pumping out the air, carriages were to be propelled with the velocity of a cannon-ball. Another proposal was made by Medhurst: he speedily devised means of propelling his carriage in the open air, and of making a communication between the interior of his propulsion-tube and the outside, preserving it at the same time air-tight. The opening was to be closed by a hydraulic apparatus called a water-valve.

Beautiful as Medhurst's scheme was in theory, it was at that time impracticable, and his experiments were unsuccessful: the water-valve refused to exclude the air from the tube. In this state was the contrivance when taken up by Mr. Pinkus, who suggested the rope-valve, which likewise failed to keep the tube air-tight, and was in turn abandoned.

Another inventor came forward in the person of Murdoch's pupil in the Soho factory-Samuel Clegg. The valve invented by him, in conjunction with Mr. Jacob Samuda, of the Southwark Ironworks, gave the final touch to Medhurst's proposal, and led to the construction of the Kingston and Dalkey, the Croydon, and several other atmospheric lines. These lines at last yielded to the locomotive, and ceased to exist.

Murdoch and Vallance proposed the use of a pneumatic tube for the transmission of parcels. With them the motive power and parcel-carriage were both to be in a tube. It is the same with regard to the arrangement of the Pneumatic Despatch Tube of the present day. Medhurst and Vallance proposed to use a pump; but the Despatch Company now attain the same object with an artificial blast, or wind produced by means of a revolving fan.

Mr. Latimer Clark used pneumatic tubes for several years as a means of intercommunication between the Electric Telegraph Company's offices at Lothbury and their branch stations at Cornhill, the greatest length of the tube being three-quarters of a mile. Any one wishing to send a message by telegraph-say to Edinburgh from Cornhill-the message is written down on a piece of paper, rolled up in a small gutta percha box, and placed in the tube; by the pressure of the atmosphere it is quickly blown through the tube, just like a pea out of a pea-shooter, and falls out of the end of the pipe at Lothbury; the box is opened, and the paper with the message written upon it is handed over to the operator at the telegraph instrument in connection with Edinburgh, and the message is instantly sent.

The Pneumatic Parcel Despatch tube, delineated at the head of this article, p. 433, Fig. 401, is now working most successfully between the arrival platform

of the Euston Square Station and the North-Western District Post Office in Eversholt Street; and it is better, for the sake of simplicity, first to explain the arrangements which are made for the purpose of sending carriages containing letters to and fro.

On entering the building erected for the necessary machinery at Euston Square, the engineer in charge points out the pneumatic tube, which is very much like an elliptical gas-main, 33 in. by 30 in. wide, and laid at an average depth of about 9 ft. below the road.

The pipes are made in 9 ft. lengths, with socket joints filled in with lead to keep them quite air-tight, and on the inside-at the bottom of the tube-are cast-iron rails 2 ft. apart. The car to run on these weighs nearly 8 cwt., and is about 8 ft. long, and runs upon four wheels 20 in. in diameter. We have thus, in a very few words, described almost the plant and rolling stock of a Pneumatic Despatch Railway. The car, when placed in the tube on the rails, is blown from end to end, backwards and forwards, as may be required. As we have already seen, air has weight, and this brings it under the influence of the laws of centrifugal force, which give it a tendency to fly off with more or less pressure, according to the velocity with which it is whirled round from a centre. It is this well-known law which is taken advantage of in working the pneumatic tube. At the side of the tube in the small building at Euston Station is a hollow iron wheel working in an air-tight box. This wheel is 21 ft. in diameter, with a thickness of about 2 ft. at the nave or centre-a thickness which gradually diminishes towards its outer circumference, so as to give it the same cubical contents at the rim as at the middle. This wheel is connected with a steam-engine of about 17-horse power, which turns it at a velocity of from seventy to ninety miles an hour, when the air which is drawn in through the centre is thrown off from its periphery with a force which gives a pressure of from 5 to 7 oz. on the square inch,-very nearly the pressure of a hurricane, and all of which, by opening a valve at the end of the tube, is driven through it with almost irresistible velocity. The cars when on the rails inside the tube almost fill it, and expose a surface of nearly 5 ft. square to the blast. They are driven along at the rate of nearly thirty miles an hour.

The whole apparatus is of the most simple, cheap, and effective character, and reflects great credit upon its engineer, Mr. Rammel, for the ease and certainty with which the air from the wheel sends one or more carriages, heavily laden, from one end to the other. For demonstration at the Polytechnic, a little model of wood was constructed about 20 ft. long. There were two carriages; the passengers consisted of a party of white mice, and they were blown from one end of the tube to the other by means of the blast of air from a fan-blower which was set in motion.

The company proposed to lay down a line of 48-in. tubes to form pneumatic stations in connection with the Camden Town Station of the London and North-Western Railway-a central site in Holborn-the Smithfield new market; in Gresham Street, in connection with the large carrying firms for goods and parcels; the General Post Office; Covent Garden Market; and the new terminus of the South-Eastern Railway at Charing Cross. It is stated that Messrs. Pickford alone convey 400 tons of parcels a day through London, at a cost of ninepence per ton; whereas the Pneumatic Company could do the same work quicker at a penny a ton a mile, and yet gain largely by the undertaking. Between the Pneumatic Despatch and the Underground Railway, which should amalgamate, the days ought to be fast approaching

when the ponderous goods vans that now fly between station and station shall disappear for ever from the streets of London. If this could be brought about by the Pneumatic Despatch Tube, it would be of great service to the public.

In a brilliant leader of the 9th November, 1865, "The Times" thus speaks of the pneumatic principle, which, unhappily, in these depressed engineering times seems to be in abeyance:

"Every dog has its day, and even the elements have their turns. Earth, air, fire, and water contend which shall render the greatest service to man, and enjoy the foremost place in the continual triumph of nature and art. In the single matter of locomotion, earth first was everything, and man trudged, rode, or drove. Then water had its turn, and man paddled, rowed, sculled, drifted, or, with earth's aid, punted or was pulled. Then air lent a wing, and the sail carried him across gulfs and oceans. Then fire, or rather steam, the child of fire and water, enabled him to beat the winds and currents, first on water, then on land. At this time we can hardly see by what infatuation we land lubbers allowed the stormy ocean to take the lead of terra firma in the use of steam for locomotion. We were actually laughing at the prejudices of old skippers when we had not a thought of steaming by land. But now comes a new move, whereof no man can see the end, though it begins timidly and awkwardly. Air is now the performer. It comes upon the scene with much modesty, as if knowing, itself to be suspected of wildness, treachery, and caprice. It only asks to operate in strong iron tubes, and tunnels of masonry in the solid ground. Like the ass of Scripture, which is not as our degenerate specimens, it wants the bridle, not the whip. We have only to raise the wind, a process easier in these days than when Lord Bacon put 'impressions of the air and raising of tempests' among the magnalia naturæ. The wind once raised, it will go as we direct it, but still a prisoner, and only revolving to and fro in its subterraneous labyrinth. The notion is so simple that when the thing is once done everybody will ask why it was not done before. Boys will break windows, and savages before this have pierced the tough skin, with peashooters. Indeed, everybody knows the power of wind under due control. Everybody has seen the 'lorry' with half-a-dozen men or more hurrying on under no other propulsion; and woe to a full-sized railway carriage if it be caught by a too favourable gale. But it is quite plain that the friction of an ordinary carriage on rails cannot be a greater obstacle than the resistance of the water to a laden ship, which nevertheless is soon overcome by even a moderate breeze. So this is nothing more than land sailing, with two simple differences as compared with sea and river sailing. The track and the wind must be fixed and in accord. In fact, the ship must be blown through a tube."

ACOUSTICS.

HE science which treats of the nature and laws of sound has received con

Tsiderable wenchor fan nature and laws of soythagoras determined

that notes of music varied precisely in the ratio of the length of the strings used. Two hundred years after his time, Aristotle wrote upon sound, and affirmed that the number of vibrations performed by strings or by the air in pipes is inversely as their lengths; and that sound is transmitted to the ear by similar vibrations communicated to the atmosphere. Sixteen hundred years after Christ, Galileo rediscovered what had been known to the ancients,