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THREE:The successful application of the pneumatic system at the Mont Cenis Tunnel in Italy, and at the Hoosac Tunnel in America, has demonstrated the value of the system where the air not only served to transmit power to operate the machinery but to ventilate the mines at the same time. Air brakes for railway trains are another example illustrating the advantages of pneumatic transmission; the force being multiplied at the points where it is applied, so that the connecting pipes are reduced to a small size, the velocity of the air making up for a great force that formerly had to be communicated through rods, chains, or shafts. The principal object attained by the use of air to operate railway brakes is, however, to maintain a connection throughout a train by means of flexible pipes that accommodate themselves to the varying distance between the carriages. Presuming that the flow of air in pipes is not materially impeded by friction or angles, and that there will be no difficulty in maintaining lubrication for pistons or other inaccessible parts of machinery when driven by air, there seems to be many reasons in favour of its use as a means of distributing power in manufacturing districts. The diminished cost of motive power when [56] it is generated on a large scale, and the expense and danger of maintaining an independent steam power for each separate establishment where power is employed, especially in cities, are strong reasons in favour of generating and distributing power by compressed air, through pipes, as gas and water are now supplied.This assumed, the scheme is complete, so far as the relative movement of the hammer-drop and the valve, but there must be some plan of giving motion to this added mechanism. In many examples there may be seen parts of machinery which continue in motion after the force which propels them has ceased to act; cannon balls are thrown for miles, the impelling force acting for a few feet only; a weaver's shuttle performs nearly its whole flight after the driver has stopped. In the present case, it is therefore evident that an independent or subsequent movement of the valves may be obtained by the momentum of some part set in motion during the descent of the hammer-head.

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