pneumatic machinery, aside from results due to the elasticity of air, is analogous in operation to hydraulic machinery.

water may be considered as a rigid medium for transmitting power, corresponding to shafts and gear wheels; air as a flexible or yielding one, corresponding to belts. there is at this time but a limited use of pneumatic apparatus for transmitting power, but its application is rapidly extending, especially in transporting material by means of air currents, and in conveying power to mining machinery.

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.

air seems to be the most natural and available medium for transmitting and distributing power upon any general system like water or gas, and there is every probability of such a system existing at some future time. the power given out by the expansion of air is not equal to the power consumed in compressing it, but the loss is but insignificant compared with the advantages that may be gained in other ways. there is no subject more interesting, and perhaps few more important for an engineering student to study at this time, than the transmission of power and the transport of material by pneumatic apparatus.

in considering pneumatic machinery there are the following points to which attention is directed:—

1. the value of pneumatic apparatus in reaching places where steam furnaces cannot be employed.

2. the use that may be made of air after it has been applied as a motive agent.

3. the saving from condensation, to which steam is exposed, avoidance of heat, and the consequent contraction and expansion of long conducting pipes.

4. the loss of power by friction and angles in conducting air through pipes.

5. the lubrication of surfaces working under air pressure, such as the pistons and valves of engines.

6. the diminished cost of generating power on a large scale, compared with a number of separate steam engines distributed over manufacturing districts.

7. the effect of pneumatic machinery in reducing insurance rates and danger of fire.

8. the expense of the appliances of distribution and their maintenance.

in passing thus rapidly over so important a subject, and one that admits of so extended a consideration as machinery of transmission, the reader can see that the purpose has been to touch only upon such points as will lead to thought and investigation, and especially to meet such queries as are most likely to [57] arise in the mind of a learner. in arranging and erecting machinery of transmission, obviously the first problem must be, what kind of machinery should be employed, and what are the conditions which should determine the selection and arrangement? what has been written has, so far as possible, been directed to suggesting proper means of solving these questions.

(1.) in what respect are air and water like belts and gearing, as means to transmit power?—(2.) what are some of the principal advantages gained by employing air to operate railway brakes?—(3.) name some of the advantages of centralising motive power.—(4.) are the conditions of working an engine the same whether air or steam is employed?