MECHANICS AND USEFUL ARTS. 43 



THE PNEUMATIC DISPATCH. 



The principle of forcing packages, etc., through a tube or conduit 

 by means of atmospheric pressure, is about to be applied practically 

 and upon a large scale in London (see Annual of Scientific Discov- 

 ery, 1861 and 1862) for conveying or pumping the mail from Easton 

 Square (a great railroad depot) to a post-office station some miles 

 distant. The tubes to be used for this purpose, and which are to be 

 laid underground, are about three feet in diameter, and of the form 

 of a horseshoe. At the bottom of the two sides of the tube is a 

 slight projection, which docs duty as a line of rails, on which the 

 carriages roll along. The latter have a board behind and in front, 

 which fits into the tube, but by no means in an air-tight manner. It 

 is said that this is not in the least necessary, and that it was a great 

 mistake on the part of former schemers in atmospheric railways to 

 encumber their tubes too much with wadding and bolstering, which 

 led to a greater loss by friction than was gained in power. To allow 

 for the inequalities of the tubes, there is more than half an inch 

 space between the outer shell of the boxes and the inside of the iron 

 pipe, and it is found that even with this margin they travel at the 

 rate of forty miles an hour. The tube is exhausted by an apparatus 

 called a " centrifugal disc," consisting of a hollow wheel, twenty-one 

 feet in diameter and but a few inches in thickness, which in its centre 

 literally sucks up the air and discharges it at the outer edge. This 

 is effected by a division of the disc into a number of small chambers, 

 which act like so many fans in gathering and emitting the air. It is 

 a very simple and beautiful contrivance, which is found to work ad- 

 mirably, much more efficient and very considerably cheaper in its 

 action than an air-pump. A small steam-engine puts this disc in 

 movement to the time of from two hundred to three hundred revolu- 

 tions a minute. It requires but a short time to exhaust the air of a 

 tube several miles long to a sufficient extent to propel a whole train 

 of letters and parcels. Judging from the success of the experiments 

 already made, the scheme bids fair to be realized before long on a 

 grand scale, and produce a revolution in the dispatch of letters as 

 great at least as that of the introduction of the penny-postage. 

 There seems no reason, indeed, why our letters should not be carried 

 to us in pipes underground as well as our gas and our water. 



THE MONT CENIS TUNNEL. 



As the work on this great tunnel is now advancing, day by day, 

 with such a regularity and success as to render it one of the most 

 wonderful of engineering feats in the world's record, a brief review 

 of the history and progress of the enterprise will not be found unin- 

 teresting. 



The tunnel, it may be premised, is in the course of construction 

 under the auspices of the Sardinian government, and is intended to 

 subserve the purpose of railway communication between Piedmont 

 and Savoy. It passes beneath what is known as the Frejus Ridge, in 

 the vicinity of Mont Cenis ; has an average depth of about a mile 

 below the surface, and a length of about eight miles. As shafts a mile 



