MECHANICS AND USEFUL ARTS. 55 



placed at an angle of forty-five degrees, has been erected, and gives 

 direct motion through the medium of a crank to a large disk of sheet- 

 iron. The disk runs on tubular bearings, and narrows from about 

 two feet six inches in breadth at its centre to three inches at its 

 circumference, its diameter being eighteen feet. Its interior con- 

 tains simply four arms, to which the sheets of iron are fastened, and 

 which serve as fans or exhausters. Through the hollow bearings, 

 upon which the disk is made to rotate at a speed of from one hundred 

 and fifty to two hundred revolutions per minute, a communication 

 exists with a vacuum chamber below, and, by the laws of centrifugal 

 action, the latter is speedily exhausted, to a certain extent, of air. 

 The speed, in fact, of the disk determines that extent, and a water 

 barometer registers it. The air rushes out with considerable force 

 from the periphery of the disk. Between the vacuum chamber and 

 the pneumatic tube, which is two feet nine inches high, by two feet six 

 inches in breadth, and a transverse section of which resembles that of 

 the Thames tunnel, there are fitted valves with hand levers for open- 

 ing and shutting them. These may be said to comprise the whole of 

 the motive and propelling agencies of the pneumatic system. 



The tube through which the dispatch trucks are drawn is not cir- 

 cular in form, but of a section resembling that of an ordinary rail- 

 way tunnel ; the internal height being two feet nine inches, the width 

 at the springing of the arch (the top being semi-circular) two feet 

 six inches, and at the springing of the invert (for the tube has a seg- 

 mental bottom) two feet four inches. The tube is of cast iron, in nine- 

 feet lengths, each weighing about one ton, and fitted into each other 



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with an ordinary socket joint, packed with lead. Within the tube, 

 and at the lower angles on either, side, are cast raised ledges, two 

 inches wide on the top, and one inch high, answering the purpose of 

 rails for the wheels of the dispatch trucks to run upon. The latter 

 are made of a framing seven or eight feet long, inclosed in sheet iron, 

 and having four flanged wheels, twenty inches in diameter each. The 

 whole truck is so made that its external form, in cross section, con- 

 forms to that of the tube, although it does not fit it closely, an inter- 

 vening space of an inch or so being left all around. Some light India 

 rubber flanges or ringes are applied at each end of the truck, but even 

 these do not actually fit the inner surface of the tube, a slight "wind- 

 age " being left around the whole truck. There is no friction, there- 

 fore ; and, singular to say, the leakage of air does not interfere with 

 the speed of transit. This can only be accounted for by the large end 

 area which the carriages have, in comparison with the small area of 

 leakage space and the comparatively low vacuum required. The first 

 experiment made was by loading a carriage with one ton of cement 

 in bags, and entering it into the open end of the tube. Upon a given 

 signal the engineer to the company caused the starting valve to be 

 opened, the water barometer showing a column of seven inches in 

 height, and the disk running at the rate of one hundred and fifty rev- 

 olutions per minute. In fifty seconds later the carriage with its con- 

 tents found its way into the engine house, through a door at the end 

 of a tube, which it forced open, and then ran forward on rails to a 

 butt placed to stop its progress. Next two tons weight were placed 

 in one of the carriages, and its transit occupied eighty seconds, under- 



