PNEUMATIC DISPATCH. This name is given to a mode of sending parcels, mail bags. or telegram papers through a tube by atmospheric pressure, or by a partial vacuum. Early in the present century, Mr. Medhurst couceived the idea of some such contrivance. He proposed to air-tight tunnels, with carriages moving through them ou rails; '; and these carriages were to be propelled by compressed air from behind, or else by suc tion in virtue of 'a vacuum formed in front of them. Medhurst was laughed at by his contemporaries as a visionary; but his speculations were called to mind in later years, and led to the attempts noticed under ATMOSPIIERIC RAILWAY.
In 1861 was announced a pneumatic dispatch project, based on a reconsideration of the causes of failure iu the earlier schemes. The conveyance of passengers and of bulky goods was not here contemplated; par&ls and mail-bags were the articles held chiefly in view. To test the theory, a quarter of a mile of iron tube was experimentally laid down near Battersea, with a fair average of gradients and curves purposely given to it. The tube was about 30 in. in diameter; and it was found easy to propel a train through the tube consisting of two iron carriages of 7 cwt. each, at a rate of 30 in. an hour.
After ninny financial disconragements, a pneumatic dispatch company obtained capital in 1802, and began operations in 1S63. The experimental tube was removed to London, and laid down beneath the roadway of Seymour street, Euston square, from the Elision station of the London and North-western railway to the n.w. district post-office in Ever sholt street—a distance of one-third of a mile Mail-bags being successfuly transmittal in this way, the company commenced in 1864 the construction of a tube on a larger scale, and this has since been completed. The tube is laid down from Euston square.to St. Martin's-le-Grand, by way of Tottenham court road, Holborn, and Newgate street—a distance of 2/ miles. The tube is of large size, nearly 4+ ft. in diameter, laid down at as small a depth beneath the carriage-way of the several streets as the water mid gas pipes will permit. It is chiefly of cast-iron; but some portions on a sharp curve are of
brick. There is a large engine-house ou the s. side of Holborn, near Lincoln's Inn Fields, to supply all the power for working the whole tube in both directions. Rarefied air iu one-half of the tube draws a train of iron carriages, laden with parcels and mail bags, from Euston station to Holborn; and compressed air drives them through the other length of tube from Holborn to the general post-office—there being suction in the one case, and pressure in the other. A reverse action brings trains in the other direc tion. The necessary amount of rarefaction in the one case, and of compression in the other, was determined by experiment; but both are produced by means of a revolving fan of peculiar construction and large dimensions worked by a powerful steam-engine at the Holborn station. If this mode of transmission were to come into general use, there would be great saving of time in the delivery of letters and parcels, and a material lessen ing of the number or parcels and mail vans and carts in the overcrowded streets of the metropolis. It is to be regretted that a work so successful in a scientific and engineering point of view should still remain undeveloped in a commercial sense. The postmaster general, the railway companies, and the groat carriers, have made no practical working engagements with the pneumatic dispatch company; and this costly tube, with the engine-house in Holborn, has now remained silent and unused for several years. This of course has checked any extension of the system into other districts. We cannot resist the conviction that a better result will present itself sooner or later.
The problem of passenger conveyance within a pneumatic tube was shown to be practicable by Mr. Hammel', in an experiment tried at the crystal palace in 1864; but nothing further has been done in the matter.