012 TEAXSACTIO^'S OF THE AMERICAN INSTITUTE. 



the tubes. Even a puppy or a poodle may be ticketed uuliarmed, 

 and as speedily as a satchel. The object attained, it is claimed, is 

 very high velocity with perfect safety. 



Its Practicability. 



The great practicability of the pneumatic dispatch arises from that 

 remarkable property of the atmosphere named elasticiti/, and from 

 which it derives the power of exerting pressure in every direction 

 and in all variations of temperature, altitude, or volume. The weight 

 of a column of air upon each square inch of surface, at the level of the 

 sea, is nearly fifteen pounds. Air compressed will, by virtue of its 

 elastic property, exert a force as many times its own weight as the 

 power employed to compress it, and will always expand to its original 

 volume. This property in air, as well as in all aeriform bodies, is 

 essentially different from liquids and solids, the latter losing by 

 long compression a portion or all of their elasticity. It has been 

 found, by careful experiment, that air will rush into a vacant space 

 at a velocity of about 1,400 feet per second. If air is compressed 

 into half its volunie the pressure in the vessel or tube is equal to 

 double the pressure of the external atmosphere, and, if permitted to 

 rush out, it will do so with a velocifv^ equal to that at which, under 

 ordinary pressure, it would rush into a vacuum, viz. : 1,400 feet per 

 second, diminished, of course, bj the amount of friction among the 

 particles of commingling air. There is nothing new in the principle 

 of obtaining motion by means of a partial vacuum, nor is there any- 

 thing incredible in the assertion that letters and small parcels can be 

 transmitted from one place to another by a current of air in a shorter 

 time than by any other means. 



The Operations of the Tube ix London. 



The pneumatic tube, as a means of intercommunication, was first 

 put into practical operation in London upon a small scale by the 

 Electric and International Telegrai)h Company in 1860. To avoid 

 inconvenience of repeating long messages, written papers, received 

 at Cornhill, in the stock exchange, and other subsidiar}^ stations, 

 were blown through tubes to the central station in Lothbury. The 

 company was so highly satisfied with the working of the tubes, and 

 found so much convenience from their use, that they shortly put 

 down others to moi'e distant stations. The apparatus was of the 

 simplest kind, consisting of an exhausting engine, the dispatch tubes, 



