MECHANICS AND USEFUL ARTS. 27 



own invention, by using three floats, he had rendered this retro- 

 grade action impossible, as all the air drawn in can pass in no 

 other than a forward direction, where it is required for use. 



THE PNEUMATIC DESPATCH IN PARIS. 



The tube connects the telegraph stations at the Bourse and the 

 Grand Hotel, and is the first instalment of a complete system 

 throughout Paris. The method adopted is the reverse of our 

 own, namely, the elasticity oi' compressed air in place of a partial 

 vacuum, so that neither an air-pump nor a steam engine is re- 

 quired. The power used is water from the reservoirs of the city 

 of Paris, which give an ascension of rather more than 50 feet. 

 There are three vessels, made of iron plate, and measuring each 

 about 1,200 gallons ; the first of these receives the water and 

 effects the compression, the two others are the receivers of the 

 compressed air. As the water arrives, the air within the first ves- 

 sel is of course forced into the other two, which are connected 

 with it by a valve opening inward. When the first vessel is filled 

 with water, another cock is opened, the liquid is allowed to run 

 off and the air to enter by means of a valve provided for the pur- 

 pose ; the operation is then repeated, and the effect is the produc- 

 tion in the two condensers of a pressure equal to about two at- 

 mospheres. The tube that connects the two stations is of cast 

 iron, about 3,500 feet long, and 2k inches in diameter, having 

 its termini in two chambers with tightly-fitting doors, which allow 

 the piston despatch-box to be placed or withdrawn from the tube 

 without difficulty. This carrier is a small brass cylinder, 4 or 

 5 inches long, closed at one end, and with a movable cover at 

 the other. It will contain about 40 despatches in envelopes. Five 

 minutes are found to be sufficient in practice for the piston to 

 make the double journey. The time occupied in the passage of 

 the despatch-box in one direction is sufficient to produce the, 

 necessary pressure for the return. The Engineer. 



DAM ACROSS THE ST. LAWHENCE. 



Damming the St. Lawrence is the topic of the day with the 

 citizens of Montreal. Monstrous as the undertaking seems, engi- 

 neers have laid it out, and capitalists are about to apply to parlia- 

 ment for a charter incorporating a capital of 2,000,000 of dol- 

 lars for the purpose. The water-power to be obtained by a suc- 

 cessful accomplishment of this work would be greater tlian any 

 other in the world, and could not fail to build up a mighty manu- 

 facturing metropolis around the present nucleus called Montreal. 

 At the same time, the city would acquire, what it must soon have 

 by some means, a head of water and a pumping power adequate 

 to its own supply. 



The arrangements of nature to facilitate the gigantic work are 

 quite interesting. The Lachine Rapids, just above the city, are 

 said to afford a fall of 25 feet in about a mile. They are divided 

 longitudinally by a series of islands running their entire length, 

 and forming with the northern bank of the river a natural enclos 



