78 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



equals a fall of 15 metres in height. Three iron-plated troughs are 

 for this purpose placed close to each station ; one receives the water 

 which compresses gradually as it fills up the trough the air situated 

 above and forces it into the two others. By emptying the first cistern 

 by means of a cock communicating with the outer air, then leaving it 

 to fill again with water from the pipes, the same experiment can be 

 repeated several times in succession, and the compressed air can thus be 

 interned in the two other cisterns at the necessary pressure. Three 

 minutes suffice to obtain this result, and the piston, driven along the 

 tube by the force of the compressed air, reaches its destination in 

 90 seconds, which gives a mean velocity of 12 metres a second. 



It is evident that all allowance being made for the expense of 

 pipes and apparatus, the same system could be advantageously 

 applied to the transport of letters and small parcels to every part 

 of a city like London. There certainly would follow great economy 

 of time in the expedition and distribution of such increasing and 

 brisk correspondence. 



hi the United Kingdom, transmission of messages by means of 

 pneumatic tubes is largely adopted, and in London, in connection 

 with the General Post Office, a large system has been established. 



The idea of using a pneumatic tube for message purposes emanated 

 first from Mr. Latimer Clark, the engineer of the Electric and Inter- 

 national Telegraph Company, who, in 1854, laid a tube from the 

 Central Station to the Stock Exchange, and by means of a vacuum 

 produced by a hand-puinp the carriers were drawn through. Sub- 

 sequently compressed air and steam power was used, so that carriers 

 could be made to move in either direction ; the advantages of this 

 plan were so great that it rapidly extended, and at the present time 

 most of the important provincial towns are provided with tubes, 

 whilst in London alone there are twenty-five tubes, representing a 

 length of nearly eighteen miles. 



The system adopted in this country, where speed is so essential, is 

 that of " radiation " from the Central Station. Tubes are laid direct 

 to the different branch offices, and where the traffic is great two tubes 

 are laid. 



The tubes employed are of lead, 2 inches in diameter, and 

 manufactured in as long lengths as possible about 29 feet. The 

 tubes are laid in iron pipes to protect them, and the joints are most 



