ACOUSTIC TELEGRAPHY, 
27 
watcher below could easily see the isards without alarming 
them, and he described their movements to his comrade so as 
to indicate when the prey was within reach. It seemed that 
these mountaineers had made use of the Thread Telegraph 
ever since they had seen it in operation at Tarbes, when the 
idea of applying it to their pursuit occurred to them. 
Is it possible that the String Telegraph suggested the idea of 
the electro-magnetic telephone ? This is not certain ; but it 
was only in 1854 that M. Bourseul conceived the idea that 
speech might be transmitted electrically. But before the tele¬ 
phone was thought of, we remember that long ago those who 
were engaged in the construction and maintenance of the tele¬ 
graph lines were able to correspond with each other at a 
distance by means of the posts erected along the road. By 
striking one of these posts with a stone, and timing the blows 
so as to reproduce the Morse signals, we were often able to 
transmit orders to distances of several miles. 
This is a circumstance well known to telegraph superinten¬ 
dents ; and as the posts are generally accessible, our readers can 
easily avail themselves of this method of correspondence. The 
sonorous vibrations are propagated with a velocity of 5,544 
yards per second through the iron wire used for telegraph 
lines, whilst the vibrations attain in air the velocity of only 
350 yards per second. 
The speaking-tubes that are used in nearly all large esta¬ 
blishments are, properly speaking, telephones. They are 
generally made of metal, and when they are not too long, the 
voice is carried from one end to the other, with all its tones 
and inflexions. The known voices of different persons can 
always be recognised. 
The diameter of the pipes is generally f inch for ordinary 
lengths ; but as the length becomes greater, or as the bends 
are more numerous, the voice is less distinctly heard. Beside, 
by the settlement of buildings, the tubes become displaced or 
broken, and often after a certain period they fail to make the 
voice heard. 
By increasing the diameter of the tubes as they are longer, 
the communication may be made at a greater distance, but the 
question of cost has then to be considered. 
During the siege of Paris, M. Casanova arranged a speaking- 
