112 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK n. 



speaking-trumpets, the one furnished and the other deprived of its 

 bell, the first carried the ticking of a watch placed inside it double 

 the distance of the second. Thus, the explanation is inexact, or at 

 any rate incomplete. It is probable that the strengthening of the 

 sound in speak ing- trumpets depends chiefly on the form of the 

 column of air in the interior, and that the walls themselves and the 

 reflection of their surfaces have little influence a view also confirmed 

 by another experiment of Hassenfratz, who covered the tube with 

 woollen stuff, without weakening the sound or its range. The influ- 

 ence of the bell is not explained. 



The speaking trumpets used at sea are about 2 metres in length, 

 the diameter of the bell being 30 centimetres. In England, much 

 longer ones, which carry the voice to a distance of nearly 4 kilometres, 

 are used. When an inarticulate sound only is made a good speaking- 

 trumpet may be heard at 5 or 6 kilometres distance. On ships, the 

 masters also use whistles for transmitting orders to the sailors. \\ 7 e 

 shall again meet with this acoustic instrument further on, its uses are 

 so numerous and its sounds attain such great intensity when they are 

 produced by gteam as in locomotives. 



III. MUSICAL TELEPHONE FOR TRANSMITTING MILITARY ORDERS 

 IN THE ARMY OR AT SEA. 



The idea of employing sounds as a means of military signalling is 

 doubtless very ancient. It is known that the Gauls posted at distances 

 within range of the voice sentries charged with transmitting orders or 

 communicating military news. But they had no particular system 

 which ensured the secresy of the communications as in the musical 

 telephone of M. Sudre, which we will explain. 



As early as the year 1817 this physicist entertained the idea of sub- 

 stituting musical sounds for ordinary language by diversely combining 

 a certain number of musical notes, and ten years later, he proposed 

 the adoption of his system for the transmission of orders in the army. 

 Instead of using the seven notes of the gamut, he confined himself to 

 the five notes C, G, C, E, and G, the sounds given by the regulation 

 trumpet. Some experiments were made in 1829 at the Champ de Mars, 

 in 1841 in the Mediterranean fleet, and in 1850 from the Champ de 



