THE TELEGRAPH. 
The agents capable of being used in telegraphy are four in 
number :— 
1. Motion of translation. (The pneumatic telegraph is the 
most perfect type of this kind of transmission.) 
2. Sound. (The acoustic telegraph aided by electricity has 
produced the telephone.) 
3. Light. (Optical telegraphs, applied more particularly to 
armies in the field and to shipping, offer an advantageous 
method of communicating in cases where the electric telegraph 
cannot be used.) 
4. Electricity, which in the electrical telegraph supplies the 
quickest and most complete method of rapidly transmitting 
despatches to very great distances. 
Motion of translation as effected by railways, even if we 
suppose the speed to be the highest practicable, does not 
attain a velocity sufficient to render this means of transporting 
written correspondence comparable to telegraphy. 
Pneumatic tubes have largely aided telegraphy for short 
distances in large downs, and the rapid motion that can be 
imparted by pressure to a current of air enclosed in an under¬ 
ground tube far surpasses the speed of a railway train. 
There is now reason to believe that this quick method of 
transmission is capable of being extended to great distances, 
and we shall no doubt before long have pneumatic tubes with 
relays, capable of passing despatches over considerable distances 
without interruption. The despatch of letters by post might 
thus be accelerated to a very great extent. 
Sound obviously presents greater capabilities of speed than 
any motion of translation. As everyone knows, it passes through 
the air at the rate of 1,100 feet per second, while through water 
and solid bodies its velocity is still greater. 
But although sound was used by the ancients in tele¬ 
graphy, it was seldom otherwise than as a simple conventional 
signal, and in this way its range never exceeded the distance of 
1,000 yards. It has been reserved for our own age to produce 
the telephone, which transmits the sounds of articulate speech 
to very great distances. There is indeed reason to expect that 
it will not be long before the telephone will have been so per¬ 
fected as to carry the sounds of articulate speech to distances as 
great as those passed over by the electric telegraph. 
