68 
THE TELEGRAPH. 
the block system , by which the signal allowing a train to be 
sent cannot be given until notice has been received that the 
preceding train has reached the next post in advance. The 
signals received, which are shown by the upper or black needle, 
correspond with the despatches of trains, and the signals sent 
with the arrival of trains. 
As each intermediate office has to correspond in two 
directions, two apparatus similar to that just described are 
necessary. 
In order to be able to distinguish the direction from which 
the signals are sent, a “ tam-tam,” formed of a spiral rod of 
steel, is substituted for one of the bells (Fig. 31). 
As we have already stated, London is the only town in 
England where tubes on the Messrs. Siemens’ system have 
been used. Tubes on the systems of Messrs. Clark and Yarley 
are laid in five other towns of the United Kingdom, namely, in 
Liverpool, Manchester, Glasgow 7 , Birmingham and Dublin. 
The practical results show that, as regards speed, it is of 
advantage to use tubes of large diameter. They show also 
that, other things being equal, the speed is very different, 
according to the way in which the tubes are fitted up. 
Derangements.— Stoppage of a carrier. — Derangements may 
occur during the passage of a carrier, resulting from an 
accident to the conveying pipe, to the carrier, or to the 
apparatus at the offices. 
\ Accidents to the machinery are quickly attended to, and the 
carriers can never stop long for that cause. If the repair 
requires some time, the corresponding office can by blowing 
into the tube send back the carriers to the office from which 
tiiey came. 
When anything has gone wrong with the carriers or with 
the line, sometimes the carriers may be urged on to their 
destination by increasing to the greatest possible extent the 
pressure of the air behind them. If this fails, the place at the 
obstacle must be dug out, and it is possible to determine with great 
precision the place at which the tube must be taken up. For 
this object, the difference of the pressure of the air in the 
reservoirs is observed when they are successively put into 
communication with lines of known length, and w 7 ith the 
portion of the line out of order. The application of Mariotte’s 
