52 
THE TELEGRAPH. 
connected with the vacuum. The atmospheric pressure then 
moves the carrier forward. The operations for moving it in 
the other direction are just the same. The opening of the 
stop-cock is asked for by a whistle at the end of a tube of J-inch 
diameter, which extends from one office to the other, each being 
provided with such a tube. 
Clark’s system as modified by Varley was applied in sixteen 
directions, but one of these is not in use. Seven of the tubes 
are provided with valves invented by Mr. Yarley, the others 
have valves of a similar arrangement made by Mr. Wilmot. 
Except this difference, the arrangement is the same in both 
cases; the tubes and carriers are identical, and the same engines 
are used for all the lines. 
A single tube is laid between the central station and each of 
the offices connected with it (unless the traffic be so great as to 
require the addition of a second apparatus similar to the first). 
The carriers containing the telegrams that are to be delivered 
are forwarded from the central to the distant office by means of 
exhaustion effected at the latter. The transmitting and receiv¬ 
ing valves are situated at the central office only. The tubes 
are made of lead ; they are 1^ inches in diameter in some lines, 
and 2 \ inches in others. Their thickness is \ inch in the first 
case, and | inch in the second. They are in 6-yard lengths. 
The joints by which the lengths are connected together have to 
be made with great care, in order that there may be no un¬ 
evenness in the inside. For this purpose a steel mandrel, 
previously warmed, is fitted into the ends of the two tubes. 
The mandrel has exactly the diameter of the opening, and when 
the joint has been soldered round it, is drawn out by means of 
an attached chain. 
Before the tubes are joined together, they are brought to the 
required diameter by causing a steel mandrel with a rounded 
front edge to traverse them. This mandrel is fastened to a 
chain wound round a barrel. Previous to this operation the 
diameter of the tubes is slightly less than that of the mandrel. 
When they have thus been adjusted, they are carried to the 
places where they are to be fixed, protected by wooden cases 
lest they should lose their shape. 
The tubes are buried in the ground to the depth of about 
2 feet, and are protected by cast-iron covering pipes of sufficient 
