PNEUMATIC TELEGRAPH V. 
S3 
diameter to hold their soldered joints. The iron is ± inch 
thick, and the pipes have their socket joints filled in with hemp 
and covered with lead. The smallest radius usually given to 
the bends of the pipe is 8 feet. For carves the cast-iron pipes are 
made in two portions, and the upper one is fixed by means of 
bolts passing through .collars, after the leaden tubes have been 
laid. 
There is no water reservoir in the course of the tube ; the 
little moisture produced by the compressed air is absorbed 
by the felt covering of the carriers. Collecting wells, like 
those represented in Fig. 26, are, however, placed at certain 
intervals. 
The carriers (Fig. 18) are made of gutta-percha : they are 
i inch thick and 5f inches long; the external diameter of the 
Fig. 18. 
gutta-percha cylinder is If inches for tubes of 2\ inches 
diameter. The front part of the carrier is made thickest, in 
order to resist the shocks it undergoes at the end of its 
journeys. An elastic band \ inch wide surrounds the carrier 
in the direction of its length, and partly covers the open end, 
to prevent the telegrams from falling out in transit. A 
wrapping of common felt covering the whole prevents the 
gutta-percha from becoming heated and softened by direct 
friction on the interior of the tube. This covering is funnel- 
shaped at the open end, so that the felt may be forced by the 
pressure of the air against the inside of the tube, and thus 
completely close it. A number of felt discs attached to the 
front end of the carrier also aid in making it into a piston, 
and in protecting the gutta-percha from the effects of shocks. 
These carriers last a long time. After two months’ use the 
felt covering and the elastic band are renewed. The cost of 
doing this is 2d. for the felt, 2d. for the elastic, and 2d. for the 
labour. 
