THE TELEGRAPH. 
56 
clerk appointed for that purpose signals to the central office by 
means of a common electric bell. On receiving this signal, 
the operator at the central office presses the stud b' ; the 
vacuum reservoir is thus put into communication with the 
cylinders l and k through the pipes m and g, the piston of the 
cylinder L opens the valve m, and the piston of the cylinder k, 
by disengaging, as we have seen, the piston-rod of the 
cylinder n, closes the valve admitting the compressed air, and 
at the same time opens the valve v. The atmospheric pres¬ 
sure is then re-established in the underground tube. 
This apparatus works with much regularity, but it is compli¬ 
cated, and its cost amounts to £60. Mr. Wilmot has simplified 
the arrangement of the valves. Apparatus of his construction is 
working on nearly all the new lines in London. The valves 
are represented in Figs. 22 and 23, where t is the pipe forming 
the prolongation of the underground tube. 
To receive a carrier the lower end of the tube is closed by 
raising the hinged clack-valve c, wffiich is covered with india- 
rubber ; then the cock v is turned, by which communication 
is established between the vacuum reservoir, the tube t, and 
pipe s ; the latter is exhausted and the clack-valve thus being 
kept close by the atmospheric pressure, the carrier is drawn 
