1872.] Engineering. 393 
Clifton Tunnel.—This tunnel, which will be about one mile in length, is 
being constructed with the view of affording railway communication between 
Bristol and the new docks now in course of construction at Avonmouth. The 
tunnel will be straight, and on an incline of r in 64 throughout its length. 
The tunnel is being bored from each end, and, no water having been met with, 
the working down hill is just as easy as working up hill. A top heading is 
being constracted about ro feet wide and 8 feet high, and the widening 
out of the tunnel is being carried on at some distance behind, the face 
of the full-sized tunnel being generally about 30 yards behind the face 
of the heading. One of Captain Beaument’s diamond drilling-machines is 
being used for boring holes in the face of the heading, the rock being removed 
by blasting. The enlargement of the tunnel to the full size necessary is done 
by hand labour in the usual manner. The holes which are bored by the drills 
are about 2} inches in diameter, having a core of 14 inches in diameter, and 
from 3 feet to 3 feet 6 inches deep. The core is easily removed, and the hole 
is then left beautifully straight and circular. Some difficulty is experienced in 
getting out the central piece of the heading; but after this is done the 
remainder comes down easily. The explosive employed is dynamite, which 
is found to answer much better than powder. The material through which 
the tunnel is constructed is very hard. The air-engine which drives the drill 
works at about 130 revolutions per minute with the air at 40 lbs. The air is 
compressed by a steam-engine at the top of the shaft, whilst the exhaust-air 
from the boring machine ventilates the tunnel perfectly. 
Subaqueous Foundations.—The method of sinking cylinders for foundations 
by the use of compressed air was first suggested by Mr. John Wright, of 
Rochester, about twenty years ago, when Resident Engineer upon the 
Rochester bridge. Recently Mr. Wright has made certain modifications 
in his previous invention, in order to provide a means by which tunnels can be 
constructed on the beds of rivers or in other waters, to obviate the necessity 
of having any thickness of earth between the top of the tunnel and the 
bottom of the river. For this purpose, caissons or bells open at the bottom, 
and from which water is kept out by air under pressure, are employed, and a 
continuous foundation can be formed. The caisson having been lowered into 
position where a section of a foundation is to be constructed, the water is 
forced out in the usual manner, and the work of the foundation proceeded 
with, the section of the work being made somewhat shorter than the diameter 
of the caisson, in order that the latter may be lifted and moved a distance 
after one section of the work has been completed. Upon a continuous 
substructure thus formed, the tunnel is constructed in horizontal courses in a 
similar manner, except that instead of the sections being made solid, they are 
now formed of a sufficient width for the purpose, a bell of suitable size being 
employed. While the upper part of the first course of blocks is being built 
separately within the bell, a portion of the invert, or so much of the tunnel as 
is to be contained in the height of the lower block is formed, the hollow of the 
invert thus made being filled in with puddle to the full height of the block. 
The sides of the tunnel may also be formed in suitably shaped blocks, 
care being taken that the space contained within the sides is filled with clay as 
the work proceeds, until the tunnel is completed by arched blocks forming the 
roof; and when the joints are all made good, the puddle can be removed, 
leaving the work finished. The caisson or bell can readily be raised and 
shifted from place to place by a traveller, running upon rails supported on 
piles, the traveller being fitted with a suitable engine and travelling gear. 
inside the bell a tank or hopper is fixed, and from this a shaft rises through the 
top of the bell to above the surface of the water, this shaft being open at each 
end, and dipping a short distance below the mouth of the hopper. The shaft 
and hopper are always kept filled with water, by means of which the escape 
of air from the bell will be prevented. Within this shaft is conducted a set 
of revolving buckets, by which all earth excavated from the bottom of the 
bell and thrown into the hopper can be drawn up. 
VOL. II. (N.S.) 3 E 
