the earth that was being traversed. Water put into the shaft helped
somewhat, but the great stress to be exerted had to be effected by
means of powerful jack screws. These were placed directly upon the
tubbing, and bore against strong beams whose extremities were inserted
into the masonry.

As a usual thing it is not easy to use more than four or six such
jacks, since the number of beams that can be employed is limited,
owing to the danger of obstructing the mouth of the shaft. Yet twelve
were used by Mr. Chavatte, and this number might have been doubled had
it been necessary. As we have seen, the frame, K K (Pl. 1, Fig. 3),
was provided with an oak circle traversed by 32 bolts. The length of
these latter was two inches and a quarter longer than they needed to
have been, or they were provided with wooden collars of that
thickness. Later on, these collars were replaced with iron bars that
held the wood against which the jacks bore in order to press the
tubbing downward (Pl. 1, Figs. 10, 11, 12, and 13).

Mr. Chavatte's great anxiety was to know whether he should succeed in
causing the first section of tubbing to traverse the four feet of
gravel; for in case it did not pass, he would be obliged to employ a
second section of smaller diameter, thus increasing the expense. He
was persuaded that the coarse gravel remaining in the side of the
shaft would greatly retard the descent of the tubbing. So he had
decided to remove such obstructions at the proper moment through
divers or a diving bell. Then an idea occurred to him that dispensed
with all that trouble, and allowed him to continue with the first
section. This was to place upon the dredge two claw-bars, T (Pl. 2,
Fig. 3), which effected the operation of widening with wonderful ease.
To do this it was only necessary to turn up the bags, and revolve the