A depth of 111 feet had now been reached, and the material encountered was solid white chalk.  From this point the work proceeded with a diameter of 13.9 feet to a depth of 450 feet.  The small shaft had been sunk directly to a depth of 475 feet.  At 450 feet the diameter was diminished by three inches.  Then an advance of a foot was made, and the diameter reduced by one and a half inch.

The reason for this reduction in the diameter and change in the mode of deepening was as follows: 

The Chaudron moss-box, when it chances to reach its seat intact, and can consequently operate well, undoubtedly makes a good wedging.  But how many times does it not happen that it gets injured before reaching its destination?  Besides, as it often rests upon earth that has caved in upon its seat during the descent of the tubbing, it gets askew, and later on has to be raised on one side by means of jacks or other apparatus.  Under such circumstances, Mr. Chavatte considered this moss-box as more detrimental than useful, and not at all indispensable, and so substituted beton for it, as had previously been done by Mr. Bourg, director of the Bois-du-Luc coal mines.

[Illustration:  FIGS. 1, 2, 3, 6 AND 4.—­Details of dredge.  FIGS. 5 6 AND 6.—­Details of maneuvering lever.  FIG. 7.—­Mode of lengthening the axis of the dredge.  FIG. 8.—­Hooks for lifting the dredge bags.  FIG. 9.—­Arrangement of valves in the beton box.  FIG. 10.—­Device for centering the tubbing.

PLATE II.]

This engineer likewise suppressed the balancing column, which is often a source of trouble in the descent of the tubbing, and forced his tubbing to center itself with the shaft through a guide with four branches riveted under the false bottom that entered the small shaft (Pl. 2, Fig. 10).  Mr. Bourg so managed that there remained an empty space of ten inches to fill in with beton.  Mr. Chavatte had at first intended to proceed in the same way, but the two last tubbings, upon which he had not counted, forced him to reduce the space to 53/4 inches.  Under such circumstances it was not prudent to employ the same means for guiding the base of the tubbing, because, if the central shaft had not exactly the same center as the large one, there would have been danger of throwing the tubbing sideways and causing it to leak.  Seeing which, Mr. Chavatte strengthened the lower part of the base ring and placed it upon another ring tapering downward, and 271/2 inches in height (Pl. 1, Fig. 5).  The object of this lower ring was to force the tubbing to remain concentric with the shaft, to form a tight joint with its upper conical portion, and to form a joint upon the seat with its lower flange, so as to prevent the beton from flowing into the small shaft.