[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. After the shaft was pumped out, digging by hand was begun with a diameter of 12 feet. After descending 20 inches an 8x10 inch curb was laid, in order to consolidate the earth and prevent any movement of the tubbing. Then the excavating was continued to a depth of 311/2 inches, and with a diameter of 93/4 feet. At this point another curb was put in for consolidating the earth. Finally, the bottom was widened out as shown in Fig. 7, so that three basal wedged curbs could be put in. This done, the false tubbing was put in place; and finally, when proceeding upward, the last ring composed of twelve pieces was reached, the earth was excavated and at once replaced with a collar composed of twelve pieces of oak tightened up by oak wedges. Each of these pieces was cemented separately and in measure as they were assembled. Through motive of economy no masonry was placed under the base of the three wedged curbs. In fact, by replacing this with a wedged curb of wood traversed by six bolts designed to fix the cast iron curb immediately above, Mr. Chavatte obtained a third curb that he would have had to have made of cast iron. * * * * * ON THE ELEMENTARY PRINCIPLES OF THE GAS-ENGINE.[1] [Footnote 1: A paper read before the Gas Institute, Manchester, June, 1885.] By DENNY LANE, of Cork.