—The heading of the tunnel, shown on [Plate XII.], like that of the Hoosac, was driven to the full width of the tunnel. It is clear from theoretical considerations, and experience has confirmed the conclusions, that the method of taking, with machine drills, the whole width of the excavation at once conduces to rapidity of advance, and to economy of explosive. In the example under consideration, three tram lines were laid up to the face. The carriages carrying the drills were run upon the two outside lines. These carriages were simply stout frameworks of oak, each having in front three horizontal iron bars, on which the drills were clamped in a way that ensured easy lateral and vertical motion. After the firing of a blast, all hands were set to shovel the dislodged rock into the middle between the machine lines for the purpose of clearing the latter as soon as possible to make way for the machines to be brought up for the next boring. The lines being thus cleared, drilling was recommenced, and the broken rock removed in wagons upon the centre line of rails. The heading being 26 feet wide, there was ample room, and, a convenient system of switching having been adopted, no delay was occasioned by a want of wagons.

The system followed was that of centre cuts, and subsequent squaring up. It consists in first blasting out an entering wedge or “key,” about 10 feet deep in this case, in the centre, and afterwards squaring up the sides by several blasts. In the Musconetcong heading, twelve holes were first drilled, as shown in the [drawing], and marked C, A being the floor of the heading. These holes were drilled with from 11⁄2-inch to 23⁄4-inch “bits,” in two rows of six, 9 feet apart on the face, and angled to meet at the bottom. They were charged with 25 lb. of No. 1 and 50 lb. of No. 2 dynamite, and fired simultaneously by electricity. The No. 1 dynamite was used in the bottom of these centre holes; in all the subsequent blasts in squaring up, No. 2 only was used.

As soon as the cut was out, a second round of holes was started for the first squaring up, as shown in the [drawings], where they are numbered 1, 1, 1, 1, &c. In these and in the subsequent rounds, numbered 2, 2, 2, 2, &c., and 3, 3, 3, 3, &c., the resistance to be overcome is, of course, not so great as in the cut. In the first and the second squaring-up rounds, from 50 lb. to 60 lb. of dynamite was used, and, in the third round, this quantity was increased to 80 lb. or 90 lb., the resistance becoming greater as the roof arch falls at the sides. In this third round, there were generally one or two additional roof holes; these are not shown in the drawing, as their position varied, according to the lay of the rock. The top holes in the first round are also intended to bring down any roof not shaken by the cut, and these are therefore angled sharply towards the centre, and bored from 12 feet to 14 feet deep. In the plan, [Plate XII.], the number 3 indicates the cut holes, and 4, 5, and 6, the squaring-up rounds. The holes of the first squaring round were always drilled about a foot deeper than the cut holes; when blasted, these generally brought out an additional foot of shaken rock at the apex of the cut. The following table shows approximately the number and the depth of the holes required, and the quantity of dynamite used for a linear advance of 10 feet.

No. of
Holes.		Depth
of
Holes.		Total
Depth
of
Holes.		No. 1.No. 2.
ft.in.ft.lb.lb.
Cut12 106 1262550
1st square up812096..55
2nd„812096..55
3rd „612072..85
Additional roof holes2- 100 -18....
80
36..40825245

The cut holes being 10 feet 6 inches deep, the blast usually brought out about 9 feet full, which, as explained above, was increased to 10 feet in the subsequent rounds. The cross section being about 175 square feet, in an advance of 10 linear feet, there are about 65 cubic yards of rock to be broken; this gives on an average 0·4 lb. of No. 1 and 4 lb. of No. 2 dynamite, and a little over 6 feet of drilling per cubic yard.

The “bench” was kept from 150 yards to 200 yards back from the face of the heading, to avoid interruptions from the heading blasts, and to allow plenty of room for handling the wagons, and for running back the machines to a safe distance, previously to firing. The system adopted in removing the bench is shown on [Plate XII.] First, six top holes, from 12 feet to 13 feet deep, were drilled and blasted; their relative positions are shown in the drawings, A being the centre line, B, the sides in the enlargement, B′, the sides of the heading, C, the face of the bench, and 1, 2, 3, 4, 5, 6, the holes. These six holes lifted the greater portion of the rock; what was left was broken by several horizontal holes. These two sets of holes, at the top and at the bottom, gave an average advance of about 9 feet. The following table shows, for that advance, the number of feet drilled, and the quantity of dynamite burned.

No. of
Holes.		Depth
of
Holes.		Total
Depth
of
Holes.		No. 2
Dyna-
mite.
ft.ft.lb.
Top holes6127262
Bottom holes4104045
Totals1022112107

The sectional area of the bench being about 306 square feet, an advance of 9 linear feet gives about 102 cubic yards of rock to be removed. The quantity of dynamite used was therefore 1·05 lb., and the depth of boring 1·1 foot, per cubic yard of rock broken.

Three machines were used at this bench, two on the top and one below. The holes were commenced with 23⁄4-inch bits, and terminated by 11⁄2-inch bits. The rock was a tough syenite.