on three-legged stools or stands for vertical or steep-inclined drilling, and on timber frames for horizontal or slightly-inclined working. The arrangement through which they were made to discharge the compressed air by which they were driven into the general atmosphere, did great service in keeping the air pure just in the places where most of the men congregated, and when in good working order, they drilled a large number of holes in the course of a day.
_Figs._54_to_59,_Page_31.png)
MODE OF FIXING UNDER BEDPLATES ON PIERS.
In these two caissons the tendency to go down hill manifested itself strongly, and powerful timber struts and hardwood wedges covered with steel plates had to be employed to keep the caisson in its position, or to force it back when it had moved. These were set on prepared faces on the rock, as shown in the sketch, Fig. 52, and their action upon the caisson will be readily understood.
The actual blasting of the rock under the cutting edge does not appear to have injured the steel edge in any way; but large chips of rock lodging between the outside face and the iron plating caused some indentations, which were not, however, of any consequence.
On several occasions photographs were taken in the air-chamber, some of them requiring an exposure of fifteen to twenty minutes; but they were not very successful, owing to the changes in the atmosphere and the uncertain light of the arc lamps. Whenever the air pressure increased to a slight extent the atmosphere became quite clear and transparent; then the air would rush out at some point under the caisson edge with a noise like distant thunder, and a great wave of cold water came rushing back. This caused a dense white fog to suddenly rise in the air-chamber, which obscured everything for a few moments and then gradually disappeared again.
Through the gaps left in the heaps of sandbags a number of strange visitors used to make their appearance, attracted, no doubt by the glare of the lighted chamber, which at night could be distinctly seen from above—such as salmon, dogfish, octopus, many other fish, crabs, and a large number of lobsters. One of the latter a—large specimen—got very excited in the chase after him, and leaped up nearly the full height of the chamber in his frantic endeavours to escape—finally jumping into an empty skip, whence he was promptly transferred to the boiling-pot.
With regard to the working hours of the men employed after the caisson had once settled down on the rock, work was carried on day and night, the only stoppage being from 6 P.M. on Saturday till midnight on Sunday; but the air compressors had to be kept going all the time, and the full pressure of air maintained. Watchmen were also kept constantly on duty in the chamber when work was not carried on. At first the men worked in eight-hour shifts, with eight hours off, later on six-hour shifts with six hours off, and finally, in the higher pressures, with four-hour shifts with eight hours off.
The total excavation for the four piers on Inchgarvie, and the four piers on Queensferry, and the four piers on Fife, are given in a tabular statement further down, together with other quantities.
The total of the wages paid by the sub-contractors to their employés, including managers', engineers', and time-keepers' salaries, amounted to 1l. 15s. per cubic yard of rock excavated in the Inchgarvie south-east caisson, and to 21. per cubic yard in the Inchgarvie south-west pier.
Neither of the Inchgarvie caissons was carried down to the full depth contemplated (see Fig. 53), as it was found that only a very small area remained to be filled up when the caissons had got to the depth where they were left. These places were carefully levelled and stepped, and iron plates were fitted in to close the gap between the caisson edge and the rock, and the spaces were then carefully built up with concrete bags and the whole grouted with cement at slack water. Upon this foundation concreting was commenced, and the whole chamber gradually filled with concrete in the manner described for the Queensferry caissons.
In Table No. VII. are a few data of interest in connection with these two caissons.
| South East Caisson. | South-West Caisson. | |
| Launched | March 30, 1885 | May 29, 1885 |
| Towed to its berth, Inchgarvie | May 15, 1885 | July 16, 1885 |
| Caisson and shield ready for lowering | February 29, 1884 | September 10, 1884 |
| Touched ground at low water | May 25, 1885 | July 29, 1885 |
| First descent to chamber | May 26, 1885 | August 5, 1885 |
| Commencement of sinking | May 27, 1885 | August 6, 1885 |
| Level below high water at commencement | 51 ft. | 64 ft. |
| In final position | August 15, 1885 | October 1, 1885 |
| Level of same below high water | 63 ft. 9 in. | 72 ft. 1 in. |
| Commenced concreting chamber | August 19, 1885 | October 2, 1885 |
| Finished concreting chamber | September 1, 1885 | October 10, 1885 |
| Caisson filled to granite level | September 15, 1885 | October 30, 1885 |
| First granite laid | September 19, 1885 | October 30, 1885 |
| Pier completed | November 20, 1885 | February 22, 1886 |
The Circular Granite Piers.
The foundations below low-water level of ten out of the twelve circular piers, vary both in size and considerably in depth, but above that level they are exactly alike. The two exceptions are the two north piers on Fife already described. Their foundations start at 7 ft. below high water, and they are only 45 ft. in diameter under the necking course. In all other respects they do not differ from the remaining ten piers. In all these the granite masonry starts at low-water level, or 18 ft. below high-water level, with a diameter of 55 ft.; rises with a regular straight batter of 1 in 101⁄2 to a height of 12 ft. 8 in. above high water, where the diameter is 49 ft., and terminates in a necking and a coping course with a somewhat rounded top at exactly 18 ft. above high water. The courses of granite, of which there are nineteen, are rock-faced, while necking and coping are of dressed granite, and they vary in thickness from 21 in. in the lower to 16 in. in the upper courses, while above those the necking is 19 in. and the coping about 3 ft. in. thickness.
