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Rh it up, but this should not amount to more than the natural batter of the outside shell. A few inches one way or the other is of no consequence in so large a caisson, as the pier raised upon it could easily be centred as much as that to either side, but it is just as easy and much better to keep it right from the first. During the sinking of No. 4 caisson, owing to an error by one of the surveying staff, the caisson got out of position to the extent of 10 in., but it was brought back to its true position in less than three days, though as much as that could not be done in every case.

The excavation and the rate of progress varied naturally with each caisson, and while for instance the first, sunk through a depth of 38 ft. of material, took ninety-six days to finish, the last was done in sixty-nine days, through over 30 ft. of nearly all stiff material. Table No. V., on the preceding page, gives principal dates and figures of interest in connection with these four Queensferry caissons.

The north-west caisson is the one which met with the accident we have described. Hence the time elapsed between launching and sinking Table No. VI. shows the levels of strata below high water through which these caissons were passed during sinking. As all strata were more or less on the slope, the mean height is here indicated, except with regard to extreme depth of excavation, which is, of course, level. In the case of the north-west or tilted caisson, a considerable quantity of