Compressed Air Work And Diving 1909

44 COMPRESSED AIR WORK. order to keep the water out from Libove the roof of the working chamber, and so give the necessary floating power. It is important that a caisson should be pitched level; if this is done in the first instance, there will be no trouble afterwards with it getting out of position or level. When a caisson has to be starteel below low water level, careful soundings must be taken, and if the site is not level, the high ground must be dredged away or the low ground levelled up. The King Edward Bridge caissons were stink in deep water by being gradually fillecl up with concrete, and as the cutting' eclo-e touched the bottom at each low water, it was observeel which part of the cutting- eclo-e was sinking deepest into the mud, then, as the rising ticle lifteel the caisson, ballast was shovelled in at the soft pkices, so that eventually when the caisson ceasecl to rise with the ticle it was practically level. If the diameter of the caisson is small in proportion to the height, it is best to gauge the level by means of hanging plumb bobs, but if the diameter is large in proportion to the height, this should be done by levelling with the ordinary level and staff. The author has found it a good plan with rectanøular caissons, when they are surrounded by staging, to have wooden blocks at each corner with coach screws screwed into them. By placing a few washers underneath each and then adding to them or taking one or two away, the four heacls can be easily brought to the same level. The levels can then be taken with reference to a horizontal joint on the caisson by levelling out to the skin of the caisson by means of an ordinary carpenters level about 2 ft. 6 in. long. This enables the levels to be taken at any moment by a foreman, and also saves the expense of having a chainman constantly in attendance. These