Having descended only three-quarters of an inch before ? —Only three-quarters of an inch; it just pressed it a little into the ground with the dead pressure of 30 tons. We then removed the cap, and put on the top of' the pile 100 tons of rails ; but we could get no depression, except some three-quarters of an inch, which was done by the little com pression that you would have from the weight of the edge of the cylinder on the gravel. That is the general mode of sink ing these cast-iron cylinders. But as it will be obvious to the Committee, in the event of our meeting with, say, the trunk of an old tree, or a very large stone, we could not proceed any further, and we have had to devise many means of get ting over any difficulty of that kind. In the case of the bridge at the Nene, we have had to go through not only a layer or two of gravel, but through 2 feet 6 inches of solid rock, and that rock not lying in a horizontal position, has offered difficulties which, under other circumstances, be very expensive to overcome. To enable us to get through any unforeseen matter, it is necessary to get into the cylinder and excavate any material that may be within it, and cut through the obstruction ; and, to do that, we have devised a means by which we convert the cylinder virtually into a diving-bell ; that is to say, we fix a cap on the top of the cylinder, and the air-pumps are constructed so that they are, when required, compressing-pumps, and we can pump just enough air into the cylinder to make it counterbalance the pressure of the column of water without, by which means we keep the work perfectly dry, and the men can get at it just as well as if they were working in this room.
" What is the greatest depth to which you have driven a single cylinder ?—I think the greatest depth to which we have driven a single cylinder is about 19 feet ; but one has been driven in the Goodwin Sands 65 feet by the same process.
"The Committee understand that the cylinders are not single, hut are piled one upon each other to the required depth?—Exactly so : they are generally used in nine-feet lengths ; the piles for the bridge at IZochester are of two diameters, they are six feet and seven feet, and they are cast in lengths of nine feet, with flanges at the top and bottom, which are accurately turned and fitted together, so that they drop on to one another ; there is a projection.
"The external water will be found to be effectually excluded by such a mode of junction ?—Perfectly ; we never have a drop through them ; they require nothing more than a single coat of paint, and when we use one of the castings we clean the flange carefully and give it one coat of good red lead paint, and put another down upon it, which is prepared in the same way ; they never leak a drop.
" Do you recommend as a general principle the adoption of a cylinder or of a square form ?—Generally a cylinder, for several reasons.
"Will you state the reasons ?—ln the first place, because it is the cheapest form to construct in the preparation of the casting itself ; and, in the second place, because it is better capable of bearing pressure, and therefore can be cast with a much less quantity of material in it ; the object in a founda tion being to get the largest bearing surflice at the least possible cost ; in the third place, because we have found in practice that it is difficult to sink square caissons close together, because, having a very small space between them, one having been sunk, it is very apt to make it difficult to sink an adjoining one ; we have no ground between them to work upon.
"The Committee understand, likewise, that there is round each cylinder a girdle of timber, which is necessary in order to keep the cylinder in its perpendicular position ?—Yes ; I have made use of piles upon which temporary frames are fixed, and put two rows of what we call wellings, forming a square space, in which the cylindrical pile is placed and driven by means of the pressure on the cap of the cylinder.
" Are the Committee to understate] that the surflice, or the bed of the river, is in the first instance level, in order to receive the cylinder ?—Not at all ; we deal with it as we find it.
"You use no mechanical means, except in the experiment to which you have adverted, of 30 tons and 100 tons of actual weight ; you have recourse rather to physical means of exhausting the air, and of admitting the pressure of the atmosphere f—Yes, because it is so much cheaper. It is a serious job to put 30 tons on to a pile, whereat: a simple cast iron cap, as I have before described, put on to the top is so exceedingly easy." hi another part of his evidence, Mr. Fox says, " There is no doubt that the cheapest foundation you can put in is to use the largest size cylinders, so as to have them within the compass of ordinary means of moving about. If' you had a pile two 11;et in ditunete• it would bear a certain load, sup po.ing it to be in any semifluid foundation ; if you double the diameter, you would only double the weight of the cylinder. but it would carry four times the load." Again, —and this is peculiarly applicable to foundations on sands, If' you give me a piece of ground, and first there is a layer of mud, and then a layer of gravel, and then a layer of rock, and then a sheet of cast-iron, and then anything else, I will puts cy tinder through it. I also will bring up the foundations of the bridge to•ow-water level, for something like the same cost as would have been expended under the old plan, in the mere material used for that purpose, saving coffer-dams altogether." Mr. Bush's " Light of all nations," though it failed so sig. pally, deserved notice. This was again a proposal to con struct a lighthouse on the Goodwin, ever a locality for experi ments of this nature. The site at last chosen was, for any good effect to be derived from it, as bad as could possibly be, being on the middle of the sand, and consequently calculated rather to lead vessels into danger, than to preserve them from it.