406 Sloan's Architectural Review and Builders' Journal. [Dec, it would be well to have the screw-thread two feet six inches, or, even three feet in diameter. When all the principal corners of the foundations have been thus piled, intermediate piles should be placed, at every five feet from each other, all around. Another, or inner row should now be placed three feet from the row just described, opposite to each other; and white-oak timbers, 8 x 12 inches, set into flanged chambers, cast to receive them, and these latter screwed securely d >wn on the heads of the piles. The oak bridging will thus bind the piles lon- gitudinall}' and vertically. The trenches may now be filled up with concrete, until flush with the upper surface of the bridgings. The bedding-stones may now be laid down, as before described. The foregoing system of screw-piling might be economized, by using but a single row of piles, placing them in the centre of the foundation, and having bracketed arms across them, to sustain the longitudinal timbers, or bridgings. The former, however, is the more advisa- ble for heavy buildings. The trouble of managing these screw- piles is not so great, as might first seem to the builder. The use of a square truck, or hand-car, running on a tempo- rary track, with a capstan in the centre, would make the work as eas}-, at least, as that of ordinary pile-driving. In the construction of foundations, in doubtful localities, or where the sub- strata is varied ; being, in one part sound and compact, and, in another, sandy or soft, Screw-Piling certainly would be most desirable, not having to descend so low as the ordinary pile, under such circumstances, and not re- quiring more than half the number of piles, in an}' case. Inverted Arching. There are other methods of securing a good foundation, one of the best of which was introduced by the Romans ; and was much practised up to the dis- coveiy, or invention of concrete, scarcely forty years ago. "We allude to the sys- tem of Inverted Arching. This was, no doubt, a secure way of building, as the experience of hundreds of years so clearly proves. But the piers, in many cases, had to be constructed on piles ; and the inverted arches themselves to be protected by counter-arches, which sustained the superstructure, thus form- ing a circle, or an ellipse, either open, or filled up, at the discretion of the architect. This mode of construction was much used in stone-bridge building; and gave gtjpat resistance to the settling- down of the superstructure. The Thames Tunnel is formed on the same principle, only the arches are necessarily transverse, instead of longi- tudinal ; and Nature, herself, presents us with the lesson in the sectional form of the egg. Concrete. The introduction of Concrete, as a material for foundations, was decidedly a great triumph of scientific skill ; and, it is remarkabl}' strange, how little, com- paratively, the American builder knows, about that admirable material. Our architects do not pay it that attention it merits, and has so amply received from English and French architects. It is true, our military engineers, the late General Totten among them, have experimented much in it ; and developed its powers of cohesion and resistance. But the econom}' of its composition, in conjunction with those powers, ought to be the subject of the architect's study. This idea of taking the precaution of piling when using it, displays a timidity, arising from sheer ignorance of its very n at ure ; as, if piling has to be resorted to, there is no use whatever for concrete ; and, if concrete is to be depended on, there is no necessity for piling. Mix concrete, in its proper propor- tions of materials ; give it suitable width, with a batter, or slope, at the edges ; and then make it not less than eighteen
