186S-] Foundations. 351 of coarse gravel, sand, and cement, or water-lime, in the following manner : Take ten bushels of gravel and six of sand, and rake them up intimately together. This thoroughly clone, in a large box made for the purpose, and placed near as possible to the trenches, take three bushels of cement, in a dry state, and mix it up with the gravel and sand, carefully and well. Now pour on water, over the whole, turning and mix- ing the mass very briskly, slide out that side of the box next the trench, and empty the contents into their place, over the before-mentioned course of dry gravel. Proceed and do likewise with every part of the trenches, until the concrete, throughout, is on a level with the tops of the piles. When sufficiently set or hardened, the whole surface will be ready to re- ceive the "planking," which is simply two pieces of white oak, or other durable timber, three inches in thickness, and ten or twelve inches in width, spiked on to the two tiers of piles. These two are to be continued in their length by others, taking care to break joints, and to bevel them on to each other. There will be a space of four or six inches between the inside edges of these two courses of planking ; and this, together with the outsides, must be filled up with concrete, flush to the surface. The Footing- Course is now to be laid with large flat stones, three, four or five feet in length, — as the necessity of the case may call for, — and ten or twelve inches thick. They are to be laid cross- wise, and edge to edge, being carefully bedded and jointed in cement. The succeeding course must be laid lengthwise, but the stones composing it need not be quite as long as those of the former course. They are, however, to be "broken-jointed," and carefully flushed with cement and spalls. This course ought to be ten inches thick, but, in width, six or eight inches less than the former. The third course ought to be built in like manner, the width being three or four inches less than that of the preced- ing one. When the surface, or cellar bottom, is reached, the walls may be built of the width called for on the basement or cel- lar plan. We have here a foundation that is well adapted for springy or boggy soil. But, in case the site should prove to have a sub-stratum of quicksand, then the ordinary piling will not answer, as the pile-driver will utterly fail to insert the piles in this treacherous material. In such a dilemma, the architect will do well to procure Mitchell's Iron Screw Piles, which will at once master the situation, as they will enter not alone sahd, but rock: itself. This is an European invention ; and was introduced in this country about twenty years ago, by the United States Light- house Board, and liberally used in the foundations of lighthouses, under water, in rock, sand, &c. The writer being then in charge of such works, under the Corps of Topographical Engineers, had ample opportunity of noting their merits and demerits. But as architects are not called on to construct buildings in such trying locations as those whereat light- houses and pier-heads ai'e most fre- quently erected, it will be unnecessary to more than allude to the fact that, al- though admirable for entering difficult and otherwise impracticable strata, the Iron Screw Pile, where exposed to the action of ice or the force of waves driven by severe storms, is but too apt to break off at the cast iron section, or to warp and twist at the wrought iron one. However, where protected from such influences, it is very available for such foundations as we have here under con- sideration. In our next number, we will give a full description of Mitchell's Iron Screw Pile, together with hints and suggestions as to the most advisable way of using it economically and well.
