Some caissons are made with vertical sides throughout their entire height; others have an outward taper for 15 or 20 feet on the lower end. The former are not only simpler in construction, but are more easily kept in a vertical position during the sinking. Caissons are usually put together in some convenient place near the edge of the water, and then conveyed on pontoons to the sites of the piers. Great care is required in lowering them into position in the bed of the river, and guide-piles, guy-chains, and other appliances are frequently necessary to keep them vertical during the sinking. The form, dimensions, thickness of plates, cross-bracing, and general arrangement will depend upon the size and depth of the pier to be 123 constructed. Caissons for heavy work on difficult or treacherous ground require great care, not only in their construction, but also in placing them in exact position, and in sinking them correctly to their proper depth. A tilted caisson is a most difficult subject to handle, and entails heavy expenditure to restore it to a true vertical position. By making careful borings, the engineer can ascertain very closely the depth to which the caisson will have to be lowered to obtain a good firm foundation. With this information the caisson can be so constructed that the upper portion, termed the temporary caisson, commencing a few feet above the bed of the river, can be detached, and removed at the completion of the work from the lower or permanent portion sunk below the ground line. Fig. 118 gives sketches of a wrought-iron plate-caisson applied to a deep-water river pier, and lowered to its full depth by the pneumatic process; dotted lines show the air-tubes through which the excavated material is hoisted and emptied into barges alongside. Many large and important pier foundations have been constructed on the system of brick cylinders or wells, particularly in India, where the foundations for large river viaducts have to be carried down to great depths through thick deposits of soft material. These wells are built upon V-shaped curbs to facilitate the penetration when sinking. Fig. 119 is a section of a well with a wrought-iron curb, and Fig. 120 is a similar well with a wooden curb. The wrought-iron curb is made in segments for convenience of transport, the pieces forming the complete ring being bolted or riveted together at the site of the foundations. The wooden curb is composed of several thick layers of hard wood planking cut to the proper shape, and laid with broken joints, the whole being bound together with suitable bolts and spikes. In some cases the lower or cutting edge of the wooden curb is strengthened or protected by a sheathing of wrought-iron plates. [Illustration: Fig. 118] [Illustration: Fig. 119, 120, 121, 122, 123]