necessary, because they can resist tension on one side in both the metal and the concrete, if the latter be of the correct quality; i.e., the cylinders can act as beams to resist the horizontal thrust of wind and trains in the same way as do the columns of elevated railroads. Nevertheless for railroad bridges the author would advise against the adoption of long cylinders for piers, on account of their inability to resist vibration effectively. In some cases it is economical to adopt a group of four comparatively small cylinders well braced on all four faces ; but with this style of foundation it is generally customary to employ braced piers resting on the cylinders.

The diameter for a cylinder should depend not only upon the size required at the top, but also upon its height and the character of the foundation. It is sometimes governed also by the total vertical load to be carried, which should under no circumstances exceed the limit set in the specifications given in Chapter XIV. Portland cement only, and that of the very best quality, should be used for filling cylinder piers, and the filling should be done with the greatest care and thoroughness. Whenever the concrete has to be placed below water it should be done by using a trémie, and the composition of the concrete should be much richer than that for concrete laid in the dry.

Whenever a cylinder is sunk to bed-rock, it should be let into same far enough to prevent all possibility of slipping, and so as to give an even bearing all around the circumference. This is an easy matter when the pneumatic process is employed for sinking, but it is often difficult when open dredging is used. This precaution is as necessary in the case of wooden or steel caissons as it is for cylinder piers, and should never be neglected where there is a possibility of scour to or near bed-rock, or where the pneumatic process is employed.

In sinking large cylinders by the open dredging process so as to fill them afterwards with piles, it sometimes becomes necessary to put in temporary timber bracing bolted to the metal, in order to prevent the cylinders from collapsing or from getting out of shape. Most, if not all, of these timbers will have to be removed before the piles can be driven.