and that of the concrete at eighty pounds per cubic foot. The partially-filled caissons when complete weigh about fifty-six pounds net per cubic foot.

As a matter of precaution, the caissons have to be figured for side- frictional resistance of 600 pounds per square foot during sinking, or some-

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times (in extreme cases) 500 pounds per square foot. Of course, it is practicable to load temporarily the caisson as it reaches the neighborhood of its final position; but such an expedient is sometimes costly and troublesome, hence it is better to design it large enough to avoid the probability of holdup.

Some engineers have objected to relying upon side friction in supporting the load, but their contention is wrong, because it certainly does exist, and it has to be overcome before any settlement of the finished pier can