tractors contend that cofferdam piers sunk by their use are more expensive than those placed by the pneumatic process; whilst other contractors, equally experienced, declare the contrary, all however agreeing that the difference in cost by the two methods is small. The author is inclined to believe that, if a thorough set of borings has failed to indicate any sunken logs, beds of large boulders, quicksand, or other serious impediment to driving or excavating, the steel sheet-piling will involve a moderate saving in first cost for depths of foundation below ordinary low water as great as forty feet. To effect such a saving, however, requires an experienced and energetic contractor or superintendent; piling of ample size, thickness, and length; sufficient of it for three, four, or even five piers (according to the size of the job), with extra pieces to provide for damage in driving; heavy pile-driving hammers; ample pumping capacity; and a full supply of derricks, engines, and other outfit. Generally, it is the small-fry contractor who prefers the cofferdam method to the pneumatic; and he is the, one who is most likely to get into trouble from failure to anticipate and provide against difficulties in driving and excavating. His pseudo-economic disposition leads him into purchasing small, thin, and short piles; for he does not recognize that large ones will withstand battering at both top and bottom much better than small ones, that thin webs are liable to be split and bent by striking large, hard boulders, and that short lengths are almost sure to involve not only flooding the dams but also filling them with sand or silt-possibly several times during the progress of the work.

Steel-pile cofferdams have been successfully used for depths as great as fifty feet below ordinary-low-water elevation; but the conditions were unusually favorable, the material penetrated being mostly soft clay that shut out the water almost completely, thus enabling bed rock to be reached at moderate expense.

Large, strong sheet-piles, in addition to the security against injury in driving which they provide, effect an economy by permitting the waling frames to be placed farther apart, thus lessening the amount of timber to be bought and the expense of both its placement and its removal; besides the metal often has more than merely scrap value at the end of. the job, which is seldom. the case when small, light sections are employed.

Sunken logs or wrecks give endless trouble when encountered in cofferdam work, as usually they have to be shattered to splinters by dynamite before the piling can penetrate them; and under such conditions the steel sheet-piling is decidedly superior to the wooden Wakefield-piling. The latter is advantageous for shallow excavations and for cases where the bed rock is too hard to penetrate; because the ends of the piles broom, and the battered wood, by absorbing water, expands and seals the bottom of the pit.

The cofferdam method is specially applicable where clay overlies the bed rock; for it will seal the bottom of the box. Where there is no such sealing layer, it is often necessary to place clay, manure, or some other