be sheathed so as to reduce the friction during sinking. The drift-bolts should be seven-eighths-inch rounds driven into three-quarter-inch bored holes. The filling of the working-chamber with concrete should always be done with the greatest care, using extra rich concrete, so that there shall be no voids between the concrete and the roof. Portland-cement of the best quality should be used for filling the working-chamber and shafts; but it is legitimate to employ an extra-good quality of American natural cement for filling the crib in case that it be necessary to keep down the expense. However, Portland cement is always preferable.

In respect to caissons built of steel and concrete but little need be said, except that great care should be taken to design the working-chamber strong enough to resist properly the weight of the concrete above and the unequal pressures from boulders below. The metal below the roof of the working-chamber should not be less than one-half inch in thickness, and all parts near the cutting edge should have thicknesses varying from three quarters of an inch to an inch. All joints in the cutting edge should be full spliced, as should also those in the roof of the working-chamber.

Timber grillages resting on piles should have, preferably, not less than four courses of timber, although often but three and more rarely two are employed. As the grillage is generally wider than the masonry, it takes about four courses to distribute the weight uniformly (or nearly uniformly) over the piles. In case of an unusually wide grillage, more than four courses would be necessary, or else the masonry should be widened by means of footing-courses. Such grillages should be built with care, so as to have a level bottom; and all piles should be cut off to exact level, otherwise there will be unequal bearing between piles and grillage that might cause serious damage to the masonry.

Brick piers are not common in America, probably because, until lately, it has been difficult to obtain proper brick. In the author's opinion, piers built exclusively of hard-burned clinker brick and mortar of the very best quality of Portland cement, mixed  in  the  proportion  of  one  part  cement  to