drawbridge machinery. Unfortunately, though, the scales used for the drawings are generally too small to make the illustrations satisfactory.

Although, as just stated, the author has no intention of trying to cover here the subject of detailing of the machinery, there are a few details which it will be well for him to touch upon in a general way, among others the question of rim-bearing versus centre-bearing turntables. The author is decidedly in favor of the former because of the greater stability involved when the load is carried near the exterior of the pier. Turntables that divide the load between the rim and centre are not to be recommended, because the division is always more or less ambiguous. The load should always be distributed as uniformly as possible over the entire drum and among the rollers, and to do this care should be used in designing the girders over the drum so that they will have not only the necessary strength, but also the proper comparative rigidities. The greater the number of points of support the more evenly will the load be distributed to the drum and rollers; and the deeper the drum the better the distribution. Now as an extra foot of depth of drum costs much less than one foot of height of pivot-pier, it stands to reason that it is always better, whenever practicable, to make the drum much deeper than the calculations for strength and stiffness demand. The only reason for not adopting in every case an excessive depth is that so doing might place the rollers below the level of high water, and thus render the span liable to injury from drift, and the machinery to being blocked by an accumulation of mud under and between the wheels.

When the vertical distance between high water and the lowest part of bottom chord is small, the longitudinal and cross girders can be placed with their bottom flanges flush with the lower surface of the bottom chords, and the drum can be built inside of the box thus formed, so that its bottom flange angles shall be flush with the bottoms of the said girders. Or, if the vertical clearance be great enough to permit it, the box may rest on the drum at either four or eight points.

As a rule, drum diameters are made too great for economy,