too limited vertical distance between the elevations of grade and high water.

The simplest form of bascule is the ordinary heel-counterbalanced, trunnion type; and this is the kind which is generally adopted when the minimum clearance allowed above water will permit. In many cases the height is not sufficient for the heel of the span and the counterweight to clear the water or the pier-tops, and then the span must be lengthened and a water-tight pit must be provided into which the said heel and the counterweight may descend. The expense of construction thus involved is very great; and, consequently, the more complicated and unsightly types, having towers and counterweights above the roadway, are resorted to.

Sometimes cases occur in which the height above water is insufficient for a simple, heel-balanced bridge of the ordinary type without water-tight pits, and where the adoption of unsightly towers and counterweights is barred for aesthetic reasons. Again, in the usual heel-counterbalanced trunnion structures, it is obligatory so to dispose the counterweighting that the center of gravity of the entire moving mass shall lie upon the axis of rotation; and this generally necessitates the location of a considerable portion of the counterweight above the deck, to the detriment of the appearance of the bridge. Under such conditions it is necessary to provide a heel-balanced, trunnion structure, in which the coincidence of the center of gravity and the axis of rotation are not obligatory, and by which the employment of either pits or unsightly towers is avoided.

These desiderata can be accomplished by a partial balance of span-weight, completing the said balance by means of a counterweight supported on a beam pivotally connected at one end to the heel, and supported at its other end by a roller or truck which passes forward and backward on a track when the span opens or closes, according to a series of patents lately taken out by Mr. Thomas Ellis Brown, Jr.

Either of the two primary types of bascule may be divided into three general classes, viz., trunnion, rolling-lift, and roller-bearing. All of these are good, but none is best for all conditions, nor can it be said absolutely that one is always more economical than another. Each has its good points and each its bad ones; and some are fitted for one location and not for another.

The rolling-lift is sometimes the cheapest, as has been shown often by competitive bids on different types submitted by Contractors; but it is not good practice to adopt it when the pier foundations are of piling, on account of the shifting of the center of gravity of the load on the piles as the span rolls backward and forward, and because of the possibility of pier settlement. The extension and compression of the outer piles, caused by such shifting, has a tendency to crack the superimposed masonry. The great advantage of this type is its retreating bodily out of the way of passing vessels.