as compared with the bascule, when long flanking-spans are employed; but for crossings over canals or canalized rivers the cost of the substructure usually has little effect on the comparison. A crossing where the piers rest on piles or sand is especially favorable to the vertical lift, since the total loads for that type are less than those for the bascule. Rolling-lift bascules are not well adapted to such foundations. Deep foundations are usually unfavorable to the swing, on account of the large base of the pivot pier.

Advantage can be taken of a badly-skewed crossing by the vertical lift; for both the span and the towers may be skewed with very little extra expense, while at least one end of the bascule will have to be squared, thus lengthening the span and increasing the cost. It is true that a similar advantage can be taken with the swing by making both ends skewed, but that would prevent the reversing of ends. However; there is not often any real necessity for such reversal.

A channel that has a tendency to shift will give the vertical lift a great advantage over either of the competing types, because it is the only one of the three which permits a change in the location of the opening span without necessitating excessive expense.

A requirement for high wind pressure militates greatly against the bascule, because it involves an augmenting of the power and, consequently, also the cost of the operating machinery; but it affects hardly at all the cost of either the vertical lift or the swing.

In a modern, first-class structure, where a concrete deck should be used to eliminate fire risk, the vertical lift and the swing can be employed; but the bascule cannot, if there is to be paving on the concrete. While a concrete deck without other paving could be used on a bascule, the span would be so heavy that a vertical-lift bridge would always be the cheaper, except for a very high lift with a short span.

The use of a block pavement or other heavy type of deck favors a vertical lift as compared with the bascule, since, for the latter, extra expense is required properly to fasten the blocks so that they shall not fall off. Furthermore, the greater weight of the deck favors the vertical lift.

The widening of the deck lengthens the moving span in any swing, and increases the size of the pivot pier and the cost of the pier protection. In a skewed crossing it augments the length of a bascule. Since a wider deck involves a heavier structure, this factor also favors the vertical lift.

In respect to quickness of operation, this condition does not affect materially the comparative economics of vertical lifts and single-leaf bascules; but both the double-leaf bascule and the swing are at a disadvantage, since they take fully twice as long to operate as do the other types.

The question of flanking spans is of such importance that the author has found it necessary in his practice and in his economic studies to divide the vertical-lift bridge into two distinct types-one where there are fixed spans flanking the movable span, and the other where there are not.