the deck become lighter than the counterweights by reason of the drying of the timber in the floor and screens, it will be necessary to add to its weight by loading it; but this condition is not likely to exist, for what weight is lost by drying will be fully made up by accumulated dirt in spite of all the precautions that may be taken to keep the floors clean.

Whether this proposed structure will ever be built is problematical, although there is a fair chance of its being finished some day with modifications tending to cheapen the work. It would be a great satisfaction to the author to complete this bridge because of the novel design for the lifting deck.

Floating draws are a type of structure that cannot be recommended except as a temporary expedient. The author had occasion once to design one of them, but the necessity for its use did not develop, so it was not built. The objections to floating draws are as follows

1.   Trouble from rise and fall of water, necessitating constant adjustments.

2.   The depression of the draw under the live load and the consequent changing of the grade.

3.   Possible disaster from injury by ice or drift.

4.   Trouble from leakage.

5.   Clumsiness of method of opening and closing the draw.

As there are no advantages to offset these disadvantages, unless it be possibly a small saving in first cost of span, it is not likely that there will be much call for floating draws.

In concluding this chapter, it may be well to summarize somewhat and indicate what kinds of draws should be used at various crossings.

For streams bearing a moderate amount of traffic with crossings located in country districts or in unimportant cities, rotating draws are the cheapest and consequently the most appropriate; but for great traffic and for important cities bascule and lift bridges are the best, the former for spans up to about one hundred and fifty feet, and the latter for longer spans. The choice between the bascule and the lift for all doubtful cases should be determined simply by the question of first cost.