the draw has to be opened only a few times per season, as is the case with bridges over most Western navigable streams.

Everything considered, whenever there is any choice between a high and a low bridge for the crossing of any important Western river, the author favors the low bridge, not so much because of its lower first cost, but on account of the smaller expense for maintenance.

The different kinds of revolving draw-spans recommended are described in detail in Chapters XV and XVII. They may be operated in various ways, for instance by man-power, steam, electricity, gas or gasoline engines, or water. Wherever an unfailing supply of electricity is available, that source of power is the best and cheapest. Steam is appropriate for large, heavy draws where electricity is not available. Gas or gasoline engines are best suited for comparatively small spans in country districts; and water-power can sometimes be employed to advantage where there is a fall of water near the bridge.

It does not pay to use storage batteries for operating drawbridges. Concerning this question the author feels that he can speak as an authority, for he once made the experiment, and it was a failure. For a while the machinery worked to perfection, but soon the batteries began to leak, and the leakage gradually increased to such an extent that the batteries would not hold their charge for three consecutive days; so the electrical power was given up, and the bridge has since been operated by hand.

Gasoline engines, everything considered, are probably the best source of power for operating the average draw-span. The author has lately designed some small draws to be operated thereby; but the machinery has not yet been installed, so he cannot report concerning how such engines act.

In respect to the power required to operate draw-spans, the author uses an average of the Boller formulae, viz.,

where W = total load on rollers in pounds, and v = velocity on pitch-circle of rack in feet per second.