draw, but making the anchorage always ready to resist the slightest tendency to lift the span.

The limiting ratio of length of span to diameter of drum that can be employed without using a central anchorage cannot well be determined by rule, but must always be left to the judgment of the designer. It might suffice, perhaps, to specify that, whenever the uplift on one arm only necessary to upset the draw is less than twenty pounds per square foot of floor in situations exposed to high wind-pressure, or less than fifteen pounds in other situations, an anchorage shall be adopted.

In the case of three draw-spans, which the author has designed lately for the Kansas City, Pittsburg, and Gulf Railroad Company, the span length is two hundred and twenty-five feet, and the diameter of the drum is only seventeen feet; nevertheless no central anchorage was used. In these bridges the open floor reduces the uplift, and the situations are not such that the spans will be exposed to abnormally high wind-pressures.

Heavy draw-spans should be operated by two or more pinions, and when these are placed, as they should be, diagonally opposite each other, some kind of apparatus ought to be used to equalize the pressure on the pinions, otherwise both the latter and the rack are liable to have their teeth broken. The reason for this is that it is impossible to make the toothing of the rack so perfect in the distance of the semi-circumference that opposite pinions operated by a single shaft shall at all times act equally. When electrical machinery is used, the equalizing can be done by means of duplicate motors; but with other machinery some kind of mechanical equalizer should be employed. The author several years ago designed one for the East Omaha draw, which worked to perfection. It was made by cutting the engine-shaft and attaching to each end a bevel-

gear wheel. These bevel-gear wheels engage with two small pinions which are inserted between the spokes of a large spur-wheel that turns loosely on the engine-shaft. If we assume the pressures on the main rack-pinions on each side of  the  drum  to be constantly  equal to each  other, the two