ample light and space to insure the comfort and safety of the attendants, as parts dangerous or even difficult of access will otherwise almost surely be neglected.

The operating machinery, the function of which is not only to transfer the motion of the motors or prime movers to the bridge structure, but also to retard and arrest that of the span, is dependent for the size and strength of its parts on the power or torque of the said prime movers and the resistance of the retarding mechanism or brakes; for the maximum operating stresses to which these parts will be subjected are obviously those which the motive power or brakes can put upon them—and usually those produced by the brakes are the more severe.

Breakdowns of machinery rarely occur from the accelerating force of the motive machine, while many do occur from excessive retardation or brake resistance; and this is easily accounted for, as the maximum torque of the motive machine can readily be determined and cannot be exceeded, while the retarding or brake forces are dependent on frictional resistances, more or less indeterminate, and are liable to great variation with slight changes of condition, rapidity of application, and maladjustment, and also to a very natural tendency on the part of designers and operators to provide unnecessary brake strength. In the author's opinion, the operating brake-resistance should not exceed the starting torque of the motors.

Economics, therefore, requires that the conditions of traffic should be carefully studied, in order to determine as nearly as possible the actual service the bridge must render, as well as the resistances to motion likely to be encountered, with the view of avoiding unnecessarily-great motive- power with correspondingly-larger brake-resistance, and consequent unnecessarily-high first-cost of machinery and motors.

The power and strength of the operating machinery, in the case of swing bridges having equal arms, is mainly a question of speed of operation; as it will generally be found that when the power is sufficient to overcome the frictional resistances and give the acceleration needed to perform the required movement in the required time, there will be an ample amount to overcome any unbalance of wind pressure likely to occur on the arms of the bridge. Similarly with lift bridges, if there is ample power to overcome the normal friction and give the required acceleration, there will be ample power to overcome at full speed any added friction on the guides from side wind-pressure, and, at a slower speed, any unbalanced load on the floor, except, perhaps, in situations liable to unusual snow load.

With the bascule bridge the case is usually different from that of the swing or the lift bridge; because, with the former in a raised position, the floor area exposed to horizontal wind pressure is large, especially in the case of the closed floors of highway bridges. The wind moment about the axis of a bascule bridge is, therefore, great, while the lever arm of the operating machinery is usually comparatively short; and hence the pressures and forces in  the operating  machinery,  as well as the motive power