the small wooden quarter-rounds at the ends by which the men take hold.

All machinery shall be so arranged that the span can be turned completely around in either direction, and so that it is reversible in every particular.

END-LIFTING APPARATUS.

The ends are to be lifted and locked by means of a toggle mechanism to be operated by screws at each end of the span. The entire machinery is to be made strong enough, with the previously specified intensities of working-stresses, to exert all upward force on each end of each truss equal to the assumed uplift in case of mechanical power; or to transmit to the end rollers the greatest force that the men can exert on the hand levers, assuming that as many men will be applied thereto as are required for the turning-machinery, and that each man exerts a horizontal thrust of one hundred and twenty (120) pounds, straining the metal twice as high as in the case where the power is mechanical.

In case of mechanical power, all the teeth and shafting must also be figured on the assumption that the entire available capacity of the machinery is required merely to start motion, and that under this condition the metal is strained twice as high as herein specified.

The size of screw required is to be determined by the following formula:

where d = diameter of screw at base of threads, and P = axial pressure on screw.

The axial pressure is to be determined for the two following cases, the greater pressure thus found being adopted:

Case I —

where R = total assumed upward reaction at one end of span, h' = greatest rise of ends when end lifts are applied, and h = travel of nut on screw necessary to produce the rise h'.