The value of v is to be determined by assuming a certain time t, in seconds, for turning the draw one fourth (1/4) of a revolution. Let l = distance in feet of the centre of pressure on one arm from the centre of the drum; then

For Mechanical-power Turning-machinery the greatest H.P. required is to be determined as follows:

Case I.— (a) By Formula (1) determine the H.P. required for turning the span in the least time in which it is probable that the said span will ever need to be opened.

Case II.— (a) By Formula (1) determine the H.P. required for turning the span in twice the time assumed in Case I. (b) By Formula (2) determine the H.P. required for operating the draw against the unbalanced wind load in twice the time assumed in Case I, and add together the two amounts of H.P. determined by (a) and (b). The sum will be the greatest H.P. required for Case II.

The greatest pressure on the teeth and torsion on shafts found for these two cases are to be used, the metal being strained on the extreme fibre as hereinafter specified; but the said teeth and shafting must also be figured on the assumptions that the entire available capacity of the machinery is required merely to hold the draw from turning under an excessive unbalanced wind-pressure, and that under these conditions the metal is strained twice as high as hereinafter specified.

For Hand Turning-machinery the H.P. required to turn the span in the least time in which it is probable that it will ever need to be opened by man-power is to be found by the formula previously specified; then the number of men required to perform this work is to be determined by assuming that six (6) men are equivalent to one

H.P. In proportioning all parts of the hand-operating machinery there shall be assumed on the levers as many men as are required by the above method, each man exerting a horizontal thrust of one hundred  and  twenty  (120) pounds. Under such  conditions  the