required (and, consequently, the cost), will generally be determined by the wind pressure to be resisted rather than by time of operation and acceleration. It is thought that the tendency of engineers is arbitrarily to assume unnecessarily-high wind-pressures, without regard to the actual conditions and situations, and that, if more careful studies of the surroundings were made, and if the pressures provided for were more in accordance with those likely to occur in actual practice, considerable economy would result.

The author believes that in but very few situations are bascule bridges liable to be operated under anything like such wind pressures as are often specified, and that due consideration is not given to the fact that, even though the toe of the span when high in the air may be under considerable wind pressure, this pressure does not extend to the heel of the span, and that on the whole surface the total pressure is far less than the velocity of the wind would seem to indicate. This is especially so when, as is often the case, the banks are high, or covered by trees or buildings, and the bridge is sheltered from all but winds in the direction of the channel. The author believes, too, from both his own experience and conferences with other bridge specialists, that, in general, bascule bridges capable of slow operation against a uniform pressure of ten pounds per square foot, or even less, and at full speed against two pounds per square foot will answer all requirements.

It is quite usual with bascule bridges to specify holding or brake power sufficient to resist such abnormal wind pressures as fifteen or twenty pounds per square foot over the whole floor surface. To meet such a condition with normal unit stresses would require excessively-strong and prohibitively-costly machinery; and it is believed that under such specifications one is warranted in using as low factors of safety as will ensure unit stresses within the elastic limit, for such pressures may never occur with the bridge raised during its entire lifetime.

Such a holding power requires a severity of brake action not only undesirable but dangerous in normal operation: and. therefore, if provision for such holding power must be made, it should be by means of auxiliary brakes, to be used only in case an emergency should arise.

The economic importance of the use, as far as possible, of machinery and apparatus of standard manufacture cannot be too strongly emphasized, not only in view of the lower first cost, but also in the matter of maintenance; because, where special designs are adopted requiring special patterns, those patterns must be stored pending future repairs, which, occurring after long lapses of time, will involve delay in the location of the required patterns, if they can be found at all, and extra expense and time in the reproduction of the parts from them. The designer of the bridge, therefore, should provide ample space for machinery with a view to the kind to be installed, as often the use of special apparatus may be necessitated by the lack of a few feet or even a few inches of room.