it would be economical to omit the said chains. These can be made of cast-iron links bored for small pins, so as to keep their pound price as low as possible.

It may be all right to omit the buffers entirely and trust to the automatic brakes to stop the span, but the author prefers to adopt the additional precaution of using the buffers as a safeguard, in case that anything should go wrong with the operation of the brakes. He has tried two kinds of buffers, viz., oil and air, and prefers the latter on account of greater reliability and cleanliness.

In respect to the character of base for pavement, it will generally be economical to use the lightest practicable, consistent with proper requirements for strength and stiffness. The modern, stiffened-buckle-plate floor with a thin layer of concrete thereon supporting three-inch wooden blocks, described in Chapter XXI, will save the lifting of considerable weight; and the consequent reduction in cost of ropes, sheaves, counterweights, and capitalized power will generally more than offset the extra cost of the lighter base.

From motives of economy alone, it is better to place the machinery house in one of the towers instead of on the span; because it takes extra truss-metal to carry it in the latter place, and this extra metal and the weight of the house with its machinery augment the cost of ropes, sheaves, counterweights, and power. But generally the operator obtains a much better view of passing vessels from the middle of the span than he could from the tower or anywhere else, consequently it will then be better to put the house on the span in spite of the extra expense involved by so doing.

Finally, in respect to the determination of number and sizes of supporting cables, it may be stated that the greater the number of cables the smaller their diameter and the smaller the legitimate diameter of the sheaves; but the greater the number of ropes the wider the said sheaves. Again, multiplicity of ropes means a multiplicity of expensive details for their connection; hence the determination of main-rope diameter is a question that generally has to be solved by good engineering judgment based upon experience rather than by economics pure and simple. The author's usual practice is to adopt four ropes per corner for loads up to 250,000 lbs., eight from 250,000 to 1,250,000 lbs., and sixteen for greater loads.

There is a combination of vertical lift and cantilevers that has lately proved to be economic. The author evolved it many years ago, but did not publish anything concerning it, preferring to await the psychological moment for, utilizing it. The opportunity did not present itself until 1918, when, as a member of the Board of Advisory Engineers to the Public Belt Railroad Commission of New Orleans, he prepared a low-level-bridge layout with a vertical-lift span for a proposed crossing of the Mississippi River near that city. He had adopted for the movable span a clear opening  of  three  hundred  feet;  and, when trying to obtain an informal