of modern wheel-loads it has permitted the pavement to crack and eventually to break up. It is probable, though, that Mr. Byrne's correction of the defect will permit in future, with perfect safety, the employment of this type of flooring.

When the electric railway tracks are entirely separated from the roadways, it is economic to use the ordinary open floor of wooden ties with proper guard-rails, because the danger from fire with such a floor is not great; and even if some of the ties were to burn, no injury of any importance would be done to the metalwork. Such fires do not spread rapidly and are readily extinguished; but when a plank base with a wooden block pavement thereon catches fire on a windy day, disaster is almost sure to ensue.

There are two legitimate ways of trimming down the weight of metal in the floor-system to a minimum, viz., employing nickel steel or some other alloy for the metalwork, and supporting the cross-girders at intervals by means of intermediate trusses and cables. In adopting the first expedient, care should be taken not to reduce the depths of the joists and stringers below the limit set by good practice-otherwise their deflections would be great enough to injure the pavement. In respect to the second expedient there are both pros and cons concerning the advisability of adopting many lines of trusses and cables. The advantages are as follows:

First. The saving in weight of metal in the cross-girders.

Second. The reduction in cable cross-section due to the lightening of the cross-girders.

Third. The better distribution of load on the piers with its consequent saving of both shaft masonry and base.

Fourth. The lowering of the grade line of the structure (due to the shallowing of the cross-girders), with its consequent reduction in cost of climb for everything that crosses on the structure.

Fifth. The shortening and consequent cheapening of the approaches due to the smaller height to be surmounted.

The disadvantages are as follows:

First. The somewhat greater weight of metal due to the larger number of parts in trusses, cables, and steel towers. In long, heavy spans this really is not a disadvantage; because, on account of ease of erection, it often pays to employ comparatively light members.

Second. The somewhat greater total width of deck called for to allow space for the extra trusses and cables; but this is often not disadvantageous, because great width of structure is necessitated by great span-length.

Third. In accordance with the theory of probabilities, the more numerous the trusses and cables the greater should be the total live load per lineal foot assumed for the design; for with only two lines of cables, practically the entire width of deck must be loaded in order to produce the maximum stresses in either cable, while with several lines of cables,