be taken for about four-tenths of the span-length and not for the full length, as is the case for simple spans.

In cantilever bridges judgment must be used in determining the lengths for equivalent uniform loads for the different portions of structure. For instance, in Type-A cantilever, shown in Fig. 12a, for the suspended span its total length is to be used; for the cantilever arms the sum of the length of the suspended span and that of the portion of the cantilever arm that is loaded for maximum stress on the member considered; and for the anchor arms the total length of the loaded portion of one such arm for direct loading and that of one cantilever arm plus suspended span for reverse loading. A skilful bridge designer can often legitimately economize in his work by making a careful study of the live-load question and cutting down the loading as hereinbefore indicated.

Similarly a study of the question of impact allowance and the utilization of the very latest information thereon will result in a certain amount of economy. For instance, since "Bridge Engineering" was written, the author has found it advisable to modify the impact allowances recommended in Chapter VII thereof in the following particulars:

First. For electric-railway loads use the same impact as for highway loads. This modification is based upon some late experiments by Prof. Turneaure.

Second. When a reinforced-concrete base is used for the pavement of a highway bridge with similar slabs for the footwalks, the impacts given may be reduced twenty-five (25) per cent.

Third. For reinforced-concrete arches and girders, without earth filling, the impacts may be reduced fifty (50) per cent.

Fourth. For earth-filled, concrete arches, they may be reduced seventy- five (75) per cent.

The determination of the width of deck that it is best to employ is an economic question of salient importance. A narrow roadway tends to induce slow travel, owing to the necessity for reducing the speed of automobiles in narrow places so as to avoid collision. For two lines of travel at high speed a width of twenty-two feet is advisable, although one of twenty feet is often considered sufficient because it is practicable to pass most auto-vehicles on a width that is two or three feet less. There are many bridges in country districts that are only eighteen feet wide and some as narrow as sixteen feet; but these do not meet with the approval of automobile drivers, because a load of hay will almost block the structure—besides, when one is driving at high speed, he does not like to be compelled to slow down when he is approaching a bridge upon which there is another vehicle.

A thirty or thirty-two-foot width of main roadway is better than one of twenty or twenty-two feet, in that it will permit a fast vehicle to turn out and pass a slowly-moving one; but there is always the danger of running into an automobile of the other line that is trying to accomplish the same purpose. A roadway of forty or, preferably, forty-two or even forty-