lutely. But if there should arise a case in which the choice of carbon steel or alloy steel for the floor system is debatable, the said diagrams might be considered as only approximately correct; and in such a case some special computations of weights and costs of metal might become necessary.

Attention is called to the phenomenally short time required for the solution of any economic problem in the use of alloy steels for bridgework by means of Figs. 5a, 5b, and 5d when the elastic limits and the pound prices erected of the steels to be contrasted are known. Hereafter it will be unnecessary for anyone desirous of employing an alloy of steel in the design of a bridge to wade through the two papers previously mentioned or the author's other writings on the subject of alloy steels, because the economic results of all his previous investigations are concentrated into the three diagrams last indicated.

The use of alloy steel in bridgework is only in its infancy, for thus far there have been very few bridges built of it. In 1903 the author began his economic study of the question of "Nickel Steel for Bridges," and the investigation required more than three years to complete. He found that good, reliable bridge-steel could be manufactured with an elastic limit of 60,000 lbs. per square inch by adding to the usual charge of molten metal 3 1/2% of nickel; and as a result of his findings several large bridges were constructed of that alloy. A few years later the great demand for nickel in the manufacture of armor plate for ships-of-war enhanced the price of that metal to such an extent as to make it too expensive to employ in bridge construction; and the advent of the Great War in 1914 sent the price soaring. Although the cessation of hostilities has decreased the demand for the alloying metal, its price is still unsettled and probably has not yet been sufficiently reduced to warrant its employment for bridges-nor in fact, has there been any call of late for metallic bridges of importance. Just as soon, though, as the general business of the country attains once more a sound condition, there will be a request for some large bridges of long span, because a number of them are even now being seriously considered; and, when that time arrives, the question of alloy steels for such structures will become a paramount issue, and either nickel or some other suitable alloying agent or agents for strengthening bridge metal will be greatly in demand.

Since the time when nickel became too expensive to use in bridges, several alloy steels, other than nickel steel, have been either exploited or suggested, the principal ones being Mayarí steel, purified steel manufactured by the electro-metallurgical process, aluminum steel, vanadium steel, and silicon steel. On account of the great cost of nickel and the other alloying metals, there is a tendency on the part of a few American bridge specialists to employ high-carbon steel in  important constructions. In  the  author's  opinion, this is a dangerous policy to adopt, because  high-carbon  steel is brittle and, therefore, unsuitable for bridgework. He  has  never  been  willing  to  use  it  in  any  of  his  constructions,  notwithstanding  the  fact that the specifications of his "De Pontibus," written in 1897,  per-