be about ten and a half pounds, which, at the present unit prices of material and labor, are worth in place about 84 cents, showing that to-day for simple-truss spans exceeding 600 feet in length it would be economic to adopt the buckle-plate floor.

As this economic question is likely to arise at any time, and as its solution is largely dependent upon the relative unit prices of concrete and structural steel in place, the author has prepared the diagrams shown in Figs. 21a and 21b, which indicate for simple-truss and cantilever spans, respectively, and for both carbon steel and nickel steel, the amount of extra metal that will be required to support one excess pound of extraneous load uniformly distributed.

Just as the MS. of this book was about to go to press, the author's attention was called to a light aggregate for concrete, termed Haydite, being named after its discoverer, Mr. Stephen Hayde, a well-known citizen of Kansas City, Mo. He claims that concrete made of it weighs only 108 lbs. per cubic foot, as against 150 lbs. per cubic foot for ordinary 1 : 2 : 4 concrete of small broken stone or gravel, and that the lighter concrete not only is practically impervious to water, but also that it gives some 25% greater resistance to compression than its heavier competitor. If these claims are correct, there should be quite a demand for Haydite for the bases of highway-bridge floors, especially as the extra cost of the aggregate is moderate. Mr. Hayde and his business associates contemplate retaining the author to make a thorough investigation of the suitability of Haydite for pavement bases in bridge construction and to show by diagrams and otherwise the economics involved thereby. Until some engineer of standing tests the new aggregate and vouches in print for its characteristics, the profession will have to take the discoverer's claim cum grano salis; nevertheless, the author has great hope of its being proved to be all that is claimed. If such be the case, its use will result in a real boon to the builders of modern, first-class, highway bridges, especially those of long span.