dling and depositing the concrete-such as chuting it long distances, using gravity mixers, allowing insufficient time for mixing, putting too much water in the mixture so as to avoid most of the expense of tamping and working the aggregate back from the face of the forms, permitting the employment of too large pieces of broken stone which fail to pass between the reinforcing bars, laying concrete in freezing weather, and placing concrete under water when there are available other methods which would permit depositing it in the air at a little greater cost.

At the time when the "Final Report of the Joint Committee on Concrete and Reinforced-Concrete" was presented, the above condition prevailed, but since then much important research work on concrete manufacture has been done, and there are now several publications which give valuable data and point the way to better concrete structures—notably the records of the elaborate series of experiments made by Prof. Duff A. Abrams. But as this treatise is intended to cover solely the field of economics and not that of general engineering practice, the reader is referred for further data on the manufacture of proper concrete to the well-known "Report" before mentioned; the "Transactions" of the American Railway Engineering Association, the American Society for Testing Materials, and the American Society of Civil Engineers; and the "Bulletin" of the Structural-Materials Research-Laboratory of the Lewis Institute.

The author is indebted for a number of valuable suggestions on concrete manufacture to his friend, J. J. Yates, Mem. Am. Soc. C. E., Bridge Engineer of the Central Railroad Company of New Jersey, and Chairman of the Committee on Masonry of the American Railway Engineering Association.