The data for this chapter were furnished mainly by the author's old friend and occasional associate in professional work, Thomas Ellis Brown, Mem. Am. Soc. C. E.; but some valuable suggestions as to items to be considered were given by Major Leon L. Clarke,who, for many years before the Great War and for a short time after his return from France, where he rendered effective and distinguished service to the Allied Cause, was the author's principal assistant mechanical engineer, and as such devoted his entire attention to the designing and installation of machinery for operating movable spans.

As long ago as 1892, when the author was retained by the City of Duluth, Minn., on his first design for a vertical-lift bridge, he recognized the necessity for some expert aid in solving certain important mechanical problems; and, consequently, he looked the country over, in order to find the highest American authority on the mechanics of lifting great weights. The result of his search proved that, even at such an early date, Mr. Brown was universally acknowledged to be the best authority on elevators and their machinery; and, therefore, the author retained him. From that incident there resulted a friendship and a somewhat desultory association which have proved very satisfactory and beneficial to both parties thereto.

With the help of two such experts as Mr. Brown and Major Clarke the author feels that he has done his best to treat one of the most difficult branches of engineering economics.

The kind of power and type of machinery suitable for the operation of movable bridges depend greatly upon the nature of the design of the structure, the available space for apparatus, and the local conditions of fuel or power supply, and are, therefore, contingent upon the ruling features of the particular case; hence no hard-and-fast rules of economics therefor can be formulated. On that account the contents of this chapter will be limited to a dissertation concerning general conditions and the offering of a few pertinent suggestions.

The selection of motive power is largely dependent on the location of the bridge with relation to sources of power-supply. When located in or near a large  city  or  town,  electric  current,  either direct  or alternating, is almost

310