A three-per-cent grade is almost the extreme limit for anything like a profitable operation; even a two-per-cent grade is one in which the operating people look forward to reconstruction and elimination. Yet there are short lengths of line up in the mining camps of Colorado, where grades of more than four per cent are operated; and it is a matter of railroad history that away back in 1852, when the Baltimore & Ohio Railroad was being pushed through toward Parkersburg, and the great Kingwood tunnel was being dug, B. H. Latrobe, the chief engineer of the company, built and successfully operated a temporary line over the divide at a grade of ten per cent--528 feet to the mile. A locomotive which weighed 28 tons on its driving-wheels carried a single passenger car, weighing 15 tons, in safety and in regular operation over this stupendous grade for more than six months. The ascent was made by means of zigzag tracks on the so-called switchback principle. That scheme succeeded earlier planes operated by endless chains; an instance of which is the quite famous road of Mauch Chunk, originally operated for coal, and now a side scenic trip for passengers. Other planes of this sort, you will remember, were in operation at Albany and Schenectady on the old Mohawk & Hudson route, now a part of the New York Central lines; but all of them involved a change of passengers and freight to and from their cars, and the zigzag switchback was considered quite an advance in its day. Two of these ancient switchbacks are still in regular use for passengers and freight--one at Honesdale, Pa., and the other at Ithaca, N. Y. The matter of grades being settled, and with it as a corrollary the question of minor curves, minor details next claim attention. Perhaps the water supply along the new line is defective. Then arrangements must be made for impounding, and perhaps suitable dams and waterworks will be built for this purpose. The water must be soft, to protect the locomotive boilers; if hard, an apparatus is erected for the softening process. Grade crossings are to be avoided, highway crossings being built, wherever possible, over or under the railroad. A railroad crossing another railroad at grade is an abomination not to be permitted nowadays. The universal use of the air-brake has permitted a reduction of the "head-room,"--the necessary clearance between the rail and overhead obstruction--from 20 feet to 14 feet. The old "head-room" was necessary to protect the brakeman who worked atop of the box-cars. This reduction of six feet in clearance was a matter of infinite relief to engineers, particularly in the bridging of one railroad over another. * * * * *