_Percussion Drills._--The first American percussion drill was patented by Mr. J. J. Couch of Philadelphia, Penn., in March, 1849. In May of the same year, Mr. Joseph W. Fowle, who had assisted Mr. Couch in developing his drill, patented a percussion drill of his own invention. The Fowle drill was taken up and improved by Mr. Charles Burleigh, and was first used on the Hoosac tunnel. In Europe Mr. Cavé patented a percussion drill in France in October, 1851. This invention was soon followed by several others; but it was not until Sommeiller’s drill, patented in 1857 and perfected in 1861, was used on the Mont Cenis tunnel, that the problem of the percussion drill was practically solved abroad. Since this time numerous percussion drill patents have been taken out in both America and Europe. A percussion drill consists of a cylinder, in which works a piston carrying a long piston rod, and which is supported in such a manner that the drill clamped to the end of the piston rod alternately strikes and is withdrawn from the rock as the piston reciprocates back and forth in the cylinder. Means are devised by which the piston rod and drill turn slightly on their axis after each stroke, and also by which the drill is fed forward or advanced as the depth of the drill-hole increases. The drills of this type which are in most common use in America are the Ingersoll-Sergeant and the Rand. There are various other makes in common use, however, which differ from the two named and from each other chiefly in the methods by which the valve is operated. All of these drills work either with direct steam pressure or with compressed air. Workable percussion drills operated by electricity are built, but so far they do not seem to have been able to compete commercially with the older forms. No attempt will be made here to make a selection between the various forms of percussion drills for tunnel work, and for the differences in construction and the merits claimed for each the reader is referred to the makers of these machines. All of the leading makes will give efficient service. It goes almost without saying that a good percussion drill should operate with little waste of pressure, and should be composed of but few parts, which can be easily removed and changed.