_Drill Mountings._--For tunnel work the general European practice is to mount power drills upon a carriage moving on tracks in order that they be easily withdrawn during the firing of blasts. Connection is made with the steam or compressed air pipes by means of flexible hose which can easily be attached or detached as the drill advances or when it is moved for repairs or during blasts. Two, four, and sometimes more drills are mounted and work simultaneously on a single carriage. In America it has been found that column mountings have been more successful for tunnel work than any other form. The column mounting made by the Ingersoll-Sergeant Drill Co. is shown in Fig. 12. In using this form of mounting no tracks or other special apparatus is required; it is not necessary, as is the case with the carriage mounting, to remove the débris before resuming operations, but as soon as the blasting has been finished and the smoke has sufficiently disappeared the column can be set up and drilling resumed. [Illustration: FIG. 12.--Column Mounting for Percussion Drill: Ingersoll-Sergeant Drill Co.] _Rotary Drills._--Rotary drilling machines, or more simply rotary drills, were first used in 1857 in the Mont Cenis tunnel. The advantages claimed for rotary drills in comparison with percussion drills are: (1) That less power is required to drive the drill, and the power is better utilized; (2) once the machines work easily they do not require continual repairs, and (3) in driving holes of large size the interior nucleus is taken away intact, thus reducing work and increasing the speed of drilling. Rotary drills are extensively used for geological, mining, well-driving, and prospecting purposes; but they are very seldom employed in tunnels in America, although successfully used for this purpose in Europe. The reason they have not gained more favor among American tunnel builders is due to some extent perhaps to prejudice, but chiefly to the great cost of the machine as compared with percussion drills, and to the expense of diamonds for repairs. Those who advocate these machines for tunnel work point out, however, that under ordinary usage the diamonds have a very long life,--borings of 700 lin. ft. being recorded without repairs to the diamonds. [Illustration: FIG. 13.--Sketch of Diamond Drill Bit.]