The system applied by Dru is worthy of attention, not so much on account of the novelty of the invention, or of any new principle involved in it, as on account of the contrivances it contains for the application of the tool, "_à chute libre_," or the free-falling tool, to Artesian wells of large diameters. It has been already explained that under Kind's arrangements the trepan was thrown out of gear by the reaction of the water which was allowed to find its way into the column of the excavation; but that it is not always possible to command the supply of the quantity necessary for that purpose; and even when possible, the clutch Kind adopted was so shaped as to be subject to much and rapid wear. Dru, with a view to obviate both these inconveniences, made his first trepan similar to that shown in Fig. 101, in which it will be seen that the tool was gradually raised until it came in contact with the fixed part of the upper machinery, when it was thrown out of gear. The bearings of the clutch were parallel to the horizontal line, and were found in practice to be more evenly worn, so that this instrument could be worked sometimes from eight days to fourteen days without intermission; whereas, on Kind's system, the trepan was frequently withdrawn after two days' or three days' service. We take the following complete account of the system from a paper read by M. Dru at the Conservatoire des Arts et Métiers, Paris, 6th June, 1867. It will be seen from Figs. 158, 159, that the boring rod A is suspended from the outer end of the working beam B, which is made of timber hooped with iron, working upon a middle bearing, and is connected at the inner end to the vertical steam cylinder C, of 10 inches diameter and 39 inches stroke. The stroke of the boring rod is reduced to 22 inches, by the inner end of the beam being made longer than the outer end, serving as a partial counterbalance for the weight of the boring rod. The steam cylinder is shown enlarged in Fig. 160, and is single-acting, being used only to lift the boring rod at each stroke, and the rod is lowered again by releasing the steam from the top side of the piston; the stroke is limited by timber stops both below and above the end of the working beam B. The boring tool is the part of most importance in the apparatus, and the one that has involved most difficulty in maturing its construction. The points to be aimed at in this are,--simplicity of construction and repairs; the greatest force of blow possible for each unit of striking surface; and freedom from liability to get turned aside and choked. [Illustration: Elevation showing Beam connected to Rods, Fig. 159.] [Illustration: Steam Cylinder. Fig. 160.] [Illustration: Single Chisels. Figs. 161, 162.]