If the whole length of the boring rod were allowed to fall suddenly to the bottom of a large bore-hole at each stroke, frequent breakages would occur; it is therefore found requisite to arrange for the tool to be detached from the boring rod at a fixed point in each stroke, and this has led to the general adoption of _free-falling tools_. M. Dru's plan of self-acting free-falling tool, liberated by reaction, is shown in side and front view in Figs. 170 to 173. The hook H, attached to the head of the boring tool D, slides vertically in the box K, which is screwed to the lower extremity of the boring rod; and the hook engages with the catch J, centred in the sides of the box K, whereby the tool is lifted as the boring rod rises. The tail of the catch J bears against an inclined plane L, at the top of the box K; and the two holes carrying the centre-pin I of the catch, are made oval in the vertical direction, so as to allow a slight vertical movement of the catch. When the boring rod reaches the top of the stroke, it is stopped suddenly by the tail end of the beam B, Fig. 159, striking upon the wood buffer-block E; and the shock thus occasioned causes a slight jump of the catch J in the box K; the tail of the catch is thereby thrown outwards by the incline L, as shown in Fig. 172, liberating the hook H, and the tool then falls freely to the bottom of the bore-hole, as shown in Fig. 173. When the boring rod descends again after the tool, the catch J again engages with the hook H, enabling the tool to be raised for the next blow, as in Fig. 171. [Illustration: Self-acting Free-falling Tools. Figs. 174-178.]