The boring tool shown in Figs. 163, 164, which was employed for boring a well of 19 inches diameter, weighs 3/4 ton, and is liberated by reaction, by the arrangement shown in Figs. 170 to 173; and the same mode of liberation was applied in the first instance to the larger tool, shown in Figs. 166 to 169, employed in sinking a well of 47 in. diameter at Butte-aux-Cailles. The great weight of the latter tool, however, amounting to as much as 3-1/2 tons, necessitated so violent a shock for the purpose of liberating the tool by reaction, that the boring rods and the rest of the apparatus would have been damaged by a continuance of that mode of working; and M. Dru was therefore led to design the arrangement of the disengaging rod for releasing the tool, as shown in Figs. 174, 175. In this case the cross-guide G fixed upon the tool is made with an eye for the disengaging rod M to work through freely. For borings of small diameter, however, the disengaging rod cannot supersede the reaction system of liberation, as the latter alone is able to work in borings as small as 3-1/4 inches diameter; and a bore-hole no larger than this diameter has been successfully completed by M. Dru with the reaction tool to a depth of 750 feet. The boring rods employed are of two kinds, wrought-iron and wood. The wood rods seen in Figs. 159, 179, are used for borings of large diameter, as they possess the advantage of having a larger section for stiffness without increasing the weight; and also when immersed in water the greater portion of their weight is floated. The wood for the rods requires to be carefully selected, and care has to be taken to choose the timber from the thick part of the tree, and not the toppings. In France, Lorraine, or Vosges, deals are preferred. [Illustration: Lantern. Fig. 179.] [Illustration: Boring Rod Sockets. Figs. 180-182.] The boring rods, whether of wood or iron, are screwed together either by solid sockets, as in Fig. 181, or with separate collars, as in Figs. 180, 182. The separate collars are preferred for the purpose, on account of being easy to forge; and also because, as only one half of the collar works in coupling and uncoupling the rods, while the other half is fixed, the screw-thread becomes worn only at one end, and by changing the collar, end for end, a new thread is obtained when one is worn out, the worn end being then jammed fast as the fixed end of the collar.