ge holes the striking weight is, say, 800 to 1000 lb., length
of stroke 2-1/2 to 5 ft., and speed from 20 to 30 strokes per minute.

[Illustration: FIG. 6. Kind Free-Falling Tool.]

By using the sliding link the cross-section and weight of the rods may
be greatly reduced, the only strain being that of tension. To deliver
a sharp, effective blow, however, the rods must drop with a quick
stroke, which brings a heavy strain upon the operating machinery. For
overcoming this difficulty, various "free-falling tools" have been
devised. By these the bit is allowed to fall by gravity; the rod
follows on its measured down stroke, and picks up the bit.
Free-falling tools are of two classes: (1) those by which the bit is
released automatically; (2) those operated by a sudden twist imparted
to the rod by the drillman. One of the best known of the first class
is the Kind free-fall (fig. 6). The shank of the bit is gripped and
released by the jaws J, J, worked through a toggle joint by movements
of the disk D. When the rod begins its downward stroke, the resistance
of the water in the hole slightly raises D, thus opening the jaws and
releasing the bit, which falls by gravity. On reaching the end of the
stroke the jaws again catch the shank of the bit and raise it for
delivering another blow. The Fabian free-fall may be noted as an
example of the second class (see Kohler, _Lehrbuch der Bergbaukunde_,
p. 57). Tools are sometimes used for cutting an annular groove in the
bottom of the hole, and raising to the surface the core so formed, for
observing the character of the rock.

4. _Rope and Drop Tools._--This method was long ago used in China.
Because of its extensive application in the oil-fields it is generally
designated in the United States as the "oil-well system." In its
various modifications it is often employed also in general prospecting
of mineral deposits and in sinking artesian, natural gas and salt
wells. One of its forms is known in England as the Mather & Platt
system.

The chief point of difference from rod-boring is the substitution of
rope for the jointed rods. For deep boring it possesses the advantage
of saving the large amount of time consumed in raising and lowering
the rods, as required whenever the hole is to be cleaned out, or a
dull bit replaced, since the tools are rapidly run up or down by means
of the rope with which they are operated