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144 Carbonado.
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Carbonado, in operations for gold-mining and other purposes, has caused it to rise in price from 2s. 6d. to £8 or even to .£10 per carat.
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THE DIAMOND DRILL.
If
steel is used to cut rocks, a percussive and not a rubbing or cutting
motion must be given to it ; otherwise,, owing to its deficient
hardness, the steel itself would be worn away rather than the rock, as
popularly exemplified in the ordinary grindstone. Where deep borings
for exploring purposes have to be made, it becomes a difficult
mechanical problem to construct a machine, which shall satisfactorily
impart a striking motion to a heavy steel tool. Moreover,, the
difficulties and chances of failure increase very rapidly with the
distance from the surface, whereas, with a continuous rotatory motion,
it is comparatively easy to bore to any depth. Borings have been
effected to a depth of several thousand feet with the aid of carbon,
not much greater difficulty being experienced at the end than at the
commencement of the operations.
Selected
pieces of carbon are firmly embedded in a ring of steel, called a
crown, of the size corresponding to that of the perforation which it is
desired to bore. This ring is screwed to a series of long hollow tubes,
which are lengthened as the work proceeds ; these tubes or rods are
kept rotating by steam power, and their weight is so adjusted, that a
pressure of half a ton can be brought to bear on the crown, when it is
boring a 4-in. hole in native or living rock. The crown rotates some
250 times a minute, and water is continually pumping through the hollow
rods, passing under the cutting face of the crow.i, to keep the
Diamonds cool, and to wash off and upward
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