factory, but when once a
wheel is well armed it will work for a long time without further
attention.

Sec. 77. Cutting a Section.

A wheel 7 inches in diameter may be rotated about 500 times per
minute, and will give good results at that speed. The work, as will
be seen from the diagram, is pressed against the edge of the wheel by
a force, which in the case quoted was about the weight of eleven
ounces. This was distributed along a cutting arc of three-quarters of
an inch.

A convenient cutting lubricant is a solution of Castile soap in water,
and this must be freely supplied; if the wheel gets dry it is almost
immediately spoiled owing to the diamond dust being scraped off. In
the figure the lubricant is supplied by a wick running into the
reservoir. I have used both clock oil and ordinary gas-engine oil as
lubricants, with equally satisfactory results. As to the speed of
cutting, in the experiment quoted a bit of rather friable "gabbro,"
measuring three-quarters of an inch on the face by five-eighths of an
inch thick, was cut clean through in six minutes, or by 3000 turns of
the wheel. The travel of the edge was thus between 5000 and 6000
feet, or say 9000 feet, nearly 2 miles, per inch cut.

A good solid rock, like basalt, can be cut into slices of about 3/32
inch thick. A very loose rock is best boiled in Canada balsam, hard
enough to set, before it is put against the wheel.

Instead of a grinding machine a lathe may be employed. The disc is,
of course, mounted on the mandrel, and the work on the slide-rest.
The latter must be disconnected from its feed screws, and a weight
arranged over a pulley so as to keep the work pressed against the
wheel by a constant force.

It may, perhaps, occur to the reader to inquire whether any clearance
in the cut is necessary. The answer is that in all probability, and
in spite of every care, the wheel will wobble enough to give
clearance. If it does not, a little diamond dust rubbed into the side
of the wheel, as well as the edge, will do all that is required. The
edge also, after two or three armings, "burrs" a little, and thus
provides a clearance naturally. It is not unlikely that in the near
future the electric furnace will furnish us with a number of products
capable of replacing the diamond as abrading agents. The cost of the
small amount of diamond dust; required in a laboratory is so small,
however, that it; is doubtful whether any appreciable economy w