iciently well to stop
eddy currents--at all events, sufficiently for ordinary practice. An
appreciable advantage is that slate may be purchased at a reasonable
rate in large slabs of any desired thickness. It is generally cut in
the laboratory by means of an old cross-cut saw, but it does not do
much damage to a hard hack saw such as is used for iron or brass.

Marble. According to Holtzapffell, marble may be easily turned by
means of simple pointed tools of good steel tempered to a straw
colour. The cutting point is ground on both edges like a wood-turning
tool, and held up to the work "at an angle of twenty or thirty
degrees" (?with the horizontal). The marble is cut wet to save the
tool. As soon as the point gets, by grinding, to be about one-eighth
of an inch broad it must either be drawn down or reground; a flat
tool will not turn marble at all.

A convenient saw for marble is easily made on the principle of the
frame saw. A bit of hoop iron forms a convenient blade, and is
sharpened by being hammered into notches along one edge, using the
sharp end of a hammer head. The saw is liberally supplied with sand
and water--or emery and water, where economy of time is an object.
The sawing of marble is thus really a grinding process, but it goes on
rapidly. Marble is ground very easily with sand and water; it is
fined with emery and polished with putty powder, which, I understand,
is best used with water on cloth or felt. As grinding processes have
already been fully described, there is no need to go into them here.
I have no personal knowledge of polishing marble.

Sec. 116. Conductors.

The properties of conductors, more particularly of metals, have been
so frequently examined, that the literature of the subject is
appallingly heavy. In what follows I have endeavoured to keep clear
of what might properly appear in a treatise on electricity on the one
hand, and in a wiring table on the other. The most important work on
the subject of the experimental resistance properties of metals has
been done by Matthieson, Phil. Trans. 1860 and 1862, and British
Association Reports (1864); Callender, Phil. Trans. vol. clxxiii;
Callender and Griffiths, Phil. Trans. vol. clxxxii; The Committee
of the British Association on Electrical Standards from 1862 to
Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi. (1893);

Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p.
838; Feussner and St.