I have permission to describe,
has been employed by Professor W. Ramsay. It will be sufficient if I
explain its application to the operation of graduating a tube or strip
of glass in millimetre divisions.

[17] Originally suggested by Bunsen.

The apparatus required consists of a standard metre measure,[18] divided
into millimetres along each of its edges, with centimetre divisions
between them, a ruler adapted to the standard metre, as subsequently
explained, and a style with a fine point for marking waxed surfaces.

[18] Such measures can be obtained of steel for about _fifteen
shillings_ each. They are made by Mr. Chesterman of Sheffield. They can
be obtained also from other makers of philosophical instruments, at
prices depending upon their delicacy. Those of the greatest accuracy are
somewhat costly.

[Illustration: FIG. 38.]

Fig. 38 represents the standard measure, and the ruler.

At _AA_ are the millimetre divisions on the edges of the measure, the
longer transverse lines at _BB_ are placed at intervals of five
millimetres and of centimetres. The ruler is in the form of a
right-angled triangle; it is shown, by the dotted lines, in position on
the standard metre measure at _I_; and again, with its under surface
upwards, in the smaller figure at 2. It consists of a perfectly flat
sheet of metal, about ten centimetres in length from _C_ to _C_,
sufficiently thick to be rigid, and has a ledge, _DD_ in each figure,
which is pressed against the side of the measure when using it, to
ensure that the successive positions of the edge (_LL_) shall be
parallel to each other. At _GG_ are two small holes, into which fit
small screws with fine points. These must be in a line parallel to the
edge (_LL_), so that when the ruler is in position on the scale, the
points of the two screws, which project slightly, shall fall into
corresponding cuts on the divided scales (_AA_).

To graduate a strip of glass, or a glass tube (_HH_), the surface to be
marked must first be coated with wax, which should be mixed with a
little turpentine, and be applied to the surface of the glass,
previously made _warm_ and _dry_, by means of a fine brush, so as to
completely cover it with a thin, closely-adherent, and
evenly-distributed coat of wax, which must be allowed to cool.

Fix _HH_ firmly on a table, and fix the standard measure by the side of
_HH_. If the thickness of _HH_ be about equal to, but not greater than
that of the standard