r grit, and are then
coated. They afterward pass over a series of accurately leveled wheels
running in a tank of water kept exact by an automatic regulator at a
temperature of from 80 deg. Fah. to 100 deg. Fah., by means of a small
hot water circulating system. The emulsion trough is jacketed with hot
water at a constant temperature. This trough is silver plated inside,
because most metals in common use would spoil the emulsion by chemical
action. The trough is 16 in. long; it somewhat tapers toward the
bottom, and contains a series of silver pumps shown in the cut; the
whole of this series of pumps is connected with one long adjustable
crank when plates of the largest size have to be coated; when coating
plates of smaller sizes some of the pumps are detached. A chief object
of the machine is to deliver a carefully measured quantity of emulsion
upon each plate, and this is done by means of pumps, in order that the
quantity of emulsion delivered shall not be affected by changes in the
level of the emulsion in the trough; the quantity delivered is thus
independent of variations due to gravity or to the speed of the
machine. These pumps draw the emulsion from a sufficient depth in the
trough to avoid danger from the presence of air bubbles, and the
bottom of the trough is so shaped that should by chance any
sedimentary matter be present, it has a tendency to travel downward,
away from the bottoms of the pumps. There is a steady flow of emulsion
from the pumps to the delivery pipes, then it passes down a guide
plate of the exact width of the plate to be coated. Immediately in
front of the guide plate is a fixed silver cylinder, kept out of
contact with the plate by the thickness of a piece of fine and very
hard hempen cord, which can be renewed from time to time. These cords
keep the cylinder from scraping the emulsion off the plate, and they
help to distribute it in an even layer. There would be two lines upon
each plate where it is touched by the cords, were not the emulsion so
fluid as to flow over the cut-like lines made and close them up.

[Illustration: Fig. 9.]

The silver cylinder to a certain extent overcomes the effects of
irregularities in the glass plates, for the cylinder is jointed
somewhat in the cup and ball fashion, and is made in two or more
parts, which parts are held together by lengths of India rubber.

The arrangement is shown in section in Fig. 9, in which A is the hot
water jacket of the emulsion