t differ, or there is only four or five degrees variation,
the atmosphere of the room is very moist and means should be taken to
expel the superfluous quantity.

HEAD RESTS.--The button head rest with chair back clip, A fig. 19--is
much the best for travelling artists, as it can be taken apart, into
several pieces and closely packed; is easily and firmly fixed to the
back of a chair by the clamp and screw a and b, and is readily adjusted
to the head, as the buttons c, c and arms d, d are movable.

Sometimes the button rest is fixed to a pole, which is screwed to the
chair; but this method is not so secure and solid as the clip and
occupies more room in packing. Both the pole and clip, are furnished
in some cases with brass band rests instead of the button; but the only
recommendation these can possibly possess in the eyes of any artist, is
their cheapness.

[Illustration: Fig. 19 (hipho_19.gif)]

For a Daguerreotypist permanently located the independent iron
head-rest, B fig. 19, is the most preferable, principally on account of
its solidity. It is entirely of iron, is supported by a tripod (a) of
the same metal and can be elevated by means of a rod (b) passing
through the body of the tripod, to a height sufficient for a person,
standing, to rest against.

[Illustration: Fig. 20 (hipho_20.gif)]

GALVANIC BATTERY.--This article is used for the purpose of giving to
imperfectly coated plates a thicker covering of silver. The form of
battery now most universally employed for electrotype, and other
galvanic purposes, is Smee's--Fig. 20. It consists of a piece of
platinized silver, A, on the top of which is fixed a beam of wood, B,
to prevent contact with the silver. A binding screw C is soldered on
to the silver plate to connect it with any desired object, by means of
the copper wire, e. A plate of amalgamated zinc, D, varying with the
fancy of the operator from one half to the entire width of the silver
is placed on each side of the wood. This is set into a glass vessel,
P,--the extreme ends of the wood resting upon its edge--on which the
acid with which it is charged has no effect. The jar is charged with
sulphuric acid, (common oil of vitriol) diluted in eight parts its bulk
of water. The zinc plates of the battery have been amalgamated with
quicksilver, and when the battery is set into the jar of acid there
should be no action percieved upon them when the poles F, G, are not in
contact. Should any actio