If they do not 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.
[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.

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
[hipho_20.gif]
of a piece of platinized silver, A, on the top of which is