s. Now, in the magnet-house, we see light and chemistry doing
the tasks before performed by human labor; and doing them more
faithfully than even the most vigilant of human eyes and hands. Around
the magnets are cases of zinc, so perfect that they exclude all light
from without. Inside those cases, in one place, is a lamp giving a
single ray of prepared light, which, falling upon a mirror soldered to
the magnet, moves with its motions. This wandering ray, directed toward
a sheet of sensitive photographic paper, records the magnet's slightest
motion! The paper moves on by clock-work, and once in four-and-twenty
hours an assistant, having closed the shutters of the building, lights a
lantern of _yellow glass_, opens the magnet-boxes, removes the paper on
which the magnets have been enabled to record their own motions, and
then, having put in a fresh sheet of sensitive paper, he shuts it
securely in, winds up the clock-work, puts out his yellow light, and
lets in the sunshine. His lantern glass is yellow, because the yellow
rays are the only ones which can be safely allowed to fall upon the
photographic paper during its removal from the instrument, to the dish
in which its magnetic picture is to be _fixed_ by a further chemical
process. It is the blue ray of the light that gives the daguerrotypic
likeness--as most persons who have had their heads off, under the hands
of M. Claudet, or Mr. Beard, or any of their numerous competitors in the
art of preparing sun-pictures, well know.

Since the apparatus of Mr. Brooke for the self-registration of the
magnetic changes has been in operation at Greenwich, the time of Mr.
Glaisher and his assistants has been more at liberty for other branches
of their duties. These are numerous enough. Thermometers and barometers
have to be watched as well as magnets. To these instruments the same
ingenious photographic contrivance is applied.

The wooden building next to the magnet-house on the southwest contains a
modification of Mr. Brooke's ingenious plan, by which the rise and fall
of the temperature of the air is self-registered. Outside the building
are the bulbs of thermometers freely exposed to the weather. Their
shafts run through a zinc case, and as the mercury rises or falls, it
moves a float having a projecting arm. Across this arm is thrown the ray
of prepared light which falls then upon the sensitive paper. Thus we see
the variations of the needle and the variations in heat and cold