250 | PHYSICS. 
the telescope. As security against currents of air, the ‘bar hangs i in a box 
having a small aperture in its movable ‘cover to allow the passage of the 
thread, and another hole in the side opposite to the telescope for the mirror 
towards which the former is directed. Should the magnetic axis of the bar 
fall in the same vertical plane with the optical axis of the telescope, the 
image of the zero point of the scale will fall accurately in the axis or the 
cross-hair of the telescope ; if this be not the case, the image of some other 
point of the scale than the zero point will appear in the latter, and when 
the distance of the scale from the mirror is accurately determined, we can 
readily calculate from the observed parts of the scale, the corresponding 
angle made by the magnetic axis of the bar with the optical axis of the tele- 
scope, and from the latter the corresponding declination. Netther the 
inelination nor the intensity can be ascertained with the same exactness. 
See page 144 for the determination of the intensity. 
Magnetic Observatories have been erected in various parts of the earth 
since 1828, thanks to the ceaseless efforts of Humboldt, observatories in 
which uninterrupted hourly observations are made for twenty-four or 
thirty-six hours in succession, at certain epochs. : One of the most complete 
and best arranged establishments of the kind is the one in Greenwich, 
adapted also for making and registering meteorological observations. 
Pl. 27, fig. 20, represents this in ground plan, while fig. 21 presents a 
general view of the building as seen from the north. On the north side, a 
mast is erected to a height of eighty feet, intended for electrical obser- 
vations. On the right side of the drawing a ball is seen on the ground, 
which, with its lantern, is intended for induction observations, and may 
readily be drawn to the top of the mast. The small building to the left is 
intended for observations on the magnetic inclination. The box not far 
from the door of the main building contains various thermometers, and may 
be turned so as to keep constantly in the shade. The main building itself, 
built of wood exclusively, without iron, and fourteen feet high, forms a cross 
of four equal arms or wings, which are erected according to the magnetic 
meridian, and, in the clear, are twelve feet broad and ten feet high. The 
distance between the extremities of two opposite wings within the walls 
amounts to forty feet. The northern wing is separated from the middle 
space by a wall, thus forming a kind of antechamber. The letters of the 
plan indicate as follows: @ the declination magnet in the southern wing, 
b the horizontal magnet in the eastern wing, c the vertical magnet in the 
western wing, d, e, f, three telescopes for observing the three magnets, a, b, c, 
from a single point (chair), 0; g is the scale of b, h that of the vertical 
magnet, 7 a clock keeping mean time, 7 an astronomical clock, m a clock in 
the antechamber, / a barometer, n a chimney, p an alarm door bell, g a shed 
or offset for the electrical apparatus, ran opening in the roof in the direction 
of the astronomical meridian. 
The declination magnet (pl. 27, fig. 24) is a thick magnetic bar of 
hardened steel, two feet long, one inch and a half broad, and one quarter of 
an inch thick; 4 is a brass ring with two plane glasses, between which are 
cross-hairs of spider’s web ; d is the lower part of the attachment apparatus ; 
424 
