METEOROLOGY. 204 
e is a silk thread to which. the magnet is suspended. This thread rises 
eight feet nine inches high, passes over two pulleys, f and g, and is attached 
by a piece of leather to the small reel, 2, which, by the aid of a catch, 
permits the magnet to be elevated or lowered ; 7 is a copper hoop, serving 
to restrain the oscillations of the magnet. The whole apparatus is placed 
on a metal stand, which rests on a particularly firm foundation. On the 
cross-arms is a rectangular box, coated inside and out with gold paper, in 
which the magnet swings freely. 
The horizontal magnet is represented from the south-west in fig. 23. 
Here a is the magnet, ah the mirror attached, c the circle of rotation, d five 
pairs of small pulleys, e, e, two silk suspension threads passing from the 
uppermost pair of pulleys to two pulleys, f, placed seven feet nine inches 
higher; thence they proceed over the pulley g, and finally to a larger pulley 
not shown in the figure. A catch wheel is attached to this latter pulley, its 
winch being shown at h. The magnet is similar in dimensions to the one 
already described, with a similar stand, eleven feet five inches high, and is 
surrounded by a copper hoop, 7. Like the preceding it swings in a double 
box, whose south side consists partly of plate glass. A scale is attached to 
the wall of the eastern wing, about eight feet five inches south of the magnet, 
and is sighted by a telescope directed to the mirror, b, of the magnet. The 
magnet is placed in a direction perpendicular to the magnetic meridian, 
being held there by the tension of the threads, e, e. It strives continually to 
place itself in the direction of the magnetic meridian, turning the two 
threads, sometimes with greater, sometimes with less force, so that in conse- 
quence of the reflection from the mirror 3, different numbers on the scale are 
constantly appearing. 
The vertical magnet is seen in fig. 22. Here a is the magnet, b the 
mirror with adjusting screw placed on it, c a knife edge on which the bar 
turns, d one of the two agate plates on which c rests, e, e, screws by which 
the centre of gravity of the magnet and its inclination may be altered, f the 
bronze support on which the magnet rests. The latter is of like dimensions 
with the preceding, and is placed on a similar stand. 
Pl. 27, fig. 25, exhibits the electrical apparatus placed in the window of 
the antechamber. In this, a is the hook which effects the connexion of the 
conducting wire with the apparatus, 5 is a screen covering the opening in 
the window, and through which there passes a vertical rod sustaining the 
apparatus, cc is a double truncated glass cone, fastened to the upper glass 
frame on each side by beds of brass, d, d, are lamps for keeping this glass cone 
constantly dry, e is a nut inclosing the glass cone, and carrying the hollow 
copper cylinder, gg, by means of the vertical arm, f. From this cylinder 
there pass out eight lateral arms, in which conducting rods can be moved 
freely up and down, and fixed at any part by screws. At h there is seen a 
Bohnenberger gold leaf electrometer, 7 is a galvanometer for determining 
the electric currents in the atmosphere, & is an instrument for measuring the 
length of the electric spark, / is a second dry pile apparatus, similar to h, but 
less sensitive, mm straw electrometers for determining electric changes in the 
atmosphere. 
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