Oscillations of the Horizontal Needle. 5 
means of small air-tight collars of brass, and an intervening pipe, 
as shewn in Figs. 2 and 4. There is a small gauge-plate g, Fig. 
2, connected with the pipe leading to the barrels, which may be 
occasionally employed in particular experiments. 
7. By means of the forked lever //, Figs. 2, 3, 4, the suspended 
bar can be deflected, and steadily arrested at any given angle, or 
otherwise set free, at any instant of time. This method of arrest- 
ing the bar has a great advantage ; for the forked portions of the 
lever operating on opposite sides and ends of the bar, preserves 
it completely in a state of rest ; so that, when set free, an extraor- 
dinarily steady vibration is obtained, provided the adjustments 
of the various points are accurately made. To effect this, the 
forked portions //, Fig. 4, are independent of each other, and are 
moveable with friction in a tube of brass. 
8. The bar is suspended in the usual way, by filaments of un- 
spun silk, attached to a vertical rod of brass, which can be raised 
or depressed by means of a screw, through certain small distances. 
A gauge of an extremely simple kind, 4, Fig. 3, for shewing the 
state of the exhaustion, is attached to the wood frame /’, and 
there is a thermometer similarly placed on the opposite side, as 
seen in Fig. 2. 
9. The magnetic apparatus is enclosed in appropriate receivers ; 
and when the exhaustion is sufficiently complete, the air-pump, 
together with the block to which it is fixed, may be removed, if 
requisite, by unscrewing the connecting joint at 7, Fig. 2, and the 
four-milled head-screws which confine the block to the oblong 
plane P. 
10. The are of vibration is observed by means of graduated 
cards. There are two of these, mq, np, Fig. 3, placed one over 
the other, either of which may be used, according to the nature 
of the experiment. The lower one, mq, is a wide plane ring, 
having several graduated concentric circles on it ; it corresponds 
to the surface of the wood block g, and has a large central hole 
for the forked lever above described (7) to pass clear. The up- 
