84 REPORT—1849. 
B is a fifteeninch magnet, belonging to a bifilar magnetometer of Dr. 
Lloyd's construction. 5° is its stirrup. 5° a pair of light brass tubes, con- 
nected with 5? by entering a short tube attached to b*, and permitting a 
horizontal adjustment (of 4). The counterbalancing ball at one end is also 
adjustible (for poising 4° properly). 
6! (figs. 1, 3, 4, 5) is the moveable shield, composed of very light sheet 
brass, curved and attached to alittle tube, which is clamped by a peculiar nut 
and screw to the end of 48. It has a very narrow slit at its lower edge in 
the centre. R 
b+ is the usual copper damper, the upper and lower central portions being 
formed into curves for the free “play” of 6? and 03. 65 6° are its supports. 
O is a diaphragm plate, whose aperture is about an inch long horizontally 
and a quarter of an inch wide. It carries 
o!, which is the féwed shield, similar in form to 6', and attached to O by 
means of a little bolt, washers and nut (0%). It is capable of adjustments for 
horizontality, height, &e. At about three-eighths of an inch from its centre 
is a slit, somewhat larger than the slit in 6'. This shield stands at a distance 
of about a tenth of an inch from 0!, and at about one-fortieth of an inch 
higher than 0'. 
C is a glass plate admitting light into the camera. It has in front a small 
brass sliding-shutter. 
D (Plates I. and III.) is a lamp constructed on Count Rumford’s poly- 
flame principle of three flat wicks raised and lowered by rack-work. 
d' is its high squared copper chimney, provided with a glass plate about 
three-quarters of an inch high placed opposite to the best part of the flame 
(or flames). 
E is the mouth, consisting of two angular pieces (as seen in Plate V. 
figs. 1 and 2), and of two little plates attached to them, forming the lips and 
aperture e!, which aperture can be diminished or increased at pleasure after — 
relaxing the little nuts of screws which pass through oblong slits cut 
through a’. 
A horizontal aperture, of about a quarter of an inch broad, cut through 
a’, admits the light which forms the focus at e! of the moveable slit (in 5"), 
and a little vertical aperture in a' admits the focus of the fixed slit in o'. — 
The magnetic curve and zero line are produced by these foci respectively. 
F is the slider case. for receiving the sliding frame. ‘9 
f2 is a perfectly true ruler of brass, attached vertically to a’ by means of 
three screws passing thiough it, through three thick washers (or little pillars), 
and through three oblong slits in @!, &c. It is capable of adjustments for 
perpendicularity, &c. 
Jf? is a roller spring, attached to a', and acts upon the slider frame side- 
wise, pressing it gently against the ruler. ; 
J? is a pair of similar springs, acting upon the frame in front, and pressing 
the glass in the frame against the mouth. 
G is the lens tube, containing two groups of Ross’s achromatic lenses. 
nA prea sed (of sliding plates, &c.) for the support and due centring 
oO > KC. ; 
- g® is apparatus of studs, pinion, milled head, key, &c. for moving the rod 
P-s ee 5 attached to the stud at g*, and serves for the adjustments to — 
focus (of G). 
H is the sliding frame suspended in F. h' are the spring bars for re- 
taining the plates, either metallic or glass, in their proper places. A? are — 
friction rollers. 3 is a hook with a screw in it, which clamps the gut line, 
entering a hole in the top of H. ip 

