Kingdom.]

AND SURGICAL INSTRUMENTS.

North, North Central, and South Central Galleries.

423

Ponging from the four corners of a marble slab (which,rien in actual operation, would be cemented on the topa stone pillar firmly fixed in the ground, and insulated>°m the floor of the observatory): these tubes, aboutfeet long, converge alternately to four points of the°rsion plate; they thus compose a framework possessing" leat stiffness. To the suspension frame of each magnet,Plane glass mirror and a concave metallic speculum arelached. The plane mirror is for the purpose of makingye-observations with a telescope in the usual manner,j Sas-light or lamp is so placed at a distance.of about twoset in f r0 nt of each speculum, that an image of a small^ I In the Conner chimnev Rurmunflinfr the burn

the copper chimney surrounding the burner mayon the sensitive paper attached to—

, The registering apparatus. This is placed midway'-tween 1 and 2, and consists of a stand supporting hori-' n| ta!ly on friction rollers two concentric glass cylinders,0l dul the inner of which is wrapped a sheet of prepared

! photographic paper : the outer or covering cylinder keepsthe paper moist during the 24 hours it remains in action.A bent arm, attached to the axis of these cylinders, is carriedround by a fork at the end of the hour-hand of a time-piece specially constructed for the purpose. The horizontalmotion of the tracing point of light, combined with thevertical motion of the paper, traces out the magneticcurve, which, when the paper is removed from thecylinder, is developed and fixed by the usual photo-graphic processes. A third light is attached to the regis-tering apparatus, for the purpose of drawing a standardor base line on the paper ; by the varying distance of anypoint of the magnetic curve from this line, the magneticvariation is determined. At the distance at which theseinstruments are placed, an angle of ] ° is represented by2 inches on the paper; but the scale valve may be enlargedat pleasure, by placing them further apart. This instru-ment is shown in fig. 1.

Brooke’s Self-registering Magnetometer.

the declination magnet.q’ a c oncave speculum attached to the magnet.f or ’ a plane glass mirror also attached to the magnet,Who„ *’ n £ observations by a telescope, on the old method,

j^ n Required.

p.’ tae torsion plate, reading to minutes by two verniers,cap’, ? frame standing upon the torsion plate. A hooktoW* being raised or lowered by a screw, is attachedsk e j ls frame, from which the magnet is suspended by ajjpp untwisted silk fibres.

da» es ’ a glass box, in which the magnet and its appen-sarne are enclosed, to protect them from the air; for thetub e «? ur P°? e > the suspension skein is enclosed in a glassof t jj e j^hich passes through a stuffing box H, in the lid

n° iit h S t as ' buriler enclosed in a brass chimney, from whichthrom,}? cau escape, except a small pencil which passesScrew . a narrow slit K, capable of being adjusted by ateciej ?? the breadth of this slit, the breadth of thefine depends.

peiieil’ a , c ' )m bination of two plano-convex lenses. Thelight passing through K, falls on the mirror B,

and is. reflected to the cylindrical lenses; by these, theimage of the slit is condensed to a point of light on thesurface of

MM, the registering apparatus, consisting of two con-centric cylinders, between which the photographic paperis placed.

N, the magnetic curve traced by the point of light.

O, a gas-burner, fixed to the stand on which the cylin-ders rest.

P, a plano-convex prismatic lens, attached to the top of

QQ, an opaque box, which protects the photographic

paper from extraneous light. A pencil of light from Opasses through P, and is brought to a focus on the surfaceof the paper.

R, the base line, described by the point of light.

SS, the bifilar, or horizontal force magnetometer.

TT, the apparatus for producing an automatic tempera-ture compensation ; this consists of two zinc tubes, whichare clamped to a glass rod by two adjustible clamps VV,the suspension skein passes over a pulley X, and the endsare attached to two hooks WW; as the temperature rises,these hooks are approximated to each other by a quantity