7 
. 
ON THE KEW OBSERVATORY. 85 
I (Plates I. to IV.) is the pulley ov the hour-arbour of the time-piece. 
i! the gut line suspending H. 7° is the counterpoise to H. 
Kis the time-piece, with its weights, pendulum, &c., and a lever with fork, 
k2, for stopping and starting the clock at any given second. 
R} is the support of K and F, 
k? are brass tubular braces. 
P® and PS are stone pillars, whose common centres are in the mean mag- 
netic meridian (about). 
P¥ and P™ are stone pillars, whose common centres are at right angles to 
the magnetic meridian (quas?). 
Q are stone brackets fixed in P® and P for the support of V. 
R is a cross slab of stone, resting on P® and PW’. 
v\ is the cross piece of mahogany (used in Dr. Lloyd’s arrangement), 
secured firmly, with-means of adjustments, upon R by bolts and nuts, r* 
S is the torsion apparatus (of plate, &c.) (Dr- Lloyd’s). 
> s° the suspending wire, passing round the grooved wheel. 
s°, on the axis of which 02 rests “ by inverted Y*.” 
T the glass tube resting on é', which is a fillet contained in é, which is a 
neck or brass tube attached to V. 
_ X is a black marble slab, carrying A, #!, &c., and supported upon P% 
and PS very firmly, but admitting of a small adjustment (on occasion) about 
the common axis of the suspending wires, s°. 
Y (Plate IV. fig. 2) is the silvered plate (in the scale board). 
y} is the magnetic curve produced by the focus of the slit in the moveable 
shield (6'). 
y? is the zero line produced by the focus of the slit in the fixed shield (o'). 
It will be easily perceived that in the arrangement which has now been 
described no hygrometric expansions and contractions can have sensible effect 
upon the required result, and J believe that thermometric variations are 
equally unappreciable. 
The scale board (Plate IV. figs. 2 and 3), for measuring off rapidly and 
correctly ordinates formed by the magnetic or other curve with the zero 
line, is thus constructed :— 
_ Ais a mahogany board. 
a, &c. are four screws attaching it to 
B, which is another heavier board, and which it is well to clamp upon a 
sloping desk. 
_ Cis a ruler attached to B by a screw at each end, passing easily through 
an oblong aperture, and allowing a lateral free motion of the ruler upon B. 
A blank ivory scale is fixed upon C. 
eis a milled-headed screw, acting by its shoulder upon a piece which 
presses C inwards, or against the right-hand edge of Y. 
_ © and ¢% are screws passing through another ruler, 
M, &c., fixed immoveably upon B, and acting by their ends upon two little 
brass sliders which press upon the left-hand edge of Y. This fixed. ruler 
™ carries a scale of white metal, upon which divisions, representing hours, 
half-hours, quarters and five minutes, are engraved, a length of one inch 
Tepresenting one hour (for the slider H in the case F is moved by the clock 
at a rate corresponding with these values). Two spiral springs are con- 
tained in B, which cause the two sliders pressing on Y to resume their nor- 
‘mal positions when c? and ¢$ are not employed. 
_ Tis the ebony stock of the T square. 
|  @ isits blade of white metal, upon which is engraved on one of the fiducial 
| edges divisions representing fiftieths of an inch, and on the other sixtieths, 

