OBSERVATORY AT WEST POINT. 195 
tangent screw of the hour circle is attached. ‘The arrangement of the tangent screw is 
shown at (39) in Fig. T. In this figure (44) is a circular plate, seventeen inches in 
diameter, with an aperture in the centre large enough to receive the polar axis, to which 
it is firmly attached by a key bolt (43). In the circumference of this circle is a groove 
which receives the friction band with its clamp screw (42), and to this band is attached 
the arm (46) which carries the nut of the tangent screw (39). Against the upper surface 
of this circle, the sectoreal arm (40), represented in Fig. /, is held by a small truncated 
conical plate (45) Fig. 7’, which fits into a chamfered aperture (47) in the arm, and is 
securely attached by screws to the clamp circle. When the circle is clamped, and the 
teeth of the sector in gear, the hour circle can move only with the clock, or by the turns 
of the tangent screw; but when unclamped, the hour circle is moved freely by the hand 
applied to the telescope, while the sector is in connexion with the clock; and the instant 
the hand is removed, the hour circle takes up the clock movement, in consequence of the 
friction between the surface of the clamp circle and sector. This device is very conve- 
nient, as it saves the labour and time of unclamping and clamping in transferring the 
telescope from one object to another. The axis (48), Fig. S, of the endless screw, passes 
through a circular aperture in the centre of the wheel (49), and the arrangement is such, 
that by means of a small pin, the wheel and axis may be firmly united or rendered inde- 
pendent of each other, so that when the sector is exhausted by the motion of the clock, 
it is renewed by throwing the wheel out of gear with the axis, and reversing with the 
hand, by means of a winch-key, the motion of the endless screw. 
The rate of the clock is regulated by a centrifugal governor, the balls of which are 
attached to the ends of a horizontal piece, at the top of the axis of motion, and when at 
rest are supported by a second cross piece (51). Connected with the supporting rods of 
the balls, are two brass arms that carry each a small box-wood screw (52), whose axis is 
perpendicular to a horizontal friction plate (53), with which they are brought in contact 
by the recession of the balls from the axis during the motion. By unscrewing these, the 
rotation is accelerated,—by screwing them up, retarded. The whole instrument is placed 
upon a solid block of granite, of the shape shown in Figs. S and TJ, to which it is 
attached by heavy screw bolts sunk into the inclined surface (60), which is parallel to the 
equator, and secured by molten sulphur. The surface (61) is horizontal. 
The telescope, by Lerebours of Paris, is a refractor whose solar focal distance is eight 
feet and aperture six inches. It has a fine position micrometer (56), furnished with an 
illuminating apparatus for bright lines and dark field, the lines being illuminated on 
both sides. The diameter of the position circle is inches; the graduation is 
on a silver band, and reads by the aid of two verniers as low as thirty seconds. The 
micrometer is also by Grubb. (57) is a small finder, and (58) a mass of lead attached to 
the cradle to counterpoise the telescope. 
ADJUSTMENTS. 
The stone block upon which the instrument rests having been carefully cut to a frame 
prepared for the stone-cutter, was put in place by the aid of a small theodolite, whose 
axis of collimation was carefully adjusted by Polaris to the meridian. A well defined 
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