AOT) A SELECTION FEOM THE OBSEEVATIONS MADE WITH THEM. 707 
Fig. 9. The crane which carries the eastern counterpoise, on a larger scale. 
The same parts are similarly lettered in all the figures. A, the cranes which 
carry the counterpoises. B, fig. 9, a guide-wheel by which the chain is kept 
in the axis of the crane in all positions of the counterpoise. The points of 
the shaft are placed eccentrically, so that it clears the wheel B when the 
telescope passes the zenith and moves north. F is of wood plated with iron, 
and is connected by rollers with the arc E, and by a rack and pinion with 
the tube. The pinion is driven by a wheel acted upon by an endless screw 
on the bar which carries the pulley N, fig. 7. The pulley N is driven by a 
band, and porter’s wheel on the lower end of the tube, and thus the motion 
in right ascension is given. C, the principal counterpoises. D, the counter- 
poises of the stage of the fii’st gallery. The stage is raised by increasing the 
action of the counterpoises D D. This is efiected by chains attached to them, 
which pass underground to a small windlass. The counterpoises being rather 
less than the weight of the stage, it descends when the chains are relaxed. 
The stage carries the gallery G, which traverses on it in right ascension, 
motion being given to two of its wheels by a winch in the hand of the 
observer. E, an arc of cast iron made in pieces 5 feet long, not quite touching 
at the extremities, to guard against unequal expansion. Each piece was 
planed and accurately adjusted in the meridian by a transit-instrument. 
H, the second gallery. I, the third gallery. The galleries are supported by 
the beams o, Avhich are plated with hon, and they are moved in right ascen- 
sion by rack and pinion. P, tension bars to secure the iron framework carry- 
ing the wheels against which the beams press as the galleries are moved 
out. K, the cranes which carry the guy-chains, by which the counterpoises 
are constrained to move nearly in the curve of equilibrium. E, the chain 
which, passing over the pulley L, moves the telescope in polar distance. 
T, the chain of the principal counterpoise. In fig. 9 the chain M is seen, 
which raises three levers, each carrying a counterpoise. These levers succes- 
sively coming into play as the telescope approaches and passes the zenith, 
maintain the chain R in such a state of tension, that the telescope obeys the 
windlass in every position. It has been found practicable so to adjust the 
levers that the residual error of compensation arising from the imperfect 
action of the guy-chains has been rendered almost insensible. 
Fig. 11. Universal joint which bears the tube. A, bolts which secure it to the bottom 
of the tube. BB, two of the three adjusting-screws: these act against the 
carriage (5, fig. 1), directly under the ball-and-socket joint of the primary 
triangles. C, the axis perpendicular, and D the axis parallel to the horizon. 
The axis D gives motion to a large index for roughly setting the telescope 
in polar distance. E, a strong trussed framework of cast iron, secured to a 
solid stone foundation. 
