. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . Fio. 17.—Section of Melbourne reflector. bearings is relieved by a weight. The friction of the upper pivot is re-lieved by a sector pressed up against it by the action of two this way, although the moving part of the telescope weighs 18,170lb, it can be turned with a pressure of 12J tt>, acting at a radius ol20 feet. The driving sector EF is 5 feet in radius ; itscircular rim is accurately toothed to fit a square threadedendless screw E, which is turned by the driving toothed wheel
. The Encyclopaedia Britannica; ... A dictionary of arts, sciences and general literature . Fio. 17.—Section of Melbourne reflector. bearings is relieved by a weight. The friction of the upper pivot is re-lieved by a sector pressed up against it by the action of two this way, although the moving part of the telescope weighs 18,170lb, it can be turned with a pressure of 12J tt>, acting at a radius ol20 feet. The driving sector EF is 5 feet in radius ; itscircular rim is accurately toothed to fit a square threadedendless screw E, which is turned by the driving toothed wheel attached to H and acted on by a pinionconnected with a hand-wheel afiords an easy means otBetting the instrument in hour angle, ormoving the telescope quickly iu right ascen-sion. The telescope is clamped by iron bandsto the strong cast-iron cradle, which is castiwith and forms one extremity of the declina-. tion axis. The counterpoise TJ is attached tothe other extremity. There is an elegant ar-rangement for diminishing the friction of thedecEnation axis, which our limits do not per-mit us to describe, and the means for clampingand giving slow motion in declination do notrequire special notice. The reader is referredfor a fuller description to PhU. .Trails., 1869,pp. 127-161. The telescope is of the Cassegrainform, the mirror having a 4-feet aperture and30i-feet focal len^h. The best existing examples of type B areAirya equatorial at Greenwich, the equatorialat Liverpool (also designed by Airy), and thephotographic equatorial recently erected at the Paris polar axis of the Greenwich equatorial consists of six iron tubesarranged so as to form two triangular braced beams connected byvery strong elliptical wheels of cast iron, which carry the upperand lower pivots of the polar axis. These tubes are shown in sec-tion at the points T, fig. 18, which represents a section throug
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