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the advantage of equatorial mountings in general ; as well as to ex- 

 emplify the best and easiest methods of adjusting and rectifying the 

 instruments, and placing the telescopes in every respect in the most 

 favourable circumstances for yielding good results. After describing 

 how, in the history of astronomy, equatorial or parallactic stands 

 were twice taken up and abandoned again, from the erroneous 

 estimate formed of the purposes to which they were adapted, the 

 author mentioned the impracticable nature of the altitude and 

 azimuth mountings which followed ; and dated the present era of 

 the perfection and the rational employment of equatorials to have 

 commenced in 1820, when Sir J. Ilerschel, in conjunction with Sir 

 J. South, erected one of these instruments, to give, by its A. R. and 

 Declination circles, absolute places roughly ; and, by means of a mi- 

 crometer applied to the focus of the telescope, small diflferences very 

 exactly : the old error having been, to attempt to determine abso- 

 lute places with the utmost precision. 



After particularising the various merits and imperfections of the 

 two grand divisions of equatorials, viz., the Enghsh and the Ger- 

 man, and mentioning a new construction in progress for the Edin- 

 burgh Observatory, combining with the single-pier and short polar 

 axis of the German form, the advantage which the English possesses, 

 of lar^e circles, and a position for the telescope between the two 

 bearing ends of the polar axis, together with an exceeding degree of 

 firmness and stiffness, — the author proceeded to describe the six 

 errors of adjustment to which all equatorials are subject, and to 

 shew, by means of a model placed within a representation of the 

 celestial sphere, contrived for the purpose, how all the rectifications 

 might be made by means of observations of stars. 



The application of clock motion to equatorials, for the purpose of 

 keepincr a celestial object stationary in the field, was next entered 

 into ; and the plan explained by which the hitherto ungovernable 

 fits of " knocking" of the revolving balls in the later form of English 

 clocks, has been remedied in the case of the Edinburgh Equatorial ; 



viz., by havintr three pendulum-balls 120° apart, attached to the 



vertical spindle of the governor, instead of only two at 180 . 



As a proper micrometer to be used for very faint objects, in place 

 of the ring micrometer, — which the author unhesitatingly condemned, 

 as never having furnished accurate results either iu A. R, or Decl., 

 but more especially in the latter, in any person's hands, though so 



