Ml 



CTHOI.E, ASTOOyOMIOAL. 



CIRCLE, ASTRONOMICAL. 



fixed wire, and that the further correction for the micrometer is made 



" 

 Min]>1y by the addition or subtraction of * to the value already fnund 



f the horizontal point. In the Cambridge Observatory, Professor 

 Airy tabulated the reading coresjxmding to the zenith position of the 

 telescope. From, a mean of such determination* the reading of the 



zenith point is nettled, and all observation* maiV in the ordinary way, 

 reduced to altitude* or zenith distance*. For this mode of observing, 

 the micrometer should be exquisitely perfect, and carefully watched. 

 ;]ring the zenith point* deduced from low and high utars, to 

 the north and south of the zenith, Professor Airy discovered some 

 anomalies, the cautes of which have not yet been fully ascertained. 

 ('Cambridge Observation*,' 1833, p. 27.) They seem, however, to 



T.ltht*er!!ieh Altitude and Aiimnth Circle, by Tronghton and Kimm, 1890. 



*xlt In other mural circles, and possibly In 



' t : ,: ; 



The micrometer microscope*, in what IUM been said, are supposed to 



circles of a different ' screw of tli<> K' space* in the circle, a mean valuo of the measures of 

 5' hy the six microscopes is obtained, from time to time, which may >>' 

 a little greater or leu than five revolutions. In this owe, the mean of 



I* exactly adjusted. o that fire revolutions of the screw carry the cross the micrometer reading* that i, of the single minutes and parts 

 of the wim< precisely over a space of S' on the circle. It is not possible must in each obi 



to make this adjustment with perfect accuracy, nor would it V., 



mnt ; but H 1* not necesmry. By examining the value of the rum, as 



n be altered in the proportion of the value thus 



ascertained to he 6'. If, for instance, the mean value of the 5' spaces 

 on the circle were apparently f>' B'l" by the microscopes, i must be. 



they are oaUedy-th.it is, the measure* in revolution* and part* of the ' subtracted from the minute* and seconds indicated by the mean of the 



