and of the Altitude and Azimuth Instnmcnt. 1 03 



In astronomy the azimuth circle is of Httle use, except in 

 furnishing ready means of bringing the upper circle into the 

 plane of the meridian ; it has however been used for making 

 out, in conjunction with the otlier circle, the quantity of re- 

 fraction at different altitudes ; but as, perhaps, the times for 

 this purpose furnish better data than azimuths, to mention the 

 matter is sufficient. Observing altitudes of the celestial bodies 

 is a thing so familiar to every one who is in the least ac- 

 quainted with these subjects, that to describe it would only be 

 to lengthen this paper. I may however remark tliat in meri- 

 dian altitudes, there is time to get an observation with the face 

 of the circle to the east, and another with its face to the west ; 

 which together give a coUimated double result, before the 

 diminution in altitude becomes sensible. Yet I feel somewhat 

 diffident about recommending this mode of observing, although 

 I have practised it myself with success. An observer, before 

 he attempts it, should be expert both in managing the instru- 

 ment and reading off' the angles. Respecting stars near the 

 equator, there is only about one minute of time on each side 

 of the meridian, that the star would continue to be bisected by 

 the wire of a telescope that magnifies sixty times ; therefore, 

 before the double observation is attempted, the time should be 

 accuratel}' known. In truth, all hurried operations with this 

 instrument may be avoided and left to the repeating cuxle. 



Comparison of the two Instruments. 



I come now to compare the two mstruments with each other 

 in their application to the different purposes for which they 

 are designed ; and to prevent the frequent recurrence of long 

 names, I shall call the repeating circle R, and the altitude and 

 azimuth instrument A ; an expedient which, perhaps, if sooner 

 adopted, might have improved this paper. 



To find the difference of latitude between distant places is 

 a most important problem in extensive geodetic operations, 

 and for this purpose R has often been represented as equal or 

 even superior to the zenith sector : but as the latter histrument 

 has always been constructed with a powerful telesco})e, and is 

 in its nature the most simple jiossible, I beg leave to dissent 

 from that opinion. For, however an instrument may be con- 

 structed, or in whatever manner it may be used, I have no 

 faith that it can give results nearer the truth, than a quantity 

 tliat is visible in the telescope. To do this, an instrument, 

 with respect to any error that is not corrected by reversion, 

 must be periect; a thing of which I have no idea. Yet every 

 one is not of my opinion. As an instance of this, a celebrated 

 astronomer, a few years ago, in the south of Europe, made 



observation* 



