Mr Galbraith on Hater's Azimuth Circle. 



US 



We have in our possession one of those of a much more 

 powerful and perfect kind. The diameter is six inches, the 

 telescopes magnify about twenty times, and there are three 

 verniers to both the vertical and horizontal circles, each reading 

 to the accuracy of ten seconds. The scale of the level attached 

 to it shows thi-ee seconds, and a third, or at least a half of each 

 division may be easily estimated. With this instrument I was 

 inclined to believe, that observations, if carefully taken, might 

 be obtained to a very great degree of accuracy. To insure all 

 the precision possible, it was my practice to observe objects to 

 the north and south of the zenith at nearly equal distances, as 

 calculated most likely to destroy any casual errors in observing 

 or reading. The general means are deduced from successive 

 pairs to the north and south, and the final result is the latitude 

 of my residence. No. 54 South Bridge, and as it is on the same 

 parallel as the north front or side of Edinburgh College, it may 

 be also considered the latitude of that much better known 

 edifice. 



Observations by the Writer in 1835. 



The final latitude deduced is, therefore, B^"" BQ' 58".05 N. 

 from 20 series to the north, and a like number to the south, and 

 since there were from 6 to 18 readings to each, or about 12 

 at a mean, the whole 40 series of observations amounted to 

 480 readings, or 160 different observations, the circle being re- 

 gularly reversed for each pair, at the same time recording the 

 readings of the level. The observations of the pole-star were 

 calculated by the aid of the table for that purpose at the end of 

 the Nautical Almanac, while the others were reduced to the 

 meridian in the usual manner by my tables. 



What may be the accuracy of this final result I shall not 

 take upon me at present to say, but I believe it to be within 

 one or two seconds of the truth. Captain Kater was of opi- 

 nion that with his smaller instrument he could, by a mean of 



