42 



MICHIGAN ACADEMY OF SCIENCh;. 



Tims we find a shape slightly elliptical. One half the ditference of the 

 means is 0.0082r, or 0.000082 in. Since the length of the axis of the me- 

 ridian circle is 41% inches, this would produce an inequality of 0.4()r)r'' 

 or 0.0270s, the coarse circle end being the bigger. 



BEARINGS OF LEVEL. 



Eye-end 



Edge laoi 



Side Berlin 



Edge Pistor and Martins 



Means 



0.3706r 

 0.3698 

 0.3fi98 

 3625 



0.3682r 



0.36S8r 

 0.3524 

 0.3714 

 0.3634 



0.3632r 



One half the difference of the means is in this ease 0,002.5r, or 0.000025 

 in., producing an inequality of 0.1235'' or 0.0082s. 

 The weight on each pivot is approximately 34 lbs. 



8.— The Circles. 



There are two circles, one divided to 2' and one to 10'. Each is read by 

 four microscopes to 0.1". Both circles were cleaned by me with fine 

 whiting in 1893. Evidently both had been cleaned a number of times be- 

 fore, but the lines on the coarse circle are in better condition than those 

 on the fine circle. On the fine circle they are faint in several places. As 

 originally cut the lines must have been rather fine and delicate. The 

 more modern method of making them heavy is better, and the heavy marks 

 can be pointed on just as accurately, as far as I can see. 



The circles are approximately STi'o inches in diameter. Each is sup- 

 ported by ten ribs running from the central hub. The ribs and the circles 

 themselves are rather light, so that there is some distortion by gravity. 

 The ribs are numbered I, II, .... through X. The fine circle is not 

 figured. I have considered the degree mark opposite X to be 0°, that 

 opposite I to be 36°, and so on. Opposite 72° I placed a scratch on the 

 silver band. Afterwards it was found that a cross had been placed oppo- 

 site 314°, which perhaps had been used on Polaris. 



The fine circle was investigated by Dr. Briinnow to every fifth degree. 

 These results are reprinted here from the Astronomical Notices, both to 

 show the character of the circle, and for possible use with any old obser- 

 vations. The coarse circle is on the side of the clamp. 



Examination of the Divisions of the Ann ArJjo)- Meridian Circle. 



The Ann Arbor Meridian Circle was originally furnished with two 

 divided circles ; but as the one on the side of the clamp was found to be 

 slightly bent when it arrived at Ann Arbor, it is now used merely for set- 

 ting the instrument, and only the one on the opposite side of the axis 

 is used for reading the zenith distances. The four microscopes for read- 

 ing the circle on each side are fastened by strong arms to a solid circular 

 disc, which is firmly screwed to a solid brass piece let into the stone 

 pillar. This disc supports at the same time the Y-pieces, so that the 

 center of the axis coincides with its center. The edge of this disc is dove- 



