J^ An Account of the Repeating Circle^ -m 



having been read off on six 'places ; and in both parts of the 

 operation the error arising from eccentricity is, as to any sen- 

 sible quantity, totally done away. 



Description of the Altitude and Azimuth Instrument. 

 The lower part of this instrument, like the other, consists of 

 a tripod and feet-screws; which latter, being a recent contri- 

 vance, and hitherto undescribed, may in this place deserve par- 

 ticular notice. Each of the three screws is double ; that is, a 

 screw within a screw : the exterior one, as usual, has its female 

 in the end of the tripod, and the female of the interior screw is 

 within the exterior. The interior one is longer than the other, 

 its flat end rests on a small cup on the top of the support, and 

 its milled head is a little above the other. Now by this ar- 

 rangement we gain three distinct motions ; for, by turning both 

 screws together, an effect is produced equal to the natural 

 range of the exterior screw : by turning the interior one alone, 

 the effect produced is what is due to this screw: and by turning 

 the exterior one alone (which may be done, because the fric- 

 tion of the interior screw in the cup is greater than that which 

 exists between the two screws) an effect is produced, equal to 

 the difference of the ranges of the two screws. Thus, were the 

 extei'ior one to have 30 turns in an inch, and the interior 40, 

 the effect last described will be exactly equal to what would be 

 produced by a simple screw of 120 threads in an inch. This 

 is an unprovement applicable to all instrmnents that are sup- 

 ported on screws, and of course to the repeating circle. A few 

 of the last made in London possess this advantage. The ver- 

 tical axis of the altitude and azimuth instrument is fixed in the 

 centre of the tripod, of a length equal to about the radius of 

 the circle. At the lower end is centred upon it the azimuth 

 circle, in close contact with the tripod ; to the three branches 

 of which it is fastened, but in such a manner as to admit of 

 a circular motion of about 3^, which motion is governed alto- 

 gether by a slow moving screw. The intention of this motion 

 is, in geodetic operations, to bring the zero of the circle to the 

 point of commencement; and, in astronomy, to place that point 

 exactly in the meridian. In an instrument for my own use, 

 however, I could dispense with this adjustment, because I know 

 that it is easier and more accurate to read off than it is to set: 

 and from what point I begin to reckon, is a thing quite indif- 

 ferent to me. Just above the circle the axis is embraced by a 

 cone, which is also well centred upon the upper end. To the 

 lower end of the cone is fixed an entire cuxular plate, formed 

 in the strongest manner ; which not only bears the two or three 



microscopes 



