Astronomical and Nautical Instruments. 



The following is an enumeration and brief description of the instruments 

 used for astronomical observations. 



Altazimuth by C. H. G. OLSEN in Christiania. The horizontal and vertical 

 circles, of 21 cm. diameter, are graduated on platinum to 10' and read off 

 by two microscopes placed diametrically reading to 10", the single seconds 

 being easily estimated. The microscopes for the horizontal circle follow the 

 instrument in the motion about the vertical axis, the horizontal circle being 

 fixed, while the vertical circle follows the telescope in the motion about the 

 horizontal axis, the corresponding microscopes being fixed at the ends of the 

 horizontal diameter. The alidade bearing these microscopes is provided with 

 a fixed level, adjustable by screw and spring working on an arm going 

 downwards from the centre. Both circles being graduated from left to right, 

 as seen from the centre, the correction to the reading of the vertical circle 

 is positive when the right end of the level is the higher. The level is divided 

 from the middle and the angular value of a part was given by the maker as 

 4", which was found to be sufficiently accurate; consequently the difference 

 between the readings of the two ends of the bubble, multiplied by 2, gives 

 the correction to the circle-reading in seconds. The degrees and tens of 

 minutes are for both circles read off by an index, which is, for the vertical 

 circle, placed at the top. 



The microscopes of both circles were always pointed to two adjacent 

 division lines, one on each side of the central notch marking the zero of 

 single minutes in the field of the microscope. Generally the two readings 

 did not differ by more than the accidental error of pointing, a few seconds; 



