MAGNETIC OBSERVATIONS. 57 



effected by means of a clamp and tangent screw. The horizontal circle of the 

 instrument was four inches in diameter, divided to half degrees, and reading by 

 means of one vernier to single minutes. It was provided with a clamp, but no 



tangent screw. 



Readings of the position of the dipping needle were made as follows: In the 

 field of view of each microscope was a plate of glass upon which was engraved 

 three fine parallel lines, the middle one being intended to represent one of the 

 two extremities of a diameter passing through a vertical circle described about the 

 prolongation of the axle of the needle. The north microscope having been turned 

 till the centre line in its field of view coincided with the north end of the needle, 

 the vernier belonging to that microscope was read off", and recorded as the reading of 

 the north end of the needle. Then the south microscope was turned till the centre 

 line in its field of view coincided with the south end of the needle, and the vernier 

 belonging to that microscope was read off, and recorded as the reading of the 

 south end of the needle. In order to distinguish between the two microscopes the 

 letter N was scratched upon one of them, and that one was always, in all positions 

 of the instrument, used to read the north end of the needle. 



The instrument having been set up and levelled, before beginning to observe it 

 was necessary to place the plane of the vertical circle in the magnetic meridian. 

 At a few of the earlier stations this was accomplished as follows: The needle was 

 placed on the agate planes, with the side on which the letters were marked facing 

 the microscopes. Then 1. The microscopes having been turned till they were 

 nearly in a vertical line, the vernier of the lower one was set to 90 0', and the 

 vertical circle was moved in azimuth so that its face (by which is meant the side 

 on which the microscopes were) was south till the lower end of the needle was 

 bisected by the middle line- in the lower microscope; the Y's were raised and 

 lowered gently, and if the bisection of the needle was altered, it was corrected 

 by turning the circle in azimuth. Then the horizontal circle was clamped and 

 read off; and this reading was called A. 2. The vernier of the upper microscope 

 was set to 90 0', and the horizontal circle having been undamped, the vertical 

 circle was moved in azimuth its face still remaining south till the upper end 

 of the needle was bisected by the middle line in the upper microscope ; the Y's 

 were raised and lowered gently, and if the bisection of the needle was altered, it 

 was corrected by turning the circle in azimuth. Then the horizontal circle was 

 clamped and read off, and this reading was called B. 3. The horizontal circle 

 was undamped, and turned in azimuth 180, so as to bring the face of the instru- 

 ment to the north, and then the 1 and 2 processes just described were repeated ; 

 thus giving two more readings of the horizontal circle, which were called C and D. 

 Then 



A+B+0+D_ E 



4 \ 



where E is the division of the horizontal circle at which it was necessary to set 

 the vernier in order that the plane of the vertical circle might be at right angles to 



8 April, 1872. 



