58 REPORTON 



the magnetic meridian. Therefore the vernier was set at 90 -f- E, and the plane of 

 the vertical circle coincided with the magnetic meridian. However, it soon became 

 evident that this process consumed too much time, and the following, which is 

 quite as accurate and much more expeditious, was adopted : A fine line was marked 

 permanently upon the top of the instrument parallel to the plane of the vertical 

 circle; then, after the instrument had been levelled, but before the dipping needle 

 had been placed upon the agate planes, a pocket compass, with a needle about one 

 and a half inches long, was placed with its centre upon the fine line, and the verti- 

 cal circle was tumed in azimuth till the compass needle and line were parallel to 

 each other. That being the case, the plane of the vertical circle was known to be 

 in the magnetic meridian, and the horizontal circle was clamped and read off. 



The following is the method which was adopted in making observations of dip: 

 1. The agate planes, and those parts of the axle of the needle which would rest 

 upon them, were carefully wiped with a piece of chamois leather (I have since, 

 seen reason to believe that a piece of cork would liave answered the purpose better), 

 and then the instrument was set up, levelled, and the plane of the vertical circle 

 placed in the magnetic meridian by the process before described. 2. The needle 

 was secured upon a block, provided for the purpose, and magnetised by means of a ' 

 pair of eight-inch bar magnets, in such a manner that its marked end acquired 

 north polarity. It was considered to be saturated with magnetism when the bar 

 magnets had been drawn from its centre to its extremities six times, the process 

 being performed upon both of its sides, and then it was removed from the block 

 and placed in position upon the agate planes, with its face (by which is meant that 

 side upon which the letters were marked) towards the east. 3. The plane of the 

 vertical circle being in the magnetic meridian, with the face of the instrument 

 towards the east, and the needle in position upon the agate planes, with its face 

 also towards the east, the north and south ends of the needle were read. Let these 

 readings be designated respectively as $' and </>". 4. The needle was reversed 

 upon the agate planes, so as to bring its face towards the west, and its north and 

 south ends were read. Let these readings be designated respectively $"' and $'". 

 5. The horizontal circle was undamped, the vertical circle turned in azimuth 180, 

 so as to bring its face towards the west, and the horizontal circle again clamped. 

 The face of the needle now being towards the cast, its north and south ends were 

 read. Let these readings be designated respectively as <p v and <p rr . 6 Q . The needle 

 \\;i- reversed upon the agate planes, so as to bring its face towards the west, and 

 its north and south ends were read. Let these readings be designated respectively 

 as tj> T " and <p r '". 7. The time was noted, and then the needle, having been 

 removed from the agate planes, was placed upon the block provided for the pur- 

 pose, and rcmagnetised in such a manner that its marked end acquired south 

 polarity ; after which it was again placed in position upon the agate planes, with 

 ice towards the west,,and its north and south ends were read. Let these read- 

 ings be designated respectively as 4/ and $'. 8. The needle was reversed upon 

 the agate planes, so as to bring its face towards the east, and its north and south 

 ends were read. Let these readings be designated respectively as $" and i)/". 

 9. The horizontal circle was unclamped, the vertical circle turned in azimuth 180, 



