liv Intkoduction to the Makerstoun Observations, 1843. 



marks are on one face of each needle. The needle is observed in four positions with 

 one end dipping, namely, with the marked face of the needle on the same side as, 

 and opposite to, the graduated face of the circle, the latter being in the meridian, 

 first to the east, and then to the west ; as each extremity of the needle is observed 

 there are thus eight readings obtained. The poles being changed, and the other end 

 dipping, other eight readings are similarly obtained. The means of the two read- 

 ings for each position are given in this volume. 



In changing the poles, the needle was placed on a small wooden block having 

 a hole to receive the axle ; it then received eight strokes on each face (as in the 

 method of double touch) from two magnets, each 9 inches long, |- inch broad. 



79. The inclinometer occupied the pillar I, in the plan (Plate I.), to the east 

 of the declinometer pillar till May 30. 1843, when it was removed to the wooden 

 house, described No. 24, where it was placed on a strong wooden post unconnected 

 with the floor. 



80. After June 1843, many of the observations were rendered valueless by the 

 action of the lifter on the needle when the latter was in certain positions. The fol- 

 lowing notes to the observations made at the time will explain the nature of the 

 difficulty : — 



July 26. 1843. This observation made with great care, in order to determine 

 if the apparent increase of dip is real, or only due to instrumental error. Every 

 reading was determined by at least a dozen trials ; the different trials varied little 

 for the same position with one exception, in which the readings varied 10' on each 

 side of the mean ; there is, however, evidently something wrong with the lifter, as, 

 on attempting to verify a reading marked (a), after the completion of the observa- 

 tion, only 73° 45' could be obtained, instead of 74° 0'. 



August 1. 1843. As it was found that the lifter did not move freely last obser- 

 vation, it was taken out, cleaned, and bent a little, so as to fit better ; to-day's ob- 

 servation was then made, but some other source of error exists, as there is a ten- 

 dency, in lowering the needle on the planes, to move it always in one direction ; the 

 readings thus may be made to go on increasing, and, by a slight difference in the 

 mode of lowering, they may be made to go on diminishing. It cost two hours to 

 make the first reading, and the observation is not considered good. 



September 12. 1843. Previous to this observation, the agate planes were ad- 

 justed to horizontality as accurately as the small level would permit. The obser- 

 vation was bad, but chiefly in the reading marked (*), 67° 30' was the reading 

 generally gained, but after half-an-hour the needle rested at 68° 19'. 



After this date the instrument was sent to Messrs Adie and Son for adjust- 

 ment, when the needle No. 1. was also adjusted. 



The following are notes to the observations after the adjustment of the instru- 

 ment : — 



October 6. 1843. Observation not considered good — several of the readings 



I 



