1 Inteoduction to the Makerstoun Observations, 1845 and 1846. 



5ih, To eliminate the secular change approximately, if it be considerable. 

 Both the latter methods were employed in many of the determinations given in 

 Table 9. 



Inclinometer, 



82. The dip instrument was made by the late Mr Robinson of London. The 

 vertical circle is 9i inches in diameter ; it is divided to 10', the graduations counting 

 from 0° on the horizontal to 90° on the vertical ; 1'* is estimated with the aid of 

 lenses attached to a glazed case ; the vertical circle turns with a copper framework 

 on a vertical axis, centred in a horizontal circle ; the latter is 6 inches in diameter, 

 is divided to 30' and is read to 1' by means of a vernier. A sliding framework 

 carrying Ys moves within that bearing the agate planes on which the axle of the 

 needle rests ; the Ys serve to lift and lower the needle on the agates, but they have 

 been found to act very irregularly, at times giving the needle a pitch in a certain 

 direction. A level screwed to the basement plate indicates the horizontality of the 

 agates ; this was, however, also verified occasionally by means of a small level placed 

 upon them ; it was found that the level varied according as the door of the case 

 inclosing the instrument was shut or open ; it was, therefore, always tested with 

 the door shut, as it is during observations. The reading of the horizontal circle, 

 when the vertical circle is in the magnetic meridian, was obtained with the aid of 

 a horizontal needle, carried on a pivot whose arms rest on the agate planes. There 

 are two dipping needles, numbered 1 and 2, and one end of each needle is marked 

 A, the other end is marked B ; all the 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 readings 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. 



The inclinometer occupied a strong wooden pillar in the intensity house uncon- 

 nected with the floor. 



83. Observations were made on April 18 and May 2, 1843, in different azi- 

 muths, in order to determine the correction due to the irregularity of the needle's 

 axle, or perhaps to the presence of iron in the vertical circle ; these observations 

 have been already given (Table 21 and Table 22, Introduction, 1841-2.) The cor- 

 rection deduced was about — 11' for needle No. 1. A short series of similar obser- 

 vations was made, May 10, 1845, the details of which will be found among the other 

 observations ; the following values of the magnetic dip are deduced from the incli- 

 nations 9] and n of needle No. 2, observed in two planes at right angles to each other, 

 by the formula 



