lii Introduction to the Makeestoun Observations, 1844. 



<^ = 7-72 mic div. 

 The adopted value of the temperature coefficient for the balance magnet = 790 mic. div. 



It is believed that this value, which has been used in correcting all the observations 

 since the commencement of 1843, is within one-tenth of a division of the truth. 



81. The following matters should be attended to in determining the tempera- 

 ture coefficient by the previous method. 



\st, The period selected should be free from considerable magnetic irregularities. 



2(7, There should be a considerable change of daily mean temperature, the tem- 

 perature at the beginning and end of the period being nearly the same. 



3(/, The smaller the duration of the period consistently with the 2d the better. 



4fft, It will be found best, in general, to correct the daily means at first by an 

 approximate coefficient, and 



bth. 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 91 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 

 caiTying Y« moves within that bearing the agate planes on which the axle of the 

 needle rests ; the Y.? 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 



lirection. A level screwed to the basement jjlate 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 dijiplng 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 



