xxxviii Introduction to the Makerstoun Observations, 1845 and 1846. 



The horizontal screw was now turned in two revohitions, or one revolution farther 

 in than at first, when the reading was changed from — 818 mic. div. to + 1120 

 mic. div. 



Arc of vibration at commencement, 21'. Time of one vibration, S^'OO. 



Finally, the horizontal screw was turned out one revolution, as at first, the 

 mean reading becoming as at first + 160 mic. div., when 



Arc of vibration at commencement, 32'. Time of one vibration, 8s-12. 



52. These results are very consistent, and speak much in favour of the excel- 

 lence of the knife edges of the axle. It is quite certain, therefore, that the varia- 

 tions in the time of vibration observed in 1844, 1845, and 1846, were not due to the 

 varying position of the needle, since all the observations from 1844 till 1846 were 

 obtained from the needle when in positions varying less than 400 micrometer divisions. 



53. As it was believed, that during considerable disturbances when the hori- 

 zontal component of the earth's magnetism increased considerably, the north end of 

 the needle might be drawn slightly out of its position at right angles to the mag- 

 netic meridian, the following observations were made to determine whether such a 

 result would affect the time of vibration. 



January 14"* 1848. The balance needle being placed on its agate planes with its 

 magnetic axis at right angles to the magnetic meridian, the following observation 

 was made ; position of needle, micrometer reading +180. 



Arc of vibration at commencement 8''4. Time of one vibration 11^-27. 



Needle vibrated excessively by a pair of magnetic scissors. 



Arc of vibration at commencement 4'*8. Time of one vibration lls-28. 



Brought a 4-inch deflecting magnet close to the side of the balance box near 

 the west extremity, in order to draw the needle out of the plane at right angles to 

 the magnetic meridian ; after considerable vibration, always checked by changing 

 the position of the deflector, the following observation was made : 



Arc of vibration at commencement 3'-0. Time of one vibration lls-28. 



Performed the same operation with the deflector, and again observed, 



Arc of vibration at commencement 8'-0. Time of one vibration lls-12. 



Now lifted the needle by the Ys, lowered it, and observed the time of vibration 

 with a large arc. 



Arc of vibration at commencement IGG'-Q. Time of one vibration lls-27. 



