xxxviii Inteoduction to the Makerstoun Observations, 1843. 



Values of the constants and adjustments. 



52. Value of one division of the micrometer head in angular measure. 



The micrometer heads have 50 divisions numbered at the tens 1, 2, 3, 4, 5, 

 a brass needle with scales graduated to 10', at an equal distance from the centre as 

 the spider crosses in the magnet, was placed on the agate planes, and the moveable 

 wires of the micrometer brought to coincide with the graduated lines of the scale, 

 the micrometer heads being read at each coincidence. 



Left Mic. Eight Mic. 



June 21. 1841 (Table 13, Introduction, 1841-2), one division = 0'-1004 0'-1003 



Aug. 30.1841 - 0'-1002 0'0996 



Sept. 4.1843 Observations gave = 0'-1012 



Nov., 13. 1843 = 0'-1013 0'-0999 



Adopted mean value of one division, 0''I003, whence 



a = 0-00002918, e = 17° 34', k = 0-000009 



this value of k has been used in the abstracts. 



53. Time of vibration in the horizontal plane. 



The adopted mean of all the observations, Table 15, Introduction, 1841-2, 

 is 12='0. 



54. Time of vibration in the vertical plane. 



The needle being in its usual position on the agate planes, the moveable wire 

 of the left micrometer is made to bisect the spider cross ; the needle is then vibrated, 

 by means of a small piece of iron or steel, through an angle varying from 15' to 5', 

 and the periods of the cross passing the wire are estimated. When large arcs of 

 vibration have been taken, the time of vibration is deduced from the last observations 

 of the series, for the reasons stated, No. 50, 2d. The 4th conclusion given, No. 50, 

 cannot be deduced from the observations for 1843, probably because constant arcs 

 of vibration were not used, and the errors indicated 1st, 2d, and 3d, being included 

 in the results. The arc of vibration was estimated till October 18, after which the 

 arc at commencing was measured by moving the wire of the right micrometer on 

 the cross at the extremity of its arc of vibration ; the arc of vibration at the end is 

 generally about 1', being the smallest possible at which the passage of the cross can 

 be estimated with any nicety. The following is a specimen of the observations, and 

 of the method in which the results in the following Table are obtained : — 



