xxxviii Introduction to the Makeestoun 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. Right 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'*1003, 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 s -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 : — 
