Annual Vaeiations for the Magnetic Dip. 
lix 
117. The observations made in different azimuths already noticed, render it probable that the dip deduced 
above is inaccurate from instrumental causes ; the mean dip from observations in all the azimuths was less than 
that from the observations in the magnetic meridian by upwards of 10'. 
118. In order if possible to determine the true dip, the inclinometer belonging to the lioyal Society of 
Edinburgh was obtained, and observations were made with it on the Observatory dip-pillar as follow : — 
Sept. 25d 5h Needle No. 1. Dip = 71° ll'-38 ] 
26<i Ih Dip = 71° 10'-83 I Mean Dip = 71° 10'-36 
26^ - 27<i Needle No. 2. Dip = 71° 8'-88 J 
The instrument was not in good order and a considerable time was spent on the observations ; but the results 
agree very well. Observations with needle No. 2, were also made in the azimuths 30° and 120°, which gave the 
following values : — 
By the Formula for single Azimuths. By the Formula for both Azimuths. 
Azimutb 30° Dip = 71° 9'-87 Azimuths 30° and 120°, Dip = 71° 4'-2 
120° Dip = 70° 47'-7 
The observations with the Royal Society's inclinometer in tlie magnetic meridian give the magnetic dip about 
6' less than those with the Makerstoun instrument. It cannot be said that the true dip for Makerstoun is yet 
accurately determined, as the observations with the Royal Society's inclinometer in different azimuths will 
scarcely permit the assumption that it is free fi'om instrumental error. 
119. The following results are deduced from those already obtained for the horizontal and vertical components ^ 
of magnetic force. (See 1844, p. 390.) It may be remarked here, that the epochs for the horizontal component and 
magnetic dip agree very nearly in every case where both have been determined; maxima of the horizontal component 
being equivalent to minima of dip, and vice versa ; therefore, when investigations have not been made similar to 
those for the horizontal component for the magnetic dip (such as for the mean difference or disturbance), the law 
for the former may be assumed for the latter, and the values of the variations of dip in minutes may be ob- 
tained approximately from the numbers for the horizontal component by multiplying the latter by 1000. 
120. Secular Change of Magnetic Dip. — If we assume the secular change for the horizontal component 
= + 0-001318 (No. 52), and for the vertical component, as deduced from the years 1845-9, = — 0-001055, 
we find the secular change of dip = — 2'-47 ; this is considerably greater than that obtained from the obser- 
vations of absolute dip No. 116; since the secular change for the horizontal component must be near the 
truth (No. 53), and that obtained for the dip. No. 116, cannot be far from it, it is probable that the secxilar 
change for the vertical component is still considerably in error, that in fact the balance needle is still losing- 
magnetism. 
121. Effect of Disturbance on the Yearly Mean. — From the means for the days selected as nearly free from 
disturbance in the years 1844 and 1845, we find from Nos. 54 and 85, that the yearly mean of magnetic dip 
deduced from the undisturbed days is less than that from all the observations by 0-'15. The effect of dis- 
turbance, therefore, is to increase the magnetic dip, although the effect on the magnetic declination is nearly 
zero. (No. 38.) 
122. Annual Period of Magnetic Dip. — This result depends chiefly on that for the horizontal component ; 
adopting the annual period for the vertical component, deduced from the observations for the years 1843-6, 
as the best representative of that variation, and employing the annual variations for the horizontal compo- 
nent, deduced from the observations in the years 1843-6, 1842-5, and 1842-7, we obtain the following 
numbers : 
Table 56. — Monthly Variations of Magnetic Dip, free from Regular Secular Change. 
Years. 
Jan. 
Feb. 
March. 
April. 
JIay. 
June. 
.ruly. 
Aug. 
Sept. 
Oct. 
Nov. 
Dec. 
1843-6 
1842-5 
1842-7 
-0131 
-0-101 
-0-263 
-0-167 
-0-110 
-0-161 
+ 0-102 
+ 0-111 
+ 0-035 
+ 0-207 
+ 0-311 
+ 0-302 
-0-258 
-0-287 
-0-350 
-0-256 
-0-355 
-0-516 
-0-209 
-0-147 
-0-278 
+ 0-174 
+ 0-116 
+ 0-209 
+ 0-400 
+ 0-282 
+ 0-502 
+ 0-331 
+ 0-211 
+ 0-642 
+ 0-020 
+ 0-056 
+ 0-199 
-0-200 
-0-088 
-0-320 
MAG, AND MET. DBS. 1845 AND 1846. 
I 
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