Annual Variations 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 Royal Society of 

 Edinburgh was obtained, and observations were made with it on the Observatory dip-pillar as follow : — 



Sept. 25«i 5h 

 26<l Ih 

 26'i - 27<J 



Needle No. 1. 

 Needie No. 2. 



Dip = 71° ll'-38 

 Dip = 71° 10'-83 

 Dip = 71° 8'-88 



Mean Dip = 71° 10'-36 



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. 



Azimuth 30° Dip = 71° 9'-87 

 120° Dip = 70° 47'-7 



By the Formula for both Azimuths. 



Azimuths 30° and 120°, Dip = 71° 4'-2 



The observations with the Royal Society's inclinometer in the 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 from 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 secular 

 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. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



1843-6 

 1842-5 

 1842-7 



-0-131 

 -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. 



