392 Results of Makerstoun Observations, 1844. 



Diurnal Variation of Magnetic Dip. — In the mean for tlie year the magnetic dip is a maximum at 10 h 

 10 m a.m. ; it has its least value at 5 h 10 m p.m., but the value differs little from 5 h till 8 h p.m. ; it has a 

 secondary maximum at 2 h 10 m a.m., and a secondary minimum at 5 h 10 m a.m. The increase is most rapid 

 from 6 h a.m. till 10 h a.m., and the diminution is most rapid from 10 h a.m. till 5 h p.m. 



In the four months of May, June, July, and August, the secondary maximum and minimum are not at all 

 visible ; in the four equinoctial months the variation is very irregular from about 5 h p.m. till about 5 h a.m., 

 the dip oscillating about a nearly constant value ; in the four winter months the secondary becomes the prin- 

 cipal maximum, the maximum at 5 h nearly disappearing in December. The principal minimum always occurs 

 about 10 h a.m. 



The range of the mean diurnal variation of magnetic dip for the year = 2'-04 



We might proceed to consider the diurnal variation, as obtained from the selected series of 10 days and 

 5 days in each month, but this will be done with more accuracy in connection with another year's observations ; 

 at present it will be enough to give the variation for the year, as deduced from the 120-day and 60-day series. 

 The variations for these series, deduced from the last columns of Tables XXVIII., XXIX,. XLIV., and XLV., 

 and reduced so that the mean is equal to the mean of the last column of Table L.V., are as follow : — 



12 h l h A.M. 2>> 3 h 4li 5h 6 h 7 h 8 1 ' 9 h 10h llh Qh lh P , M . 2 h 3 h 4' 1 5 h 6 h 7 h 8 h 9 h 10 h ll h 



Series. ' ' t r t t t r * i t .i i / // 



120-day, 0-54 0-60 0'66 0-71 0-69 0.68 0-81 1-02 1'46 1-83 1-98 1-71 1-28 0-74 0"47 0-28 0-24 0-07 -0'04 -0-05 0"05 0-15 0"21 029 



60-day, 0-48 0-54 0-61 0-66 0-68 0-69 0-83 1'02 1-49 1-84 1-96 V76 1-30 0'76 0-49 0'34 0-29 0-04 -0-03 -004 0'01 0-11 0'24 0-30 



The means for both series give the same result ; the maximum of dip occurs at 10 h a.m., and the minimum 

 at 6 h 40 m p.m. : the secondary maximum and minimum are not exhibited in these variations ; the dip, however, 

 has nearly a constant value from 2 h till 5 h a.m. The results for the selected series, which are nearly unaffected 

 by intermittent disturbances, place the principal minimum an hour and a- half later than the result for the whole 

 series, and the removal of the disturbances seems nearly to obliterate the secondary maximum and minimum in 

 the diurnal variation. The range of the mean diurnal variation for the year from all the three series is slightly 

 above 2', which is equivalent to a variation of 6' of magnetic declination ; the range of the diurnal variation of 

 magnetic declination for the year being 7'"7, the motion in declination is rather greater than that in dip at 

 Malcerstoun. 



Ranges of the Mean Diurnal Variation of Magnetic Dip. — The ranges of the mean diurnal variation for 

 each month, as deduced from the whole series of observations, are as follow : — 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0'-78 



0'-82 



2'-12 



3'- 19 



3'-49 



3'-21 



3'-60 



3'-34 



2'-86 



2'-44 



l'-36 



0'-84 



These quantities follow the same law as that already obtained for the horizontal component. The range 

 of magnetic dip is a minimum, and has a nearly constant value, in the three winter months, December, January, 

 and February ; the range for November is exaggerated by intermittent disturbances. The range is a maximum, 

 and is nearly constant in the months of April, May, June, July, and August. When the ranges of magnetic 

 declination are reduced to their equivalent motions, as shewn by a needle freely suspended in the direction of 

 magnetic dip, it will be found that the vertical motion is greater than the horizontal motion in May and July, 

 that the amounts of the two motions differ little in the months for which the sun is north of the equator, and 

 that in the months for which it is south of the equator the horizontal motions are considerably greater than the 

 vei'tical motions. 



Diurnal Variation of Magnetic Dip with reference to the Moon's Hour-Angle. — The following Table con- 

 tains the variations of magnetic dip, as deduced from the last three columns of Tables XXXI. and XL VII. : — 



