Ix General Results of the Makeestoun Observations. 



The variations of each of these groups of years exhibit a law which may be thus stated : — The magnetic dip 

 is greatest near the eqinoxes, and it is least near the solstices. The variations for the years 1842—5 are perhaps 

 least affected by disturbances ; for these years the two maxima have nearly equal values, but the minimum at 

 the summer solstice is greater than that at the winter solstice : in the variations for the years 1843-6 the two 

 minima have nearly equal values, but the two maxima are unequal, the maximum at the autumnal equinox 

 being greatest (See Plate VI.) : and in the variations for the years 1842-7, both maxima and minima are 

 unequal ; the greatest maximum occurring at the autumnal equinox, and the greatest minimum at the summer 

 solstice. The same results are to be obtained for the annual period of the horizontal component, the differences 

 depending upon the amount of disturbance in the different groups of years. 



123. When we deduce the monthly means of magnetic dip from the observations of the two component mag- 

 netometers, made on the days selected as nearly free from irregular disturbance, in the years 1844 and 1845, 

 we find the means from all the hourly observations in those years greater ( + ) or less ( — ) than the former by the 

 following quantities. (See Nos. 57 and 59.) 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 



+ 0'158 + 0'-294 +0'-258 + 0'-196 -0'-037 -0'-026 -0'-032 + O'-06O +0'-038 4 0-348 +0'-150 +0'-356 



The effect of disturbance on the monthly mean magnetic dip is negative in the three months May to July, 

 and is positive in the remaining months. If these means be subtracted from those for the corresponding 

 months in Table 56, it will be found that the annual period from the undisturbed days has the same epochs 

 as that from the disturbed days. (See No. 58.) 



124. Annual Variation of the Ranges of the Monthly Mean Variation of Dip. — The following are the 

 ranges of the mean variation for each month, from four years' observations, as obtained from Table 57. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



I'-IO 



0'-72 



2'-06 



3'-26 



3'-82 



3'-71 



4'- 04 



3'-88 



2'-92 



2'-39 



l'-46 



I'-Ol 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



0'-53 



0'-62 



l'-85 



3'-27 



3' -2 5 



3'-26 



3'- 04 



3'-23 



The range of dip was least in February, and in the three months December, January, and February ; it was 

 greatest in July, and in the months from May to August, the range for June being slightly less than for 

 May, July, and August. The following are the ranges of the mean variation, obtained from the observations 

 on the selected days of 1844 and 1845. 



Sept. Oct. Nov. Dec. 



3'-10 l'-94 l'-38 1'05 



The range of the nearly undisturbed mean diurnal variation is least in January, and it is nearly constant 

 in the months from April to September. (See No. 93.) 



125. Variations of the Daily Mean Magnetic Dip ivith reference to the Moon's Age. — The following num- 

 bers, the means for groups of days from four years' observations, are obtained from the last column of the first 

 parts of Tables 28 and 45. 



Moon's Age. li*— 16<i ITd— 20'i 21<'— 24'i 25'i— 28"! 29i— l* 2<i— 5^ G^— Q'l 101—13* 



Variations, + 0'-069 + 0'-147 + 0'-034 - 0'-137 - 0'-123 - 0'-088 + 0'012 + 0'-085 



These numbers shew that the magnetic dip was greatest immediately after opposition, and that it was least 

 immediately before conjunction. 



126. Variations of the Daily Mean Magnetic Dip with reference to the Moon's Declination. — The follow- 

 ing numbers, also derived from four years' observations, are obtained from the last columns of Table 28 and 45. 



Day after Moon 1 ^yd id 2^ — .5'> &^ — 8<i 9* — 12* 13* — 15* 16* 19* 20* — 22* 23* — 26* 



farthest North. J 



Variations, - 0'-043 + 0'-003 + 0'-072 + 0'-004 - 0'-051 - 0'-027 + O'-OOl -I- 0'-038 



It appears therefore, that a minimum of magnetic dip occurred when the moon was farthest north, another 

 minimum occurred when it was farthest south, and maxima occurred when the moon was near the equator. 

 This law is exactly the same as that for the annual variations. No. 122. 



127. Monthly Variations of the Range of Dip. — These and the analogous results for the mean difference 

 cannot be derived from Tables 29, 30, and 46, 47, but require the conversion of all the hourly observations 



