334 



Results of the Makekstoun Observations, 1844. 



Variations of Westerly Declination with reference to the Moon's Declination. — The following are the means 

 of groups for 1844 : — 



27 days to 1 dav, Moon farthest North, 0'-50 



2 5 •■' 0'-26 



6 8 ... 0'13 



9 12 .- 0'-07 



13 days to 15 days, Moon farthest South, 0'-33 



16 ■••••■ 19 ■•'• 0'-44 



20 22 ... 0'-32 



23 26 ... 0'-43 



The principal maximum occurs about tlie time when the moun is farthest north, the principal minimum 

 after it has crossed the equator going south ; a secondary maximum is indicated after the moon is farthest 

 south, and a secondary minimum after crossing the equator moving north. 



Variations of Westerly Declination, with reference to the Moon's distance from the Earth : — 



6 days after apogee to 6 days before perigee, 0'-20 



5 days to 2 days before perigee, 0''3 1 



1 day before to 1 day after perigee 0''50 



2 days to 5days after perigee, 0'-59 



6 days after perigee to 6 days before apogee, U''68 



.5 days to 2 days before apogee, 0'"63 



1 day before to 1 day after apogee, 0'- 76 



2 days to 5 days after apogee 0''53 



In 1844, the principal maximum occun-ed about apogee, and the principal minimum about midway be- 

 tween apogee and perigee. 



Since a revolution of the perigee is performed in about 9 years, it will require observations for that 

 period in order that the variations due to changing distance may be eliminated in the discussions for change 

 of the moon's declination, and vice versa. If the variations due to one argument be much smaller than 

 for the other, a shorter series of observations may suffice ; this, however, will be determined chiefly by the 

 extent of coincidence in the results of successive years. In the case of any argument for which the varia- 

 tions are small, the large variations due to irregular causes will, in general, render a single year's observations 

 insufficient for a confident conclusion. Where, therefore, in the following discussions no extra reasons are 

 brought forward in support of the results, they should be considered as results for the year 1844 only, which 

 may be wholly or partially contradicted or confirmed by the results for other years. It has not been always 

 thought necessary to point out the coincidences or oppositions of the results for 1843 and 1844, and that espe- 

 cially, on account of the incompleteness of the series for 1843. 



Annual Variation of the Diurnal lianges of Magnetic Declination. — The following are the monthly means 

 of the diurnal ranges from Table III. : — 



Jan. 

 9'-00 



Feb. 



10'-28 



March. 



16'-21 



^\pril. 



15' -88 



.May. 



13'-48 



June. 



12'-41 



.July. 

 12'-36 



Aug. 



14'02 



.Nept, 



1.5'-22 



Oct. 

 15'-69 



Nov. 

 15'-91 



Dec. 



ll'-22 



Whence the mean diurnal range is a maximum twice and a minimum twice, in the course of the year ; 

 it is a maximum in the months of March and April, and again in the months of Septcnilier, October, and 

 November, it has its least value in the month of January, and it is a secondary minimum in the months of 

 June and July. The diunial ranges of magnetic declination were greatest about the equinoxes, and least near 

 the solstices. 



When we compare these means mth the ranges of the mean diurnal variation of each month (see Table 

 VI.), we obtain the following quantities, which are the excesses of the monthly means of the diurnal ranges 

 over the diurnal ranges of the monthly mean variations : — 



From which it appears, that those irregular causes which render the mean of the diurnal ranges greater 

 than the range of the mean diurnal variation, have their maximum effect in March and November, their 

 minimum in June, and a secondary minimum in January, or nearly according to the law stated above for the 

 annual variation of the diurnal ranges. 



