358 Results of Makeestoun Observations, 1844. 



Variation of the Horizontal Component with reference to the Moon's Age. — This law is very distinctly marked 

 in the means given in Table XXIII. The horizontal component of magnetic force is a miidmum at opposition, 

 and a maximum at conjunction. This will be more evident from the following means of groups : — 



14 days to 16 days, Fidl Moon, 0-0001 1 1 



17 ..".... 20 ..". 0-000090 



21 24 ... 0000339 



25 28 ... 0000543 



29 days to 1 day, New Moon, 0-000564 



2 5 ... 0-000561 



6 9 ... 0-000435 



10 13 ... 0-000081 



An examination of the daily means of the hoi-izontal component shews, that this law is indicated with 

 great distinctness in seven or eight lunations of 1844, — namely, in the lunations between January 5 and 

 April 3, and between August 2 and December 20. The law is distorted or exaggerated in some lunations 

 by the larger disturbances. In all probability, the mean range is exaggerated by the larger negative dis- 

 turbances which appear to have occurred about full Moon in 1844. 



Variations of the Horizontal Component with reference to the Moon's Declination. — This law is also well 

 marked in the means in Table XXIII. The horizontal component is a maximum when the moon has its 

 gi-eatest north, and also its greatest south declination ; and it is a minimum between these epochs, when the 

 moon is rather north of the equator. The following are means of groups : — 



27 days to 1 day. Moon farthest north,... 0-000441 

 2 ..'.... 5daVs 0-000328 



6 8 ,.'. 0000193 



9 12 ... 0-000332 



13 days to 15 days, Moon farthest south, 0-000477 



16 19 ..'. 0-000426 



20 22 ... 0-000228 



23 26 ... 0-000134 



This law con-esponds with that of the annual period, and may be stated, generally, as follows : — The 

 liorizontal component is a maximum when the sun or moon has its greatest north and south declination, and 

 it is a minimum between these epochs, when the sun or moon is near the equator. 



Annual Variation of the Diurnal Ranges of the Horizontal Component. — The following are the monthly 

 means of the diurnal ranges : — 



0-00 , 



The diurnal range is therefore a minimum in January, a secondary minimum in June, a maximum in 

 April, and in August; or generally, it is a minimum near the solstices, and a maximum immediately after the 

 vernal and before the autumnal equinox. In the annual variations, therefore, it appears that when the 

 horizontal component is a maximum, the diurnal range is a minimum, and vice versa. 

 The mean of the diurnal ranges for the year = 0-003760. 



'WTien the means of the diui-nal ranges are compared with the ranges of the mean diurnal variations (see 

 p. 363), we obtain the following excesses of the former compared with the latter: — ■ 



Jan. Feb. March. April. May. .lune. .luly. Aug. 8ept. Oct. Nov. Dec. 



000 I 1230 1675 2785 1612 0866 0531 0633 1099 1074 1377 1835 1750 



Whence it appears, that the causes which render the mean diurnal range gi*eater than the range of tlie 

 mean diurnal variation, have their minimum effect in June, a secondary minimum being in January; their 

 maximum in March, and a secondary maximum in November. This is tlie same result as that already 

 obtained for the magnetic declination. 



some instrumental causes, and considering t\w means ftir the last three years, commencing December 1S42, we have the following 

 quantities {Resultate, p. 77) : — 



Dec. Jan. Fob. March. April. May. .Iiinc. July. Aug. .'^ept. Oct. Xnv. 



1'93 I 657 633 637 663 653 750 760 783 770 677 710 780 



These means include the secular change, and they follow nearly the same law of variation as the means at the foot of Table 

 XXII. ; in fact, an e.vamination of the above quantities will shew that any elimination of secular change which is near the truth will 

 give the law as already stated. Taking the mean secular change from 1843 — 1845 =. 0-0018 a-year (Resultate, p. 27), and reducing 

 the values to December at the rate of O'OOOIS a-month, we obtain the following means : — 



Dec. Jan. Feb. Mart-Ii. April. May. Junp. July. Aug. Sept. Oct. Xov. 



1-93 I 657 618 607 618 593 675 670 678 650 542 560 615 



These appear to give a sufficiently distinct result; the horizontal component is a maximum in December and in June (May, 

 June, and July having nearly the same value), and it is a minimum in April, and a principal minimum in September. This seems 

 nearer to the results of the Makerstoun Observations than could be hoped for, when we consider the diflerence of instruments (Dr 

 LaMONT's having a unifilar, and tht* Makerstoun instrument having a bifilar suspension), and the consequent liabilities to different 

 kinds of instrumental error. 



