Horizontal Component of Magnetic Force. 



369 



These quantities give the same annual law as has already been obtained for the magnetic declination. 

 The maximum disturbance of the horizontal component occurs near the equinoxes, and the minimum 

 disturbance occurs near the solstices. Wliile the effect of disturbances on the hourly means is less for the 

 horizontal component than for the magnetic declination, the individual observations of the former differ more 

 fi-om the corresponding monthly means than for the latter, the diurnal range in each case being the standard 

 of comparison. 



TABLE XXXV. — Mean Difference of a Single Observation of the Bifilar Magnetometer from the 

 Monthly Mean at the corresponding hour, with reference to the Moon's Age, Declination, and 

 Distance from the Earth, for 1844. 



This Table has been formed from Table XXXTV., in the manner already described, Table II. 



Variation of the Mean Differences with respect to the Moon's Age. — From the first portion of Table XXXV. 

 it appears that the average difference is a maximum about opposition, and a minimum near conjunction. The 

 following are the means of groups : — 



So. Div. 



Sc. Div. 



1 day, New Moon, 3-85 



5 days, 3-55 



9 ... 4'32 



13 ... 5-22 



14 days to 16 davs. Full Moon, 477 29 days to 



17 20 ..'. 4-87 2 



21 24 - 3-15 6 



25 28 ... 3-46 10 



These means indicate in a general manner the result stated above. There is, however, a secondary maxi- 

 mum apparent at the time of conjunction, minima occurring before and after that epoch. 



The average difference for the 11 davs (10 — 20), including full moon, = 4-97 Sc. Div. 

 11 .'. (25-5), newmoon, = 3-60 



Variations of the Mean Differences with reference to the Moon's Declination. — A general examination of 

 the second portion of Table XXXV. shews that the average difference of a single observation from its corre- 

 sponding monthly mean is a maximum twice, namely, when the moon is about two days north of the equator, 

 and that it is a minimum twice, namely, when the moon is farthest north and farthest south. This will be seen 

 also in the following means of groups :- 



Sc. Div 



27 davs to 1 day, Moon farthest North, 3-81 

 2 ..'.... 5 days, 4-68 



6 8 ... 4-20 



9 12 ... 3-64 



Sc. Div. 

 13 days to 15 days, Moon farthest South, 3-71 



16 19 ... 4-00 



20 22 ... 4-07 



23 26 ... 4-82 



From these the principal minimum occurs when the moon is farthest south ; the difference between the 

 minima is small, and is perhaps accidental, as the result for the magnetic decUnation placed the principal mini- 

 mum when the moon was farthest north. (See p. 347.) 



MAG. AND MET. OBS. 1844. 5 A 



