Magnetic Declination. 343 



hour-angle is ll*", or an hour before its inferior transit, and of a minimum of westerly declination 4'' 30"" 

 before its superior transit. The range of the variations is less than I'S. 



When the moon is in opposition south of the equator, the variation is double, — there are two maxima 

 and two minima ; the maxima have nearly equal values, one occurring immediately before the moon's transit 

 of the inferior meridian, the other about 2^ 25>" after the superior transit. The principal minimum occurs 

 about 4'^ 30™ after the inferior transit, and the secondary minimum about 4*" before it. The range of the 

 variations is about I'-O. 



In the means for the whole 12 lunations, there are also two maxima and two minima; the principal 

 maximum occurs about half an hour before the moon's inferior transit, and the principal minimum about 

 6 hours before the superior transit ; the secondary maximum occurs about 4'' 20" after the superior transit, and 

 the secondary minimum about 4 hours before the inferior transit. The range of the variations is about I'-O. 



When we compare these results, with those already obtained for the sun's hour-angle, we find, that 

 whereas the diurnal variation due to the sun has its greatest range in the summer half-year, the reverse is 

 the case for that due to the moon wliich is greatest in the winter. It should be remembered, however, that 

 in several respects the winter is to us for the moon, what the summer is for the sun. In winter, we have 

 the greatest amount of moonshine when the moon is north of the equator. In winter, the moon is in 

 opposition to the sun, when the former is north, and the latter south, of the equator ; whereas it is the 

 sun which is north, and the moon south, of the equator, at opposition in summer. It does not seem 

 improbable, then, that the diurnal variation due to the moon may have a variable character through each 

 lunation, in a manner analogous to the change of the solar diurnal variation from summer to winter, from the 

 greatest north to the greatest south declination. If this be the case, the lunar diurnal variation will not be 

 eliminated in the usual summations for the solar diurnal variation, and, in such a fact, we might find some ex- 

 planation of the secondary maximum wliich is exhibited most distinctly in the solar diurnal variation for winter. 



The minimum of westerly declination occurs when the sun and moon are on the same declination circle, 

 — that is, about 5 or 6 hours before the superior transit ; but the maximum occurs for the moon when near 

 its inferior transit, and for the sun when past its superior transit. 



Several of the aingle lunations shew the results given above with considerable accuracy, and the verity of 

 the results has been confirmed by the discussion of the observations for 1345, which will be found in its 

 proper place. 



Intermittent Disturbances. 



Since in the process of selecting the series of 1 20 and 60 days, those days only were adopted which were 

 free from all large disturbance, the 60-day series probably being nearly free from disturbance even of the 

 smallest class, it is evident that the differences between the means for these series and the means for the whole 

 series of observations should indicate the law, and approximately the amount, by which intermittent disturb- 

 ances affect the continuous variations. The following are results of the comparisons of the undisturbed series 

 with the complete series :— 



Effect of Intermittent Disturbances on the Yearly Mean Declination. 



The mean declination for 1844, deduced from the whole series of hourly observations, = 25°'17'0G W. 



120days - 25°-17'-08 ... 



60 .." = 25°-ir06... 



This remarkable result proves that intermittent disturbances have no effect on the mean position of the 

 declination magnet for the year ; and, therefore, that in the course of the year the integrals of the disturbing 

 forces are equally positive and negative. 



Effect on the Monthly Mean of Declination. — If the days selected had been regularly distributed over 

 each month, this result might have been expected with some accuracy ; this regularity of distribution, however, 

 could not be obtained in consistence with the other condition of freedom from irregularity ; and the means for 

 the selected days in each month may be expected to dift'er from the true mean, were it from the effect of secular 

 change alone. The diflerences are small even with this drawback. The average difference (independent of sign) 

 of the mean for the 10-day series from the true monthly means being only 0''1, the greatest difference being 0'-2 ; 

 and for the 5-day series, the average difference is 0'-2, and the greatest 0'-46. It seems therefore very probable 

 that the effect of intermittent disturbances upon the mean position of the magnetic needle for a month is also zero. 



A comparison of the daily means renders it extremely probable that were a sufficient number of observa- 

 tions made use of to obtain the true mean on days of much disturbance, the results would not differ more from 

 the means for undisturbed days at the same epoch than would be accounted for by the regular laws of variation. 



Effect of Intermittent Disturbances on the Hourly Means of Declination. — The following Tables contain the 

 differences, for each month, of the hourly means obtained for the whole hourly observations, from those for the 

 selected series of 10 days and 5 days. 



