343 



examination of each day's observations in the years 1844 and 1845, 

 a series was selected, in each month, of days nearly unaffected by 

 magnetic irregularity ; the diurnal variation was then obtained for 

 these undisturbed days, and this was compared with the diurnal vari- 

 ation deduced from all the observations ; the assumption being made 

 that the mean for the whole 24 hours was unaffected by disturb- 

 ance, the differences of the hourly values would evidently shew the 

 effect of disturbance on the hourly mean position. This assumption, 

 it was found, must be as nearly as possible true for the magnetic 

 declination, because the monthly means of the selected days differed 

 little or nothing from those of all the days ; this, however, is not the 

 case for the element of dip, the disturbance appeared to affect the 

 daily or monthly mean to a small extent. Confining myself here 

 to the result for the year (referring to the volume of the Transac- 

 tions for the partial results which vary with season) ; the following 

 numbers indicate the displacement of the mean hourly positions by 

 disturbance, upon the assumption that the centre of gravity for each 

 figure is the same : — 



12h 14'> 16'> ISh 20l> 22'> C" 2h ;4h 6'^ S^ lO*" 12l» 



0-35 0'-25 0'-06 0'-15 0'-27 0'-30 0'-23 0'-31 0'-30 0'17 0'-31 0'-39 0'-35 



The diameter of the figure is little greater than 2'*0. 



In the mean figure for the year (see Plate VIII. already referred 

 to), minima occur at 4'' a.m. and about 5^^'' p.m., the maximum occurs 

 about 10^^ P.M., and a maximum occurs between &^ a.m. and 4'" p.m. 

 If, making allowance for the effect of disturbance on the position of 

 the centre of gravity with reference to dip, we suppose the centre of 

 gravity of the dotted figure for the year, raised 0'-15 on the line of 

 mean declination, or that of the continuous figures lowered as much, 

 we find the maximum effect of disturbance to occur about 10'^ p.m. 

 and lO** A.M., and the minimum eflfect about 4'' a.m. and b^ p.m. 

 This result was obtained for the magnetic declination in 1844, and 

 is given in the volume for that year. 



Motions with reference to the Moon's Hour-Angle. — These, as 

 obtained from the means of all the lunations in the years 1844 and 

 1845, and as deduced from winter lunations for 1845 only, are 

 shewn in Plate VII. The resulting figures, especially that for the 

 winter lunations of 1845, bear some resemblance to the diurnal mo- 

 tion for the month of December. 



