DECENNIAL PERIOD OF THE MAGNETIC VARIATIONS, ETC. 589 




Three-Hour Means. One-Hour Means. 
1854, == = 2:86 = 291 
1855, = = 3-24 = = 3-35 
1856, — = _ 4-05 a = 4:16 
1857, a2 213-16 a = 3-28 
1858, == = 2:93 a = 3-05 
1859, 28 aes a 
TEGO MMA pili al desi = 29% 22 = 3-21- 
1861, = = 291 _ = 3-09 
1862, Ss 87 SS hit 
NeCsva Tse yee = = 2-90 os = 3-22 
1864, LE a = 310 
From all these series, we conclude that, in general, the ratio is greatest when 
the disturbance is least, and vice versa. 
49. There is nothing in these ratios which can indicate a connection between 
the increase of mean disturbance from the hour of minimum to the hour of 
maximum in different years; nor between the increase from the hour of mini- 
mum and the howr of maximum in the year of minimum, to the same hours in 
the year of maximum disturbance. We do not know the nature of the forces 
which produce these deviations, nor their mode of action, but we may suppose 
that the latter will be the same in different years, and that the deviations should 
be related by some common law. The following result may aid in this de- 
termination :— 
(50.) Let us represent the maximum and minimum mean disturbance for 
any year by D,, and D, respectively, then we find that the following relation 
holds very nearly, 
/Dn — /D, = constant. 
The quantities, Table IV., do not give us the exact values of the maxima and 
minima of the hourly mean disturbance, since these will not occur in general at 
the exact hours of observation, but they may be taken as near approximations. 
We find from the following results the errors of the preceding equation. 
