MAGNETIC DECLINATION, AT DIFFERENT PERIODS OF THE DAY. 639 
For the mean Diurnal Variation, it is manifestly indifferent whether the secular 
change be eliminated or not ; the line DD retains in either case its position and 
form unchanged, for the sum of the monthly corrections for secular change in every 
case =0. If, therefore, the Annual Variations were substituted for the annual 
ranges in the dark vertical lines in Plate XXVII., we have reason to believe that a 
correct representation would be given of what would be the facts of the Annual and 
Diurnal Variations if no secular change whatsoever existed ; or if a secular change 
were to take place (as has recently been the case in Britain) in the opposite direction 
to that which previously prevailed, and were eliminated by the same process. 
But when the vertical lines of Annual Variation are thus placed on the Diurnal 
Variation, projected as in Plate XXVII. according to its mean declination-value, the 
complexity occasioned by the two phenomena being viewed together is considerable, 
and is greatly increased when, as in Plate XXVI., four different stations are exhibited 
on the same plate, for the special purpose of examining and comparing one class of the 
phenomena only, (those of the Annual Variation,) as presented at different stations. 
To facilitate this examination and comparison, the curve, if such it may be called, of 
Diurnal Variation, has been projected in Plate XXVI. as a straight horizontal line ; 
and in that Plate consequently the Annual Variations at the several observation hours 
may be viewed independently of Diurnal Variation, as well as of Secular Change. 
On directing our attention to this Plate, it is perceived at the first glance, that the 
range of variation at all the stations is considerably greater during the hours of the 
day than during those of the night ; and that there is a great similarity, though not 
a perfect identity, at all the stations, in the relative amount of the range at different 
hours. The amount does not progressively enlarge to a maximum at any obviously 
natural epoch, — such as, for example, at or about noon, when the sun’s altitude is 
greatest, or at the early hours of the afternoon when the temperature is greatest, — but 
it will be distinctly seen that at all the stations the increase of the range is most rapid in 
the first or second hour after sunrise, and that its extent at the hours from 7 to 9 a.m. 
(19^ 20*' and 21*') is not exceeded at any subsequent hour at Hobarton, the Cape, 
and St. Ilelena ; whilst at Toronto the great enlargement of the annual range takes 
place even earlier, the hours of 6, 7 and 8 a.m. being exceeded by none, though they 
are equalled by a second increase at noon and the two following hours. This second 
enlargement is also perceptible at the same hours, though not to the same extent, at 
Hobarton and St. Helena. 
On examining the distribution of the months at the different hours, or their 
relative positions to each other in the several vertical lines, we may perceive that 
certain months, which are found congregated at the one extremity of the range 
during the early hours of the morning, undergo a transfer towards the opposite ex- 
tremity at a subsequent period of the day; thus June, July and August, which, in 
respect to their positions in the range, may generally be grouped together, occupy 
usually one extremity of the range, — and November, December and January, which, 
4 N 2 
