184 W. A. Norton on the Variations of the Declination 
comprehensive sense ; that is as comprising all the variations that 
occur during the day, whether we compare them hour by hour, 
or by longer intervals. 
There are two classes of diurnal variations of the magnetic 
elements—viz, those which are Periodic and those which are 
Irregular, or more properly Occasional. Thus any one element, 
as the declination, regularly increases during a certain portion 
of every day, and then as regularly decreases during another 
portion of the day. Changes also occur, toall appearance fortu- 
itously, at any hour of the day; so that their occurrence cannot 
be predicted for any one hour. Still it is now known that the 
irregular variations, so called, are under the control of certain 
tain parts of the day the liability is to an increase, at other parts 
to adiminution. If we compare the amount of the declination, 
or other element, at any hour of the day, with the same at the 
same hour of the following day, we find that a change has oc- 
curred, and if we do the same throughout the year we discover 
that the element in question, or its amount at a particular hour 
increases during half of the year, from a certain day, and de- 
creases during the remaining half; in other words, it undergoes a 
regular variation, the period of which is a year. ‘The amount of 
the change of the declination, or horizontal or vertical force, 
that takes place during a day, also varies from’ one season to 
another. Besides the periodic variations whose period is a day, 
or half a day, or a year, there is another class, recently discovered, 
it 1s 
hose period is about ten years (10 to 11 years hus, 
found that the amount of the alternate increase and decrease of 
the declination durin is greater some years than others, 
and that it alternately augments and diminishes during a pe- 
riod of ten or eleven years. It is an interesting and very 1m- 
portant fact, in a physical point of view, that this period has been 
found to be identical with that of a change which has been ob- 
served to occur in the number and magnitude of the spots on the 
sun from year to year, the maximum and minimum of the one 
282; the 
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