ATMOSPHERIC MAGNETISM — GREENWICH VARIATIONS. 
101 
in the afternoon, in winter than in summer; hence the swing- is thrown forward in 
time in winter ; and though prolonged, its termination coincides with the termination 
in summer, as far at least as these two-hour observations can indicate. 
3020 . As the region precedes the sun, the degree of mean declination here ought to 
make the day-swing come on early, i. e. earlier than at Hobarton, and especially 
earlier than at Toronto, unless other causes of variation interfere. Now the begin- 
ning is earlier than at Toronto, but the end the same. Both the beginning and the end 
is an hour earlier than at Hobarton. The latter difference I believe due to the differ- 
ence of mean declination : at Toronto, I think we shall find another cause influencing 
the time ( 3032 .). 
3021 . We are to remember also that in winter the sun or warm region passes the 
magnetic meridian two hours before he passes the astronomical meridian ; and there- 
fore his effect in giving west position to the south or upper end of the needle ceases 
long before it does in summer, and perhaps even before it ceases to come nearer; 
and so the eastern after-effect on it ought to be greater, which it is. This eastern 
effect should be strengthened also, because the action of the warm region on the 
needle ought to be comparatively great after passing the magnetic meridian ; for its 
path forms an obtuse angle with the meridian before the passage and an acute one 
afterwards ( 3000 .), and therefore is more powerful. To all these causes of action will 
be added the effect for the time of the cold in the distant west ( 3005 .). 
3022 . The case of difference of direction before 19 ^ ( 3016 .) is very marked at 
Greenwich, as may be seen by looking at the Curves for the months, Plate II. The 
south or upper end of the needle goes west in May, June, July and August, from 12'* 
to 19 '*, i. e. from midnight to five hours before noon ; but in October, November, De- 
cember and January, it is eastening at the same hours. Considering first a summer 
month, as June, the upper end of the needle is westward as the sun comes onward 
(as it ought to be) until 19 '*, when he is almost in the middle of his passage through 
the east quadrant ; and in respect of distance and angular relation to the magnetic 
meridian, the warm region is then, probably, in the place of greatest power to pro- 
duce westening of the needle end*. In the next six hours the needle passes to ex- 
treme east, performing, according to the observations, a fourth of the whole swing 
in the first two hours, a half in the next two, and a fourth in the remaining two, the 
journey being no doubt with first rapidly increasing and then rapidly diminishing 
velocity. In this transit of the region, the sun is for about two-thirds of the time 
in the eastern quadrant, and one-third in the western ; and his path in the latter third 
forms almost the base of an equilateral triangle with Greenwich, having the magnetic 
meridian for one side, so that all that time it is close to and therefore has strong action 
on the needle ( 3000 .). The sun is at D in such a position as respects this angle, that if 
* It must not be forgotten, that the return from an extreme east or west position is not when the sun or 
warm region passes by a neutral line, or from one quadrant to another, but when it passes its point of greatest 
action in a quadrant (2982.). 
