ATMOSPHERIC MAGNETISM — GENERAL PRINCIPLES. 
95 
as the cold retreats, the needle will return east to its mean place ; assuming that there 
is no other action for the time than that of the cold region. The upper end of the 
free needle, therefore, at any given place will tend towards the cold region, just as 
before it tended fro7n a warm region ; and as the declination is affected, so also the in- 
clination will be. If the cold be on the magnetic meridian of a place within the 
tropics, as St. Helena or Singapore, it will increase the dip there, whilst at the same 
moment it is diminishing the dip at places south or north of it having considerable 
dip, a result which follows directly from the inflection of the lines of force into or 
towards the cold region. 
3005. The chief regions of heat and cold on the same parallel of latitude, do 
not follow each other at equal intervals of time. It is difficult to make a judgement 
regarding their interval in the atmosphere above; but the maximum of cold on the 
earth for the twenty-four hours, is assumed by many as being seventeen hours after the 
preceding noon, and only seven hours from the coming noon. This brings into con- 
sideration the joint effect of hot and cold regions in deflecting the lines of force, espe- 
cially during the forenoon and middle of the day. If a cold region be only three and 
a half hours west of a place at the same time that the warm region is three and a half 
hours east of it, it is very manifest that the joint effect of the two, for both act then to 
cause the same deflection, will be far greater than that of the heat or cold alone, or 
than any corresponding effect at other periods, for neither twelve hours after, nor at 
any other time, will there be an equivalent condition of circumstances; and so it is 
also for other combinations of hot and cold regions, the effect of which will vary both 
by position and by their extent. A free needle is held in tension by the lines, which 
are themselves governed by the hot and cold regions of atmosphere ; it probably never 
occupies its mean place, but is always in the resultant of these ever-present and ever- 
varying causes of change. 
3006. As the earth revolves under the sun, each place would have, speaking gene- 
rally, a maximum and a minimum of temperature for its atmosphere in the twenty- 
four hours. But looking at the globe as a whole, there would be one maximum and 
two minima, i. e. there would be a maximum region somewhere beneath the sun in 
his path, and a minimum in each of the polar regions ; which, as regards the twenty- 
four hours, wmuld not be at the pole, but in some place of high latitude, and perhaps, 
as before, seven or eight hours before noon. These cold regions will be very seriously 
affected in their extent and place and power by the position of the sun between the 
tropics ; for as he advances to one tropic the cold region there will diminish in extent 
and force, whilst the other will grow up in importance; and whilst they thus vary in 
their power of influencing the general direction of the lines of force, they will vary in 
their own position also, and have at different times very various relations of place 
to the sun in different months, and so produce very various effects. It is these dif- 
ferences which are made manifest to us, as I believe, in the night and morning actions 
at the numerous observatories scattered over the globe. 
