ATMOSPHERIC MAGNETISM — IRREGULAR VARIATIONS. 
75 
air together, or makes the air more dense in one region than another, or proceeding 
from one to another, balances regions which before were in different conditions, 
then every such change will be accompanied by a corresponding change in the dis- 
position of the magnetic force, to which we may perhaps hereafter be able to refer 
by means of our instruments. Even tides in the air ought to produce an effect, 
though it may be far too small to be rendered sensible. 
2955. The precipitation of rain or snow is a theoretical reason for the change of 
magnetic relations in the space where it takes place; because it alters the tempera- 
ture where such precipitation occurs, and relieves it from a quantity of diluting dia- 
magnetic or neutral matter. A chilling hailstorm might affect the needle in a sum- 
mer’s day. Clouds may have a sensible influence in several ways ; acting at one time 
by their difference from neighbouring regions of clear air, and at other times by 
absorbing the sun’s rays, and causing the evolution of sensible heat at different altitudes 
in the atmosphere at different places, or preventing its evolution more or less at the 
surface of the earth. Those masses of warmer or colder air of which meteorologists 
speak, which being transparent are not sensible to the eye, will produce their pro- 
portionate effect. And hypothetically speaking, it is not absolutely impossible that 
the hot and partially deoxygenated air of a large town like London, may affect in- 
struments in its vicinity ; and if so, it will affect them differently at different times, 
according to the direction of the wind., 
2956. If one imagines on the surface of the earth a spot which shall represent the 
resultant there of the atmospheric actions above, and can conceive its course as it 
wanders to and fro, under the influence of the various causes of action which have 
been in part referred to, whilst it still travels onwards with the sun, one may have an 
idea of the manner in which it may affect the various observatories scattered over 
the earth. I believe that its course, as regards the east and west direction of its 
wanderings, is partly told in the photographic registerings of Greenwich and Toronto, 
being there mingled in effect with other causes of variation. This spot may be con- 
centrated or diffuse ; it may pass away and reappear elsewhere ; there may even be 
two or more at once sufficiently strong to cause vibrations of the needle between them. 
2957- The aurora borealis or australis can hardly be independent of the magnetic 
constitution of the atmosphere, occurring as it does within its regions, and perhaps in 
the space above. The place of the aurora is generally in those latitudes the air of 
which has a distinct magnetic relation, by difference of temperature and quantity, to 
that at the equator, and the magnetic character both of the aurora and of the medium 
in which it occurs ties them together ; therefore, to be aware of and to understand in 
some degree the latter, will probably direct us to a better comprehension of the former. 
The aurora is already connected with magnetic disturbances and storms ; it may in 
time connect them with changes in the atmosphere in a manner not at present anti- 
cipated, and as the suggestion is founded upon principle it seems deserving of con- 
sideration. 
