Chemistry and Physics. 415 
Series. Those produced by difference of temperature were described 
in the Philosophical Magazine for 1847, but are now resumed with 
more care, and found to belong to it alone, and not to nitrogen or car- 
bonic acid ; as its temperature is raised its paramagnetic force dimin- 
, its magnetic power. Ifa mass of the air be cooled it becomes mor 
paramagnetic, if heated it becomes less paramagnetic (or diamagnetic,) 
as compared with the air in a mean or normal condition. 
The effect of the approach and retreat of the sun in his daily course 
‘i 
apidly eastward, as the sun passes by, until two o’clock, the dip then 
decreasing ; after which the needle goes west again, following the sun. 
On examining the results at Toronto, corresponding effects were found 
to occur, when the upper or south end of the needle was consi ered, 
and therefore in accordance with the hypothesis. ‘The examinations of 
the observations made at Greenwich, Washington, Lake Athabasca, 
Fort Simpson, and St. Petersburgh, are considered as adding further 
confirmation. By the aid of these observations the author restates his 
principles more minutely, endeavoring to indicate what difference, 
changes in the inclination, declination, place of the sun, land, and sea, 
&e., will produce. . 
hough the sun is the cause of those changes in the oo 
1 
Sie emt, oc | ie fl a 
which affect the lines of force of the earth, he is not assumed as 1 
centre of action as regards those lines; that is considered to exist some- 
minishes the dip at places which are within the tropics, and with little 
inclination, as St. Helena. By other kinds of observations, it appears to 
be in advance of the sun. All the phenomena indicate that the sun 
does not act directly on the needles at different places, but mediately 
_ through its effect on the atmosphere. : 
_ The author then considers the possible cause of numerous irregular 
variations, such as thos that are shown by the photographic processes 
