68 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVI.; 
be in the axis of this curve or cone, and its return, either in declination or inclination, 
to the mean is an important indication of the amount and position of the variable 
forces which influence it at such times. 
2933 . My hypothesis does not at all assume that the heated or cooled air has become 
magnetic so as to act directly on the needle after the manner of a piece of iron, either 
magnetically polar or rendered so under induction. There is no assumed polarity of the 
oxygen of the air other than the conduction polarity ( 2822 . 2835 .) consequent upon 
a slight alteration of the direction of the lines of force. The change in the magnetic 
conducting power causes this deflection of the lines ; just as a worse conductor of heat 
introduced into a medium of better conducting power disturbs the previous equable 
transfer of heat, and gives a new direction to that which is conducted ; or as in static 
electricity, a body of more or less specific inductive capacity introduced into a uni- 
form medium disturbs the equable lines of force which were previously passing 
across it. 
2934 . The sole action of the atmosphere is to bend the lines of force. The needle 
being held by these lines and, when free, being parallel to them, changes in position with 
the changes of the lines. It is not necessary even that the lines, which are immediately 
affected in direction by the altered air, should be those about the needle, but may be 
very distant. The whole of the magnetic lines about the earth are held by their mutual 
tension in one connected sensitive system, which has no sluggishness anywhere, but 
feels in every part a change in any one particular place. There may be, and is con- 
tinually, a new distribution of force, but no suppression. So when any change in direc- 
tion happens, near or distant, the needle in a given place will feel and indicate it, and 
that the more sensibly according to the vicinity of the place and the kind of change 
induced ; but the disposition of the whole system has been affected at the same mo- 
ment, and therefore all the other needles will be affected in obedience to the change 
in the lines of force which govern them individually. 
2935 . The needle is a balance on which all the magnetic power around a given 
locality fastens itself, even to the antipodes, and it shows for each place every varia- 
tion in their amount or disposition, whether that occurs near or far off. Its mean 
position is the normal position ; and as regards atmospheric changes, the fixation of 
the lines offeree in the earth ( 2919 .) is that which tends to give the lines a standard 
position (exclusive of secular changes), and so bring them and the needle back from 
their disturbed to their normal state. Hence, whilst considering the causes which 
disturb either the declination or the inclination, arises the importance of keeping in 
mind the mean position or place of the needle ( 2932 .), and not merely the direction 
in which it is moving. 
2936 . So the well-known action of the sun on the needle is, by my hypothesis, very 
indirect ; the sun at a given place affects the atmosphere ; the atmosphere affects the 
direction of the lines of force ; the lines of force there affect those at any distance, and 
these affect the needles which they respectively govern. 
2937 - I have, for the sake of convenience in considering a special action of the 
