NATURAL PHILOSOPHY. Ill 



" On a former occasion it was shown that oxygen gas was magnetic, 

 being attracted towards the poles of a magnet ; and that, like other 

 magnetic bodies, it lost and gained in power as its temperature was 

 raised and lowered, and that the change occurred within the range of 

 natural temperatures. These properties it carries into the atmosphere, 

 and the object of this lecture was to show how far they might be ap- 

 plied to explain certain of the observed variations of the terrestrial mag- 

 netic force. The earth is a great magnet ; its power, according to 

 Gauss, being equal to that which would be conferred if every cubic 

 yard of it contained six one-pound magnets ; the sum of the force is 

 therefore equal tO f 8,464,000,000,000,000,000,000 such magnets. The 

 disposition of this magnetic force is not regular, nor are there any 

 points on the surface which can properly be called poles ; still the re- 

 gions of polarity are in high north and south latitudes, and these are 

 connected by lines of magnetic force (being the lines of direction) which, 

 generally speaking, rise out of the earth in one (magnetic) hemisphere, 

 and, passing in varied directions over the equatorial regions, into the 

 other hemisphere, then enter into the earth to complete the known 

 circuit of power. A free needle shows the presence and direction of these 

 lines. In London they issue from the earth at an angle of about 69 

 with the horizon (being the dip or inclination) ; and the plane in which 

 they rise forms an angle of 23 AY., nearly with true north, giving 

 what is called west declination. "Where the dip is small, as at the 

 magnetic equator, these lines scarcely rise out of the earth, and pass 

 but a little way above the surface ; but where it is large, as in northern 

 or southern latitudes, they rise up at a greater angle, and pass into the 

 distant realms of space, from whence they return again to the earth 

 in the opposite magnetic hemisphere ; thus investing the whole globe 

 with a system of forces like that about an ordinary magnet, which 

 wherever it passes through the atmosphere is subject to the changing 

 action of its magnetic oxygen. There is every reason to believe that 

 these lines are held in the earth, out of which they arise, and by which 

 they are produced, just as the lines which originate in a magnet are 

 held by it, though not in the same degree ; and that any disturbance 

 from above, affecting them, will cause a greater change in their place and 

 direction in the atmosphere and space above, than in the earth beneath. 



" The system of lines of magnetic force, around a magnet or the earth, 

 is related by a lateral tension of lines of static electrical force ; both 

 the one and the other being easily made manifest by experiment. The 

 disturbance of the tension in one part is accompanied instantly by dis- 

 turbance of the tension in every other part ; for as the sum of the ex- 

 ternal powers of a system, unaltered at its origin, is definite and 

 cannot be changed, so any alteration, either of intensity or direction 

 amongst the lines offeree at one place, must be accompanied by a cor- 

 responding change at every other. So, if a mass of soft iron on the 

 east side of a magnet causes a concentration of the lines of force from 

 the magnet on that side, a corresponding expansion or opening out of 

 the lines on the west side must be, and is, at the same time, produced ; or 

 if the sun, on rising in the east, renders all the oxygen of the air on 

 that side of the globe less magnetic and less able therefore to favor the 



