SECT. XXXIV. CONSERVATION OF FORCE. 345 



quently the magnetic force of the atmosphere must increase from 

 the equator to the poles of maximum cold ; it must vary summer 

 and winter, night and day. Its effect upon terrestrial magnetism 

 is unknown ; but it can hardly be without some influence. M. 

 E. Becquerel observes " If we reflect that the earth is encom- 

 passed by a mass of air equivalent in weight to a layer of mer- 

 cury of 30 inches, we may inquire whether such a mass of 

 magnetic gas, continually agitated, and submitted to the regular 

 and irregular variations of pressure and temperature, does not 

 intervene in some of the phenomena dependent upon terrestrial 

 magnetism. If we calculate, in fact, what is the magnetic force 

 of this fluid mass, we find that it is equivalent to an immense 

 plate of iron, of a thickness little more than ^ g of an inch, 

 which covers the whole surface of the globe." Both the 

 conducting power of the air and its density are increased 

 by cold ; and as the sum of the magnetic forces which issue 

 from the earth on one side of the line of no dip is equal to their 

 sum on the other side, the intensity and concentration in our 

 winter are coincident with a diffusion and feebleness in the oppo- 

 site hemisphere, so that the line of no dip will move annually 

 from north to south and back again. The same holds with 

 regard to day and night. Thus the law of the conservation of 

 force is rigorously maintained ; and it is equally so in the effect 

 of the atmosphere on the magnetic lines of force, which refracts 

 them as they pass through it, in one direction in summer, and in 

 the opposite direction in winter in one direction in the enlight- 

 ened hemisphere, in the other in that which is dark. The whole 

 of the magnetic lines about the earth are held by their mutual 

 tension in one connected, sensitive system, which feels in every 

 part, even to the antipodes, a change in any particular place. 



It may be mentioned as a well-known fact, that apparent 

 anomalies have been found in the diurnal variation of the decli- 

 nation in the high magnetic latitudes of the northern hemisphere 

 when compared with their great regularity in other parts of the 

 same hemisphere, and that the magnetic storms are of much 

 greater magnitude there than in lower latitudes. Moreover, 

 although Captain Maguire's observations at Cape Barrow, in 

 the North Polar Ocean, show that the annual and diurnal varia- 

 tions of the casual disturbances or magnetic storms, as well as 

 those of the decennial period, are maintained, yet it appears that 



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