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72 



0?i Atmospheric Magnetism. 



time. As the night came on and a temperature lower than the 

 mean came up from the east and passed over, the lines of force 

 would be inflected as in P or Pj and a reverse variation of the 

 needle to that which occurred before would now take place - 



The natural effects of variation must be produced consequent 

 upon the magnetic nature of oxygen and its daily variations of 

 temperature is manifest ; but whether they cause the observed 

 variations, or are competent to do so, is a question that can only 

 be decided after very careful enquiry! Observations are now 

 made on the surface of the earth with extreme care in many 

 places, and these are collated, and the average or mean result, as 

 to direction and intensity of the earth's force, ascertained for 

 every hour and season ; and also many remarkable, anomalous, 

 and extra results evolved, A theory of the causes of any or all 

 of these variations may be examined first by the direction which 

 the varying needle does or ought to assume, and then by the 

 amount of the variation. The hypothesis now brought forward 

 has been compared by the mean daily variation for all the months 

 in the year at north and south stations, as Toronto and Hobarton, 

 and at many others near to and far from the equator, and agrees 

 in direction with the results observed far beyond what the author 

 anticipated. Thus the paths described by the upper ends of free 

 needles in the north and south hemispheres should be closed 

 curves, with the motion in opposite and certain directions, and 

 so they are : — the curves described by needles in north and south 

 latitudes should be larger in summer and smaller in winter, and 



so they are ; 



a night or cold action should 



row up in the win- 



ter months, and such is the case : — the northern hemisphere 

 oAght to have a certain predominance over the southern, because 

 of its superior temperature, and that is so; — the dispositioft of 

 land and water ought to have an influence, and there is one in 



so that in the first statement and examina- 



the right direction: 



tion of the hypothesis it appears to be remarkably supported by 

 the facts. All these coincidences are particularly examined into 

 and stated in the Philosophical Transactions already referred to. 

 The next step will be to ascertain what is the amount of change 



in the conducting power of the air for given changes of temper- 



ature, and then to apply that in the endeavor to ascertain whether 

 the amount of change to be expected is (as well*as the direction) 

 accordant with that which really occurs. * - 



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