B. TERRESTRIAL PHYSICS AND METEOROLOGY. 97 



by Nervander at Helsingfors, from 1844 to 1848, afforded 

 him an excellent point of comparison, and the means of de- 

 termining the diurnal period of the declination. The annual 

 variations could then be investigated, and finally the secular. 

 From twenty-two years of observations, between 1841 and 

 1862, he has thus been able to compile a table representing 

 the normal hourly variations of declination for each month, 

 and for the entire year ; from which it appears that the 

 greatest western declination occurs between one and two 



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o'clock A.M. in every month, being a little earlier in sum- 

 mer and a little later in winter, with a secondary maximum 

 just before two o'clock, and closely following the principal 

 maximum. The greatest easterly declination occurs in the 

 winter months between nine and ten A.M. During the rest 

 of the year it varies between seven and nine A.M. The 

 other maximum of easterly declination is not so decidedly 

 expressed, and occurs in winter and spring between ten and 

 eleven A.M. In respect to the secular variations of the dec- 

 lination, it would appear to have varied between two and 

 a half degrees west in 1727, to six and a half degrees west 

 in 1831, reaching its maximum about the year 1806, when it 

 was about nine and a half decrees west. He suggests that 



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if the increasing accuracy in our means of measurement is to 

 have any meaning at all, and not be a useless expenditure 

 of time and money, it becomes necessary now that, in any 

 place where a magnetic observatory is surrounded by an 

 assemblage of houses, there should be annually made, for 

 purposes of comparison, an independent standard determina- 

 tion of the declination at some point near by but entirely 

 outside of the buildings in question, and free from their in- 

 fluence. Wild's Hepertwium, IV., art. 1. 



ATMOSPHERIC ELECTRICITY. 



In reference to atmospheric electricity, Sir William Thom- 

 son states that if one ascend any mountain peak on a fine 

 day, and there prove that the surface of the earth on the 

 peak is negatively electrified, the result will be valuable to 

 science; and if on several days the ground is found to be, 

 all day and all night, negatively electrified, then there will 

 be a very great acquisition to our knowledge regarding at- 

 mospheric electricity. According to him, positive atnios- 



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