to the determination oj the Magnetic inclination. 165 



and from those of Humboldt and Forbes, this mean decrease for 

 the thirty-six years between 1806 and 1842 



= 3' 2"-3. 



This result agrees with Hansteen's investigations on the 

 observations made at other places in Europe, by showing that 

 the yearly decrease of inclination in Gottingen has become less. 

 If t be the number of the year, and I the inclination, we may set 



1=67° 23' 43"-122"-29(^-1850) + l"-337(^-1850)2, 



whence we obtain the following comparison of the observed with 

 the calculated values : — 



Observed. Calculated. Dift'erence. 



Dec. 1805 69 29 6 69 36 43 -7 43 



Sept. 1826 68 29 26 68 23 17 +6 9 



July 1, 1837 67 47 67 52 41 -5 41 



67 53 30 67 52 41 +0 49 



Oct. 8, 1841 67 42 43 67 42 +0 43 



June 21, 1842 67 39 39 67 40 18 -0 39 



Aug. 7, 1852 67 18 38 67 18 34 +0 4 



According to this, the annual decrease of inclination at Got- 

 tingen during twenty-two or twenty-three years has diminished 

 one minute ; in round numbers, therefore, the annual decrease — 



Hence, in all probability, the inclination in Gottingen will con- 

 tinue to decrease until the year 1895, when it will attain a 

 minimum of 



66° 37' 7", 



and from that time it will increase. 



From this investigation, Weber concludes that the inclina- 

 tion instruments hitherto employed, being more portable, will 

 continue to be used on journeys in preference to his own ; but 

 that in fixed observatories, where magnetometers are kept for 

 other purposes, it will be found advantageous to apply induc- 

 tion to observations on the inclination with the magnetometer, in- 

 asmuch as a great simplification of labour and a uniformity in the 

 treatment of all the three elements of the earth's magnetism is 

 thereby attainable. 



