DISTRIBUTION OF THE MAGNETIC HORIZONTAL FORCE AM) 



TOTAL FORCE. 



Distribution of (lie Magnetic Horizontal Intensity and Construction of Isodynamu 



Lines for 18 I .'. 



If we group the observed intensities in the same manner as the dip, the mean 

 epoch 1842.0 may likewise be assumed, and all observed intensities be reduced to 

 that date. 



Correction to Epoch. — We have tin- following direct comparisons: — 



The average annual decrease in the value of X between 1840 and 1862, is, there- 

 fore, 0.0030, or, when expressed in parts of X, equal to 0.00076. Tins result 

 agrees tolerably well with that deduced by Assistant Chas. A. Schott in the Coast 

 Survey Report of 1861, where 0.00110 was found. 



Supposing the dip to increase at the rate of 1' a year, and the total intensity to 

 remain constant, the corresponding decrease of the horizontal intensity would 

 amount to nearly the quantity found above-; we cannot, therefore, as yel decide 

 whether the total intensity remains stationary, or is slightly changing. 



1 Front Coast Survey Report of lSlil (yet in manuscript). 



1842.5, X = 4.347, Captain Lefroy. 

 1843.5, =4.292, Dr. Locke. 

 Mean, 1843.0, = 4.320 



2 In July and Nov. 1840, X = 4.160 \ 



In July and Nov. 1841, = 4.1GC I Mean, 4.166 for 1842.0. 

 In July, 1843, = 4.17 J > 



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