22 A. TANAKADATE. 



by ]:)lainmetric method, and is indicated by a dotted line in the 

 phite, where only a part of the curve is obtained the mean is 

 inferred from those of tlie neighbouring curves. 



The diurnal curve was also useful in warning the observers 

 of magnetic storms, during which the regular observations had 

 to be sustained. 



§ 7. Reduction to the Epoch 1895.0. 



In order to reduce the observed values to a common epoch, 

 we require to know the secular variation of each magnetic element. 

 It was hoped by taking observations at the stations of the pre- 

 vious survey of 1887 this might be found at each place. On 

 carrying out the work, however, it was found that observations 

 in most of the old stations were impracticable on account of 

 changes that have since taken place in their surroundings. Al- 

 though many of the names in the list of observations are the 

 same, there are only 7 places where regular observations were 

 made at the identical spot ; the values of dip and horizontal 

 intensity given in tlie list under the heading " Station 1887 " 

 were nnxde again now for the sake of reference even though the 

 surroundings had somewhat changed. Under these circumstances 

 it was considered not expedient to derive the annual variation 

 in the way first thought of. 



The method used is to find mean secular variations by 

 com})aring the empirical expressions of magnetic elements in terms 

 of longitude and latitude, which were already worked out by Prof. 

 Knott, with those of the present treated in similar manner, 

 su})posing those two sets of values to represent the magnetic 

 state of the country at the epochs corresponding to the means of 



