^rAGNETIC SURVEY OF JAPAN. 



219 



The Magnetic Constants for the Mean Station. 



Dip 



Horizontal Force 



TcT:il Force 



Declination 



V. — The Diurnal Variation. 



Plates XTT to XV show graphically the daily march «»f tlie 

 Dec]iiiati(m at various Stations, as ohserved l)y ^Ir. Tanakadate. For 

 the earlier curves comparatively few points Avere taken ; but as tlie 

 survey progressed the ol)servation of successive declinations was found 

 to he such a simple matter with the electromagnetic declinometer thai 

 Mr. Tanakadate made it a special feature of his work. Fhe chief 

 value of such observations in the present case is that fi-om them a 

 thoroughlv good mean for the day may be obtained. As a Avhole, 

 the c rves are of a character which speaks well for the accuracy and 

 sensitiveness of Mr. Tanakadate's instrument. The sensitiveness 

 indeed depends simply and solely upon the fineness of graduation on 

 the theodolite circle. In iudi^ing of the merits of the curves, we must 

 bear in mind the conditions of the experimenf. Foi' not only will 

 any possible magnetic storms disturb the general smoothness of the 

 observations, but also windv and wet weathei- will almost certainly 

 cause disturbances in measurements made by an instrument mounted 

 0!i an ordinary tripod under a teut. 



Taking the 17 lu'st and most complete sets of observations (see 

 Appendix 1>). we find for the mean daily range for tlic three months 

 beginning July 4th the value 81 This is about 1' greater than the 

 mean diurnal range obtained by Mr. Wada during his five months of 



