178 KNOTT AND TANAKADATE 



At each place, three observations at Jeast of all the elements were 

 made, one in the morning, one near noon, and one in the evening. 

 In the final tnbulation, the arithmetic means of the dips and horizontal 

 intensities are given. The mean declination is however obtained by 

 a different method. From the complete observations made at Hagi 

 (No. 72) and Hamada (No. 73), means were taken by summing the 

 24 ordinates corresponding to each hour of the civil time. In both 

 cases this mean came out lower than half the sum of the principal 

 maximum and principal minimum by (Due-tenth their difference. 

 Thus if djcV be the maximum and minimum respectively, the true 

 mean was found to be 



d + d' d — d' 

 2 Î0~~ 



This rule is applied to all the observations. In several cases the 



maximum and minimum are only inferred from the neighbouring 



points.* 



The magnetometer used by the South Party presents many 



points of novelty, both in construction and in mode of using. It 



combines the ordinary apparatus for the measurement of the horizontal 



force with a special form of declinometer invented by Mr. Tanaka- 



date and described in a paper published in the Proceedings of the Royal 



Society of Edinburgh (1884-6).+ The whole apparatus is built up 



upon a theodolite, which in its ordinary form as an alt-azimuth 



instrument serves for all the astronomical observations necessary for 



findino- the latitude, lonaûtude. and meridian of the station, and the true 



sidereal time. The instrument is shown in Plate VI, mounted for its 



* A glauce at the curves will show that this method gives a mean in neither case 

 differing by as much as half a minute from the value of the declination as observed between 

 the hours of Ö and 12 p. m., so that (if we disregard the possibility of the existence of a 

 magnetic storm at these hours) the methods adopted by the North and South Parties to 

 obtain a good mean declination give quite concordant results. [C O-. K] 



t Also in the Pàçialnilnjôlnm Zn^xjii (Vol. II). 



