760 REPORT— 1898. 



the ground was uniform and its surface plane. He expresses the results of his deter- 

 minations by tracing the course of the isogenic line for 10° 30', which passes through 

 the west and is parallel to the lines in Italy, that for 10° which passes through the 

 centre, and for 9° 30' whicli passes through the east of the island and is profoundly 

 modified by the presence of Etna, the isoclinal lines for 54°, 53° 30', 53°, 52° 30"', 

 52°, which, with the exception of the 2nd and 3rd, passing respectively to the 

 north and south of the volcano, are quite uniform, and the isodyuamic line for -252, 

 wliich has nearly the same shape as the isoclinals. By comparison of his results 

 with those obtained by Ghristini in 18S2, the author concludes that the annual 

 changes of the elements are : — 



Declination ...... — 5'6' 



Inclination --1'4' 



Horizontal Intensity . . . . + -000017 



2. On the Influence of Altitude above the Sea on the Elements of Terrestrial 

 Magnetism. By Dr. van Rijckevorsel and Dr. W. van Bemmelen. 



The object of an investigation, which lasted from 1895-97, was to see if we 

 could detect any influence of altitude above the sea on the magnetic elements. 

 For various reasons, the Iligi was thought to be the mountaiu most suitable to 

 our purpose, and in 1895 our observations were taken solely in order to discover 

 if this mountain was really non-magnetic. We found that it was so. 



In 1896 complete series of observations were taken, both on the mountain and at 

 its base, at a large number of stations. The discussion of these observations seemed 

 to show a very slight decrease of the horizontal and a somewhat larger, though 

 still very slight, increase of the vertical component. 



However, this result was very doubtful, because we found at the same time 

 that the Rigi, though certainly non-magnetic as a mass, was covered all over with 

 superficial centres of attraction, very weak, but still causing disturbances which 

 seriously interfered with our results. 



We resolved, therefore, to try again in 1897. In that year we took dips only 

 at 198 stations, one of us always observing near the base of the mountain, the 

 other, absolutely simultaneously with him, on tne top or on the slopes. Although 

 this time also the small, but numerous, local disturbances were a decided drawback 

 for the discussion, the following results were obtained : 



1. We think it is proved, that if there be an influence of the height above the 

 sea on terrestrial magnetism it is so exceedingly small as to make it unfeasible to 

 detect it with our present surveying instruments. Therefore it is decidedly 

 unnecessary to take altitude into account in magnetic surveys. 



2. If, nevertheless, it should ultimately be found that there is such an influence, 

 however small, it is more likely that it would be an increase of the vertical force 

 with altitude than anything else. 



3. On the Variation of Terrestrial Magnetic Force with Altitude.^ 

 By Professor J. Liznar. 



The author has endeavoured to eliminate the effect of the nature of the soil 

 and shape of the surface at any point, on the variation of the magnetic force with 

 height above the surface at the point, by basing his calculations on the results for 

 the 205 stations in Austria at which observations of the magnetic elements have 

 been made. He divides the stations into three groups — the first including those of 

 altitude less than 200 metres, the second between 200 and 400, and the third 

 above 400 metres. The mean value of an element for the stations of a group, he 

 considers to be the value of the element for the mean altitude of the group, almost 



' The complete paper is published in Wiener Ameiger, 1898, p. 168. 



