154 



The following Comrnunications were read : — 



1. Observations on Terrestrial Magnetism made in different 

 parts of Eui'ope, especially with reference to the in- 

 fluence of Height. By Professor Forbes. 



These observations have reference chiefly to the intensity of the 

 earth's magnetism, and were made for the most pai"t with Han- 

 steen's apparatus, in the possession of the society. 



The Jirst section of the paper refers to the method of maiiing the 

 observations, which is nearly that of Professor Hansteen. 



The secojid refers to the corrections applied. These are — 1. for 

 the rate of the chronometer; 2. for reduction of the vibrations of 

 the intensity needles to infinitely small arcs ; 3. for the eflfect of tem- 

 perature in diminishing the magnetism of the needles, which was 

 determined for each by direct experiment ; 4. for changes in the 

 earth's magnetism ; 5. for progressive changes in the needle's mag- 

 netism, which were considerable for one of the Hansteen needles, 

 but for the other very small. 



The third section contains tabular views of the results obtained 

 from different series of observations, but particularly from one se- 

 ries made in the central chain of Alps in 1832, and another in the 

 Pyrenees in 1835. The horizontal intensities at several detached 

 points, as Edinburgli, Brussels, and Paris, were also ascertained; 

 that at Edinburgh is expressed by .840, Paris being 1.000. 



In the fourth section, the results are grouped and analyzed, the 

 Alpine series being first taken, and the relations of the intensities 

 determined with respect to latitude, longitude, and height. The 

 same was done for the Pyrenean observations ; the results being 

 not merely graphically deduced, but actually calculated from as 

 many equations of condition as there were stations, by the method 

 of least squares. The following numbers were obtained : — 



Alps. Pyrenees. 



By Needle By " Flat" 



No. 1. Needle. Both Needles. 



Variation of intensity for 1' of I «„,^„„. „„,.^ „„„„,„ 



latitude, N increasing. . ) —000364 -.000505 -.000210 



Variation of intensity for 1' of) „„„„ 



longitude, E increasing, | +-000055 +.000106 -.000100 



™heig°ht!°!'""'.'' '" '""} -000033 -.000027 ' -000053 



Of these results, those obtained in the Alps, and by needle No. 1, 



are the most trust-worthy. From the variation due to latitude and 



longitude, the direction of the isodynamic lines is easily found. In 



