22 PROFESSOR FORBES'S EXPERIMENTS ON 



36. Combining these by the method of least squares, we obtain the following 



\ahies : — 



X = — .000210 



y = + .000100 



z = — .000053 

 SI' = — .0028 

 Hence it appears, that on the same parallel of latitude the intensity increases 

 in a westerUi direction, which is the reverse of the result found for the course of 

 the isodjTiamic lines in the Alps ; but, in truth, I do not attach much importance 

 to these observations, unless for the sole consideration of height, on accoimt of 

 the small area of country over which the observations were made. There were 

 probably in the Pyi-enees some som'ces of local disturbance which the observa- 

 tions on the Pic de Bergons particularly indicate, and which, having been repeated 

 ^vith coincident results, could not be owing to an en'or of obsen^ation.* At the 

 same time it is satisfactory to find that the influence due to height is the same in 

 direction, though greater in amount than that obtained in the alpine series. On 

 this subject I proceed to offer some remarks. 



37. The first experiments which seem to have had even remotely in view the 

 question of the decrease of magnetic intensity with height are those of Saussure, 

 made during his memorable stay on the Col du Geant in 1788. The observations 

 were too rude, and differ too widely from each other to deserve much confidence ; 

 but those made at Chamouni and on the Col du G^ant, which were fortunately 

 imder ahnost the same temperatm-e, agree very closely, but give a slightly greater 

 intensity to the latter, which is the effect due to the latitude.! The gi-eat dimi- 

 nution of intensity in going from Geneva to Chamouni, observed by Saussure, is 

 certainly erroneous, as the reverse has been shewTi to take place. 



38. In 1804 M. Gay Lussac performed his celebrated aerostatic ascent, and 

 from his magnectic observations concluded that no appreciable difference of inten- 

 sity existed at the smface of the earth and at the height of 23,000 feet. This, 

 however, can only be considered as referring to gi'eat and palpable change. The 

 difficulties inseparable from the experiment prevented many oscillations from being 

 observed, or great precision in the times from being attempted, whilst corrections 

 for arc, diurnal variation, and temperatm-e. were not apphed. The last of these, 



* Since this passage was written, on mentioning to Piofessor Necker of Geneva, the anomalous 

 result as to the direction of the isodynamic lines in the Pyrenees (anomalous, because differing from the 

 supposed direction inserted in Hansteen's maps, which is deduced from analogy, and not, I believe, 

 from direct observations in that country), he pointed out the curious (though perhaps accidental) coin- 

 cidence which this result offers to the views he has long entertained as to the general parallelism of the 

 lines of geological elevation, and those of magnetical intensity, which the bearing of the isodynamic 

 lines which I have given for the Alps remarkably confirms. 



t Saussure, Voyages aux Alpes, § 2103. Tom iv. 



