22 PROFESSOR FORBES'S EXPERIMENTS ON 



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

 \'alues : — 



a: = — .000210 



?/ = + .000100 



== — .000053 



U' = — .0028 

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

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

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

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

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

 probably in the Pyi-enees some sources of local disturbance which the observa- 

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

 with coincident resiilts, 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 difier too widely from each other to deserve much confidence ; 

 but those made at Chamouni and on the Col du Geant, which were fortunately 

 under ahnost the same temperature, agree very closely, but give a shghtly greatei' 

 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, observ^ed by Saussure, is 

 certainly erroneous, as the reverse has been shewn 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 surface 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, dim-nal variation, and temperature, were not applied. The last of these, 



* Since this passage was written, on mentioning to Professor 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. 



■j- Saussure, Voyages aux Alpes, § 2103. Tom iv. 



