3(>6 Professor Forbes's Experiments on 



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 obser- 

 vations were made. There were probably in the Pyrenees 

 some sources of local disturbance which the observations on 

 the Pic de Bergons particularly indicate, and which, having 

 been repeated with coincident results, could not be owing to 

 an error of observation*. At the same time it is satisfactory 

 to find that the influence due to height is the same in direc- 

 tion, 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 in- 

 tensity with height are those of Saussure, made during his 

 memorable stay on the Col du Geant in 1788. The observa- 

 tions were too rude, and differ too widely from each other to 

 deserve much confidence; but those made at Chamouni and 

 on the Col du Geant, which were fortunately under almost 

 the same temperature, agree very closely, but give a slightly 

 greater intensity to the latter, which is the effect due to the 

 latitude |\ The great diminution of intensity in going from 

 Geneva to Chamouni, observed by Saussure, is certainly er- 

 roneous, as the reverse has been shown to take place. 



38. In 1804 M. Gay-Lussac performed his celebrated 

 aerostatic ascent, and from his magnetic observations con- 

 cluded that no appreciable difference of intensity existed at 

 the surface of the earth and at the height of 23,000 feet. This, 

 however, can only be considered as referring to great and 

 palpable change. The difficulties inseparable from the ex- 

 periment prevented many oscillations from being observed, 

 or great precision in the times from being attempted, whilst 

 corrections for arc, diurnal variation, and temperature, were 

 not applied. The last of these, however, could hardly fail to 



* 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, decause differing from the supposed direction 

 inserted in Hansteen's map, which is deduced from analogy, and not, J be- 

 lieve, from direct observations in that country), he pointed out the curious 

 (though perhaps accidental) coincidence 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 con- 

 firms. 



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



