370 Proceedings of the British Association. 



safely pronounced to be beyond the limits of the errors of observa- 

 tion. Subsequently, Gay-Lussac ascended in a balloon to the alti- 

 tude of about, or perhaps beyond twenty three thousand feet, yet his 

 observations also, vi^hen due allowance was made for alteration of 

 temperature, gave no alteration of the magnetic intensity. But the 

 researches of M. Kuppfer seeming to conduct to a quite opposite 

 conclusion, and the result, as stated by him, being such as, if the 

 observations were correctly made, would give a diminution of the 

 magnetic intensity for stations whose elevation above the earth was 

 considerable, which could by no means be accounted for by ordinary 

 errors of observation, Prof. Forbes deemed this a matter of so much 

 importatice to science, that he determined to make an extended 

 series of observations at various levels among the Pyrenees and 

 Swiss Alps. Accordingly, having last summer provided himself 

 with a sufficient number of magnetic needles fit for making proper 

 observations upon magnetic intensity, and their times of vibration at 

 Paris having been accurately ascertained, he commenced his tour 

 for this purpose in the neighborhood of Barege and Bagnieres ; and, 

 from a multitude of observations which he had made and recorded, 

 he now wished to select a series of forty five observations made at 

 thirteen different stations, the elevation of which above the level of 

 the sea varied from six thousand to ten thousand feet. Before he 

 detailed these observations and their results, he described the prin- 

 ciple upon which they were conducted, and which appeared ingen- 

 ious, and well calculated to lead to satisfactory results. In each in- 

 stance, the observations were made at three distinct stations — one 

 on the summit of the mountain peak, or most elevated spot ; and 

 two at a lower, but equal level on each side of the hill, so chosen, 

 that a vertical plane would pass through the three stations, and be 

 perpendicular to the axis or length of the hill. It is obvious then, 

 that, speaking generally, any disturbing effect exercised upon the 

 needle by the materials of the hill at one of the lower stations, 

 would be opposite in kind to that exercised at the other of the two 

 lower stations ; and, therefore, the mean between these observations, 

 made at the two lower stations, would give the magnetic intensity at 

 a point immediately beneath the upper station. By a comparison 

 of this mean intensity, therefore, with the intensity at the upper sta- 

 tion, it could be readily proved whether or not the intensity dimin- 

 ished as you ascended to a greater elevation. The result of the 

 entire of this laborious course of experiments was, that, with the 



