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, when 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 
importance 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 Barége and Bagniéres; 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—oné 
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, — 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- 
as you ascended toa greater elevation. The result of the 
entire of this laborious course of experiments was, that, with the 
