Terrestrial Magnetic Intensity. 367 



be sensible, the variation of temperature being no less than 

 36° Cent., and as cold tends to increase the apparent intensity, 

 if no such increase was observed, it might plausibly be argued 

 that the real intensity had diminished. It must, however, be 

 observed, that the observations lasted only in general from 

 one to two minutes, and that in so short a period (and de- 

 pending on a single value of the elapsed time) the accelera- 

 tion due to the above-mentioned cause would hardly be per- 

 ceptible. Taking the mean result of the effect of temperature 

 ascertained by myself for No. I. and " Flat" needles, we find 

 the factor —'00037 applicable to the time for a decrease of 

 1° R. of temperature, which agrees exactly with Hansteen's 

 mean correction. If we apply this to Gay-Lussac's observa- 

 tion we find a correction of —•0108 as a factor applicable to 

 the time, for the effect of —36° Cent, of temperature. Yet 

 large as this is, amounting to T £oth part of the whole, the 

 discrepancies of observation often amount to double that 

 quantity*. Still we admit with M. Kupffer that the probabi- 

 lity deducible from M. Gay-Lussac's observations, is in favour 

 of a slow diminution. 



39. The next series of observations includes those of Hum- 

 boldt and Gay-Lussac, recorded in the Memoir es d'Arcueilf, 

 which include observations made in the Alps, though at no 

 great heights; and here no particular influence of height was 

 observed, nor was indeed looked for J. 



40. Since that period the subject seems to have met with 

 little practical attention, until the recent publication of M. 

 Kupffer' s " Voyage au Mont Elbrouz" by the Petersburg 

 Academy. From his observations with a needle by Gambey, 

 half a metre long, M. Kupffer attempts to deduce, not only 

 the fact, but the amount of the diminution with height, and 

 this upon the authority of a single experiment, and at no con- 

 siderable elevation §. In fact, all the intricate corrections 

 which this delicate observation requires were little more than 

 guessed at. The difference of geographical position of the 

 two stations (19/ in lat. 38' in long.) was allowed for by ob- 

 servations made with a different apparatus, — the effect of 

 temperature was deduced from indirect experiments, far from 



* See the details of the observations in the Annates de Chimie. An xiii. 

 (1805), torn. lii. p. 75. f Tom. i. p. 1. \ Ibid. p. 10. 



§ The observations were not made at the summit of Mont Elbrouz, as 

 stated in the Annuaire du Bureau des Longitudes, 1836, p. 288, but near the 

 foot of it, and the difference of height of the stations was less than 5000 

 English feet. The stations were " Pont de Malka," and " Hauteur de 

 Kharbis." 



