368 Professor Forbes's Experiments on 



presenting a mutual agreement ; and the whole difference of 

 level (4500 French feet) offered a very small basis for so ge- 

 neral a conclusion. But, besides this, there is an oversight 

 in M. Kupffer's deductions (first pointed out to me by Pro- 

 fessor Necker of Geneva), which tends yet further to diminish 

 the probability of his conclusions. The estimate of the effect 

 of geographical position on the magnetic intensity, M. Kupffer 

 conceives to be such, that the variation for 12' in lat. (dimi- 

 nishing from the lower to the upper station) would exactly 

 counterbalance the variation due to 38' in E. long, (also di- 

 minishing from the lower to the upper station); the one in- 

 creasing the duration of an oscillation as much as the other 

 diminished it. Now it appears from his own statement on the 

 preceding page (Memoir, p. 87), as well as from the known 

 direction of the isodynamic lines, that these variations con- 

 spire with one another, so as to render the anomaly attributed 

 to height greater than before. The upper station is S.W. of 

 the lower, the direction of the isodynamic lines is from N.W. 

 to S.E., consequently the variation of position is such as would 

 diminish the time of vibration of the needle, whilst in effect it 

 was found to be increased. From M. Kupffer's data, I find 

 that the time of one vibration of his great needle by Gambey 

 (24 s, 05 nearly) would be diminished about S, 104 for the 

 change of latitude and longitude, whilst it was observed to 

 be increased by S, 063. The anomaly, then, instead of being 

 s -063, as M. Kupffer states it (and which he attributes to the 

 effect of 4500 French feet of elevation), would be S, 167, or 

 nearly three times as great. M. Kupffer's law of an increase 

 of '000583 of the whole time of vibration, for a rise of 1000 

 French feet, will therefore, when corrected, amount to '00155, 

 and the factor for the diminution of intensity to twice as much, 

 or *0031, which is just ten times as great as my observations 

 indicate, and is so considerable, that, were the conclusion just, 

 it could not fail to be detected by the most ordinary instru- 

 ments at the most ordinary elevations. 



41. But if the anomaly be admitted to exist in M. Kupffer's 

 observations, whence does it arise? I have no difficulty in 

 answering the question. I shall not dwell upon the incom- 

 plete data from which the corrections due to temperature, 

 latitude, &c. are derived ; nor upon the entire incompetency 

 of a single observation, which unknown causes (for instance, 

 an iron mine, or the occurrence of an aurora borealis) may 

 affect. I take M. Kupffer's own statement in the geological 

 section of his work, which pronounces the whole country sur- 

 rounding Mont Elbrouz to afford one continued evidence of 



