•24 



PROFESSOR FORBES'S EXPERIMENTS ON 



long, (also diminishing from the lower to the upper station) ; the one increasing 

 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 knoA\Ti direction of the isod_ynamic lines, that these variations conspu-e 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 isodjoiamic lines is 

 from N. W. to S. E., consequently the variation of position is such as would di- 

 juinish the time of vibration of the needle, whilst in effect it was found to be in- 

 creased. From M. Kupffer's data, I find that the time of one vibration of his great 

 needle by Gajibey (24s.05 nearly) would be diminished about 0M04 for the 

 change of latitude and longitude, whilst it was observed to be increased by O'.063. 

 The anomal.y, then, instead of being 0'.063, as M. Kupffeh states it (and which 

 he attributes to the effect of 4500 French feet of elevation), would be 0M67. 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 cor- 

 rected, amount to .00155, and the factor for the diminution of intensity to twice 

 as much,' or .0031, wliich is just ten times as great as my obseiwations indicate, 

 and is so considerable, that, were the conclusion just, it could not fail to be de- 

 tected by the most ordinary instruments 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. 1 shall 

 not dwell upon the incomplete data from which the con-ections due to tempera- 

 tm"e, latitude, &c. are derived ; nor upon the entire incompetency of a single 

 observation which unkno^vn causes (for instance, an iron mine, or the occur- 

 rence of an aiu-ora borealis) may affect. I take M. Kupffer's own statement 

 in the geological section of his work, Avhich pronounces the whole country 

 surrounding Mont Elbrouz to afford one continued evidence of ancient volcanic 

 eruption,* to abound in hot ferruginous springs,! to be so intersected by Tra- 

 chj-tesj^: Lavas, ^ and Diorites,|| that there is distinct evidence of this tract being 

 nothing else than a crater of elevation, raised by the ui^heaving force of the tra- 

 chyte of Mont Elbrouz itself,^ which he states to be undistinguishable fi-om the 

 rock of Pichincha, the great South American volcano.** Any one who has the 

 slightest acquaintance with the connection between magnetism and volcanic 

 rocks will be at no loss to explain anomalies even greater than those which M. 

 KuPFFER has observed. 



42. It was from a persuasion of the entire inconclusiveness of M. Kupffer's 

 results, as well as of all preceding ones, that I undertook the experiments already 

 detailed, in the hope of compensating for the imperfections of the apparatus by 

 the number and extent of the experiments. I own that until I came to calculate 



' Voyage, p. 39. j ?• 39. P- 44, p. 55. X P. 44, p. 61, p. 65. 



§ P. 60, p. 66. II P. 63. f P. 65. •• P. 35. 



