MAGNETIC INTENSITY. 95 



Does the intensity of the magnetic force perceptibly de- 

 crease at such heights as are accessible to us, or does it per- 

 ceptibly increase in the interior of the earth 1 ? The problem 

 which is suggested by these questions is extremely complica- 

 ted in the case of observations which are made either in or 

 upon the earth, since a comparison of the effect of considera- 

 ble heights on mountain journeys is rendered difficult, because 

 the upper and lower stations are seldom sufficiently near 

 one another, owing to the great mass of the mountain ; and 

 since, further, the nature of the rock and the penetration of 

 veins of minerals, which are hot accessible -to our observation, 

 together with imperfectly understood horary and accidental 

 alterations in the intensity, modify the results, where the ob- 

 servations are not perfectly simultaneous. In this manner 

 we often ascribe to the height or depth alone conditions which 

 by no means belong to either. The numerous mines of con- 

 siderable depth which I have visited in Europe, Peru, Mexi- 

 co, and Siberia have never afforded localities which inspired 

 me with any confidence.* Then, moreover, care should be 

 taken, in giving the depths, not to neglect the perpendicular 

 differences above or below the level of the sea, which consti- 

 tutes the mean surface of the earth. The borings at the 

 mines of Joachimsthal, in Bohemia, are upward of 2000 

 feet in absolute depth, and yet they only reach to a stratum 

 of rock which lies between 200 and 300 feet above the level 

 of the sea.f Very different and more favorable conditions 

 are afforded by balloon ascents. Gay-Lussac rose to an ele- 

 vation of 23,020 feet above Paris ; consequently, therefore, 

 the greatest relative depth that has been reached .by borings 

 in Europe scarcely amounts to -p^th of this height. My 

 own mountain observations, between the } 7 ears 1799 and 

 1806, led me to believe that the terrestrial force gradually 

 decreases with the elevation, although, in consequence of the 

 causes of disturbance already indicated, several results are 

 at variance with this conjectural decrease. I have collected 

 in a note individual data, taken from 125 measurements of 

 intensity made in the Andes, in the Swiss Alps, Italy, and 



*-We may ask what kind of error can have led, in the coal-mines 

 of Flenu, to the result that in the interior of the earth, at the depth- 

 of 87 feet, the horizontal intensity had increased O'OOl ? Journal de 

 I Institut^ 1845, Avril, p. 146. In an English mine, which is 950 feet 

 below the level of the sea, Kenwood did not find any increase in the 

 intensity (Brewster, Treatise on Magn., p. 275). 



f Cosmos, vol. i., p. 159. 



