90 COSMOS. 



complicated in the case of observations which are made 

 either in or upon the earth, since a comparison of the effect 

 of considerable 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 not accessible to 

 our observation, together with imperfectly understood horary 

 and accidental alterations in the intensity, modify the results, 

 where the observations 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 nume- 

 rous mines of considerable depth which I have visited in 

 Europe, Peru, Mexico, and Siberia, have never afforded 

 localities which inspired me with any confidence. 9 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 constitutes the mean surface of the earth. 

 The borings % at the mines of Joachimsthal in Bohemia are up 

 wards 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. 10 Very different and more favour- 

 able conditions are afforded by balloon ascents. Gay-Lussac 

 rose to an elevation of 23,020 feet above Paris ; consequently, 

 therefore, the greatest relative depth that has been reached 

 by borings in Europe, scarcely amounts to T \th of this height. 

 My own mountain observations between the years 1709 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 

 Germany. 11 These observations extended from the level of 



9 We may ask what kind of error can have led in the coal mines of 

 Flemi to the result that in the interior of the earth, at the depth of 87 

 feet, the horizontal intensity had increased 0.001 ? 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). 



10 Cosmos, vol. i, p. 150. 



11 A diminution of the intensity with the height is shown in my 



