102 ON MAGNETISM. 



and dip is increased by 5". Both these would corre- 

 spond to the supposition that the magnetic power 

 is sensibly below the earth's surface. As the observer 

 was not actually separated from the earth, the validity 

 of- inference from these may be disputed. Another 

 series is that of observations in balloons, which are 

 free from every objection of that kind, but which are 

 not quite so accurate; and which are necessarily al- 

 most limited to observations of horizontal intensity, 

 as found by vibrations (Article 25). The following are 

 the results of these observations : 



Gay Lussac, 1803, at the height 4000 metres found 

 no sensible diminution of magnetic force' (Annales de 

 Chimie, vol. 52). 



Gay Lussac, 1804, at the height 6900 metres found 

 an apparent very small increase : but this was probably 

 caused by the low temperature of the needle, for 

 which no correction was applied. The dip, imperfectly 

 observed, was not sensibly altered. (A. de C. vol. 52.) 



Glaisher, 1862, found at the height 20200 feet 

 a diminution of power : but in other observations at 

 5300, 11000, and 3800 feet, found the same as on the 

 earth (Report of British Association 1862). 



Glaisher, 1864, found a diminution of about f part 

 at the height 14000 feet. (R. of B. A. 1864.) 



Glaisher and Evans, 1864, found an even larger 

 diminution at height 3600 to 5000 feet. (R. of 

 B. A. 1865.) 



It would appear generally from these observations, 

 that there is a sensible diminution of magnetic hori- 



