solid surface of the globe, are more or less magnetic, and cause 

 great variations in the direction of the magnetic needle ; nor is 

 the needle ever free from this combined influence. 



It is well known, although not always taken into considera- 

 tion, that the magnets were originally made out of a rock called 

 a loadstone ! a rock impregnated with iron in a state of oxide. 

 All primary crystalline rocks containing iron and manganese will, 

 with due delicacy, point north and south like the steel needles, 

 i. e. in the exact direction in which they are found in situ, when 

 fresh cut from the place where they were formed. 



The above facts have been more or less proved by various ex- 

 periments made in the equatorial regions. 



Indeed, these effects have been more or less observed in every 

 part of the world, — in the Straits of Magellan, along the whole 

 range of the Andes, in Mexico, North America, the Arctic 

 Regions, in Scotland, and in various parts of the continents of 

 Europe and Asia. Not only is this influence of terrestrial mag- 

 netism observed in the solid substances which constitute the sur- 

 face, but the atmosphere has been proved to be equally affected 

 by it. This last phenomenon was observed by me on various oc- 

 casions whilst making magnetical experiments in South America; 

 and the same has been observed by various persons in Europe, 

 and more especially when the aurora borealis makes its appear- 

 ance ; and, to make use of the happy term applied by Humboldt, 

 there appears to be " magnetical storms," probably arising from 

 the equilibrium of the fluid being disturbed. They are very com- 

 monly observed before and during earthquakes in equatorial 

 America. 



The two hemispheres are represented in Plates IV. and V., 

 on which will be observed the present average direction of the 

 magnetic needles, according to Barlow's ' Magnetic Chart of 

 Variations/ It will be seen that, notwithstanding the numerous 

 local disturbances which the needles must be subject to, they 

 preserve a remarkable degree of uniformity from pole to pole ; 

 and when we reflect on the delicate nature of our instruments, 

 and consider how slight an action will cause a deviation from 

 the line of equilibrium, we should, or ought to be, astonished 

 that their general direction is so near the true meridian, rather 

 than it should vary a little from it. 



