DISCOVERIES IN THE CELESTIAL SPACES. 335 



have, under especial conditions of meridian or equatorial direc- 

 tion, and of solid, fluid, or gaseous inactive conditions of the 

 bodies, confirmed this important result. Gilbert had so clear 

 an idea of the force imparted by telluric magnetism, that he 

 ascribed the magnetic condition of iron rods on crosses of old 

 church towers to this action of the Earth.* 



The increased enterprise and activity of navigation to the 

 higher latitudes, and the improvement of magnetic instru- 

 ments, to which had been added, since 1576, the dipping 

 needle (inclinatorium), constructed by Robert Norman, of 

 RatclifT, were the means, during the course of the seventeenth 

 century, of extending the general knowledge of the periodical 

 advance of a portion of the magnetic curves or lines of no va- 

 riation. The position of the magnetic equator, which was be- 

 lieved to be identical with the geographical equator, remained 

 uninvestigated. Observations of inclination were only carried 

 on in a few of the capital cities of Western and Southern Eu- 

 rope. Graham, it is true, attempted in London, in 1723, to 

 measure, by the oscillations of a magnetic needle, the intensity 

 of the magnetic terrestrial force, which varies both with space 

 and time ; but, since Borda's fruitless attempt on his last voy- 

 age to the Canaries in 1776, Lemanon was the first who suc- 

 ceeded, in La Perouse's expedition in 1785, in comparing the 

 intensity in different regions of the earth. 



In the year 1683, Edmund Halley sketched his theory of 

 four magnetic poles or points of convergence, and of the peri- 

 odical movement of the magnetic line without declination, bas- 

 ing his theory on a large number of existing observations of 

 declination of very unequal value, by Baffin, Hudson, James 

 Hall, and Schouten. In order to test this theory, and render 

 it more perfect by the aid of new and more exact observations, 

 the English government permitted him to make three voyages 

 (1698-1702) in the Atlantic Ocean, in a vessel under his own 

 command. In one of these he reached 52 S. lat. This ex 

 pedition constituted an epoch in the history of telluric mag 

 netism. Its result was the construction of a general variation 

 chart, on which the points at which navigators had found an 

 equal amount of variation were connected together by curved 



* The first observation of the kind was made (1590) on the tower of 

 the church of the Augustines at Mantua. Grimaldi and Gassendi were 

 acquainted with similar instances, all occurring in geographical lati- 

 tudes where the inclination of the magnetic needle is very considerable. 

 On the first measurements of magnetic intensity by the oscillation of a 

 needle, compare my Relation Hist., t. i., p. 260-264, and Cosmos, vol. 

 i., p. 186, 187. 



