DISCOVERIES IN THE CELESTIAL SPACES. 719 



aninvestigatecL Observations of inclination were only carried 

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

 Europe; 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 

 succeeded in La Perouses'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 periodical 

 movement of the magnetic line without declination, basing 

 his theory on a large number of existing observations of de- 

 clination 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 expedi- 

 tion constituted an epoch in the history of telluric magnetism. 

 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 lines. 

 Never before, I believe, had any government fitted out a naval 

 expedition for an object whose attainment promised such ad- 

 vantages to practical navigation, while at the same time it 

 deserved to be regarded as peculiarly scientific and physicc- 

 mathematical. 



As no phenomenon can be thoroughly investigated Ly a 

 careful observer, without being considered in its relation to 

 other phenomena, Halley, on his return from his voyage 

 hazarded the conjecture that the northern light was of a 

 magnetic origin. I have remarked, in the general picture of 

 nature, that Faraday's brilliant discovery (the evolution of 

 light by magnetic force) has raised this hypothesis, enounced 

 as early as in the year 1714, to empirical certainty. 



But if the laws of terrestrial magnetism are to be thoroughly 

 investigated that is to say, if they are to be sought in the 

 great cycle of the periodic movement in space of the three 

 varieties of magnetic curves, it is by no means sufficient that 

 the diurnal regular or disturbed course of the needle should be 



