118 Major-Genera) Sabine on the Cosmical Features 



any more particular statement resrardina: its duration could be 

 as yet little more than conjectural. These variations, therefore, 

 still retain the name at first assigned to them of secular changes. 

 That they are eminently systematic, — that in all parts of the °;lobe 

 they manifest themselves as due to a common cause, — and that 

 this cause is not traceable to any terrestrial changes with which 

 we are acquainted or can reasonably conceive, — are considerations 

 which confer a very high interest on the investigations connected 

 with this branch of masnetical science. TYe owe the first clear 

 conception of the distribution of terrestrial magnetism, and of the 

 systematic character of the changes taking place in it from epoch 

 to epoch, to two papers communicated to the Royal Society 

 towards the close of the seventeenth century by Dr. Halley, fol- 

 lowed in 1701 by his general map of the isogonic lines, or lines 

 of equal declination, which were thence long called Halleyan 

 lines *. Conducting his investigations in the true spirit of in- 

 ductive inquiry, he formed by careful research, and gave in 

 his first paper in 1683, an extensive Table of the most trust- 

 worthy observations of the declination (the only magnetic ele- 

 ment then obtainable) in different parts of the globe, and by the 

 study of these he arrived at the conclusion that they admitted 

 of explanation upon the hypothesis of four magnetical poles or 

 points of attraction, two in each hemisphere, situated not far 

 from the geographical poles, — the needle in those parts of the 

 earth which lie adjacent to any one of these magnetic poles being 

 governed thereby, the nearest pole being predominant over the 

 more remote, whilst in the equatorial regions the magnetic direc- 

 tion became more complex, inasmuch as the influence of the four 

 poles required to be taken into account. He obtained by his 

 hypothesis a representation of the phenomena so satisfactory, 

 that he expressed the conviction (using his own words) that " he 

 had put it past doubt that there are in the earth four such mag- 

 netical points or poles which occasion the great variety and 

 seeming irregularity which is observed in the variations of the 

 compass;" the paper contained the geographical coordinates of 

 the positions assigned by him to his four poles for the epoch at 

 which he wrote. In his second paper, in 1692, Dr. Halley dis- 

 cussed at large the phenomena of the secular changes of the de- 

 clination, showing that, as far as they were then known, they 

 might be explained by regarding two of the poles as fixed or 

 stationary, and the two others as subject to a gradual and pro- 

 gressive movement of translation — the northern to the east, and 



* This was the first introduction of a mode of representing in a combined 

 view a large bodv of facts collected from different sources and referring to 

 different localities, which has been since found of great value in many 

 branches of natural science. 



