64 DISSERTATION SECOND. [partij. 



he traversed the Atlantic and Pacific Oceans in various di- 

 rections, as far as the 53d degree of south latitude, and re- 

 turned with a collection of facts and observations for the 

 improvement of geography, meteorology, and navigation, far 

 beyond that which any individual traveller or voyager had 

 hitherto brought together. 



The variation of the compass was long before this time 

 known to exist, but its laws had never yet been ascertained. 

 These Halley now determined from his own observations, 

 combined with those of former navigators, in so far as to 

 trace, on a nautical chart, the lines of the same variation 

 over a great part both of the Atlantic and Pacific Oceans, 

 affording to the navigator the ready means of correcting the 

 errors which the deviation of the needle from the true 

 meridian was calculated to produce. In his different tra- 

 verses he had four times intersected the line of no varia- 

 tion, which seemed to divide the earth into two parts, the 

 variations on the east side being towards the west, and on 

 the west side towards the east. These lines being found 

 to change their position in the course of time, the place as- 

 signed to the magnetical poles could not be permanent. 

 Any theory, therefore, which could afford an explanation of 

 their changes must necessarily be complex and difficult to 

 be established. The attempt of Halley to give such an ex- 

 planation, though extremely ingenious, was liable to great 

 objections ; and while it has shared the fate of most of the 

 theories which have been laid down before the phenomena 

 had been sufficiently explored, the general facts which he 

 established have led to most of the improvements and dis- 

 coveries which have since been made respecting the po- 

 larity of the needle. 



Besides the conclusion just mentioned, Dr. Halley de- 

 rived, from his observations, a very complete history of 

 the winds which blow in the tropical regions, viz. the 



