TERRESTRIAL MAGNETISM. 61 



were the first undertaken by a government for a great 

 scientific object, viz. for the investigation of an element 

 of the earth's force, on which the safety of navigation 

 especially depends. He advanced as far as 52 S. lat., 

 and was thus enabled to construct the first extensive 

 variation- or declination-chart, which chart now supplies 

 to the theoretical investigators of the 19th century a point 

 of comparison, although not indeed a very remote one, for 

 the representation of the progressive change of position of 

 the declination lines. 



It was a happy undertaking of Halley's to connect graphi- 

 cally by lines or curves all the points on the map where the 

 magnetic decimation was the same.( 64 ) Clearness of repre- 

 sentation, and the advantage of gaining a general view of the 

 connection of detached results, were thus first introduced. 

 My isothermal lines, i. e. lines of equal temperature (mean 

 annual, summer, or winter temperature), which have been 

 favourably received by physicists, were formed in strict 

 analogy with Halley's isogonic curves. The object of the 

 isothermal lines, especially since their great extension and 

 improvement by Dove, has been to throw light on the 

 distribution of temperature over the earth's surface, and on 

 the dependence of that distribution in a great degree on the 

 configuration, extent, and relative position of the portions of 

 the surface occupied by land and water. Halley's purely 

 scientific expeditions stand out as the more remarkable, 

 because they were not designed, like so many subsequent 

 expeditions undertaken at the public expense, as voyages 

 for geographical discover y t but were strictly for scientific 

 research. Halley's stay at St. Helena in 1677 and 1678 

 had for its fruit, in addition to the data furnished to the 



