60 



COSMOS. 



ated, according to Halley, in 70° S. Int., 120° east of Green- 

 wich, and therefore almost in the meridian of King George's 

 Sound in New Holland (Nuyts Land).* Halley's three voy- 

 ages, which were made in the years 1698, 1699, and 1702, 

 were undertaken with the view of elaborating*a theory which 

 must have owed its origin solely to the earlier voyage which 

 he had made seven years before to St. Helena, and to the 

 imperfect observations of variation made by Baffin, Hudson, 

 and Cornelius van Schouten. These were the first expedi- 

 tions which were equipped by any government for the estab- 

 lishment of a great scientific object — that of observing one of 

 the elements of terrestrial force on which the safety of navi- 

 gation is especially dependent. As Halley penetrated to 52° 

 south of the equator, he was able to construct the first cir- 

 cumstantial variation chart, which affords to the theoretical 

 labors of the 19 th century a point of comparison, although 

 certainly not a very remote one, of the advancing movement 

 of the curves of variation. 



Halley's attempt to combine graphically together by lines 

 different points of equal variation was a very happy one,f 

 since it has given us a comprehensive and clear insight into 

 the connection of the results already accumulated. My iso- 

 thermal lines (that is to say, lines of equal heat or mean an- 

 nual summer and winter temperature), which were early re- 

 ceived with much favor by physicists, have been formed on 

 a similar plan to Halley's isogonic curves. These lines, es- 

 pecially since they have been extended and greatly improved 

 by Dove, are intended to afford a clear view of the distribu- 

 tion of heat on the earth's surface, and of the principal de- 

 pendence of this distribution on the form of the solid and fluid 

 parts of the earth, and the reciprocal position of continental 

 and oceanic masses. Halley's purely scientific expeditions 

 stand so much the more apart from others, since they were 

 not, like many later expeditions, fitted out at the expense of 

 the government with the object of making geographical dis- 

 coveries. In addition to the results which they have yielded 



* Edmund Halley, in the Philos. Transact, for 1683, vol. xii., No. 

 148, p. 216. 



t Lines of this kind, which he called tractus chalyboeliticos, were 

 marked down upon a chart by Father Christopher Burrus in Lisbon, 

 and offered by him to the King of Spain for a large sum of money ; 

 these lines being drawn for the purpose of showing and determining 

 longitudes at sea. See Kircher's Magnes, ed. 2, p. 443. The first va- 

 riation chart, which was made in 1530, has already been referred to 

 in the text (p. ~>C>). 



