LIEUT-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 25 



vations give as its ratio in August 1839 to 1000 at Panama, we have Mr. Douglas's 

 determinations in immediate comparison with those of Captain Belcher as follows : 



Douglas, 1830, 1833. Belcher, 1839. 



, A 



Fort Vancouver Nos. 5. and 6. 594 ; Nos. 3. and 4. 586 . . . 576 



San Francisco Nos. 5. and 6. 728 ; Nos. 3. and 4. 726 ... 711 



Monterey . Nos. 5. and 6. 752 ; Nos. 3. and 4. 740 ... 731 



S*^ Barbara . Nos. 5. and 6. 785 ; Nos. 3. and 4. not obs*^. 765 



It has been assumed in this comparison that the horizontal intensity in London had 

 the same representative value in the years to which Mr. Douglas's observations cor- 

 respond as in the year to which Captain Belchers correspond. But we know that 

 the secular decrease of the dip in London causes a corresponding increase in the 

 horizontal magnetic force at that station, and we are sufficiently acquainted with the 

 average amount of the yearly diminution of the dip to introduce it as an element of 

 calculation. Mr. Douglas's observations with Nos. 5. and 6. correspond to No- 

 vember 1828, when those needles were vibrated in London ; and with Nos. 3. and 4. 

 to January 1832, being the middle time between the observations before his depart- 

 ure, and those made with the same needles in June 1836, when returned to England. 

 Captain Belcher's determination corresponds to August 1839, when his needles were 

 vibrated at Woolwich. Taking the annual decrease of the dip in London in the in- 

 terval at 2''6*, and the value of the horizontal intensity at 480*0 in August 1839, we 

 have its value 472*7 in January 1832, and 469*7 in November 1828; omitting the 

 consideration of the secular change of the intensity itself, of which we know extremely 

 little at present. Adopting these values of the horizontal intensity at the respective 

 epochs, the American determinations become as follows, being all relative to 480 in 



August 1839. 



Douglas. Bblcheb. 



Fort Vancouver Nos. 5. and 6. 581 ; Nos. 3. and 4. 577 . . 576 

 San Francisco . Nos. 5. and 6. 712 ; Nos. 3. and 4. 714 . . 711 

 Monterey . . Nos. 5. and 6. 736 ; Nos. 3. and 4. 729 . . 731 

 S*^ Barbara . . Nos. 5. and 6. 768 ; Nos. 3. and 4. not obs*i. 765 

 There are still involved in the comparison the secular change of dip at the American 

 stations, and the secular changes of the total intensity both there and in London : 

 none of these are known sufficiently to make them proper elements of calculation, 

 though we have reason to believe that the effect of each of these causes on the 

 comparative numbers would be considerably less than that of the decrease of dip in 

 London-f-. But enough has been said to show the large proportion which in such 



* Eighth Report of the British Association, pp. 62. Q6. 



t By comparing Captain Belcher's observed inclinations with M. Hansteen's map of that element in 1780, 

 we perceive that the inclination is annually increasing on the west coast of North America, but the amount of 

 the annual change is apparently considerably less than that of the annual decrease in Europe:— with an annual 

 increase of inclination we should have a decrease in the horizontal intensity ; this corresponds with the re- 

 maining differences between the determinations of Captain Belcher and Mr. Douglas. 

 MDCCCXLI. E 



