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, 1S30, 1833. Belcher, 1839. 
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 ta Barbara . 
Nos. 5. and 6. 
785; 
Nos. 3. and 4. not obs d . 
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 Belcher’s 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. 
>v 
Belcher. 
t 
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 . 
• 7H 
Monterey . . Nos. 5. and 6. 736 ; 
Nos. 3. and 4. 729 . 
731 
S ta Barbara . . Nos. 5. and 6. 768; 
Nos. 3. and 4. not obs d . 
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. 66. 
f 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 
