186 Mr. S. Besvf'ick's further Illustrations of a Method 



North magnetic pole, lat. 70° O' : west long. 91° O'. 



South magnetic pole, lat. 75° 5' : east long. 155° O'. 

 In taking the epoch of 1492, we shall have an interval of 

 357 years, which, multiplied by the rate of revolution in lon- 

 gitude of each magnetic pole for that epoch, namely, 16^^ 

 minutes annually for the northern, and 8^ minutes annually 

 for the southern, will give 98° 10' for the former, and 49° 5' 

 for the latter : hence, if the magnetic poles be removed back- 

 wards in west longitude by so much as the amounts stated 

 above, we shall then have their relative positions for the year 

 1492 as follows: — 



1849, N.M. pole, W.long.91° O': S.M. pole, E.long.l 55° O' 



98° 10' 49° 5' 



1492,N.M.pole,W.long.lS9° 10' : S.M. pole, E.long. 105° 35 



This preliminary is all which is necessary for the adaptation 

 of our method to any given epoch, past or to come. In the 

 present case it is applied to one of the most memorable in 

 nautical astronomy — the discovery of the line without mag- 

 netic variation. The merit of this discovery belongs to Chris- 

 topher Columbus, who crossed it whilst on his celebrated first 

 voyage for the discovery of a western route to the Indies. We 

 think it would be advisable for the Committee of Magnetical 

 Observation to make this epoch the test of all methods pro- 

 posing to compute magnetic declination ; for the question 

 naturally presents itself — how shall we best prove a method 

 but by its application to the recorded observations of the past ; 

 and what observation is more memorable than the one here 

 proposed ? This eminent navigator has recorded the position 

 of this line in the year of his first voyage, September 25, 1492, 

 in the following words : — 



" 2i° west of the island of Corvo, the magnetic variation 

 changed and passed from N.E. to N.W." 



We propose to test our method by trying ivkether it mil 

 Jlnd this line in the same spot at the time stated. An explana- 

 tion of the process having been fully stated in the former 

 article, it is useless to give a repetition in the present instance. 

 It will be observed, however, that corresponding items with 

 those previously given are here arranged under corresponding 

 numbers. 

 Mean Declination of the Compass Needle on the Atlantic during 



the voyage o/" Christopher Columbusyor the epoch of Sept, 25, 



1492. 



(1.) Lat. and long, of position. 



N. lat. 27° 0' : comp. 63°. 

 W. long. 31° 30'. 



