510 



MAGNETIC INTEXSITY. 



and the general results deduced for each station. The cohimn 

 entitled Time " is that of 300 vibrations ; and the " Corrected 

 Time " is the mean of these, corrected for the rate of the chrono- 

 meter and the arc, and reduced to an average temperature of 60°. 

 The dips are those observed by Captain Fitz-Roy; except at Port 

 Famine, where, as Captain Fitz-Roy did not observe, it has been 

 supplied from Captain King's observations; and at Coquimbo, 

 where, for the purpose of computing the intensity, it has been 

 supplied by estimation from the other geographic positions on this 

 coast, at which Captain Fitz-Roy observed the dip. In the column 

 showing the time of vibration as a dipping-needle at Plymouth 

 corresponding to the periods of observation at the several stations, 

 the compensations have been introduced for the variation in the 

 intensity of the cylinder, agreeably to what has been said above on 

 that subject. The two final columns exhibit the values of the total 

 magnetic intensity at the different stations derived from these obser- 

 vations. In the first of the two columns, the values are given rela- 

 tively to the force at Plymouth, considered as unity ; and in the 

 second column, relatively to the force at Plymouth, expressed by 

 1.375 ; for the purpose of exhibiting Captain Fitz-Roy 's results in 

 direct comparison with the determinations of continental observers, 

 who have taken Paris as their basis, giving the force at Paris the 

 arbitrary expression of 1.3482. I have taken the ratio of the force 

 at Plymouth to that at Paris to be as 1.375 to 1.348, which I 

 believe will prove a very near approximation ; it is that which 

 results from Captain Fitz-Roy's observations at Plymouth, in 

 October 1836 (page 17), and mine, at Tortington, in Sussex, in 

 June 1837 (page 10) : the dip at Tortington, at the period in 

 question being 68° 57', and the intensity, compared with Paris, 

 through the medium of London, 1.368. 



