84 



OBSERVATIONS OF THE MAGNETIC INTENSITY 



Dip and Total Intensity at London and Paris. 



Place. 



Date. 



Needle. 



Dip. 



Mean Dip. 



Total Intensity. 



Year. 



Month. 



D. 



Degs. 



Min. 



Degs.i Mins. 



Lond. 1. 



Paris 1. 



London 

 Paris 



1837 



June 

 Aug. 



16 



17 



No. 1 

 2 



Lloyd No. 1 

 2 



Mean, 

 No. 1 



69 



67 



18.3 

 17.4 

 16.9 

 21.3 



69 



67 



17.8 

 21.4 



1.000 

 0.980 



1.020 

 1.000 



67 



19.6 

 23.2 



Paris 

 London 



1838 



July 

 Aug. 



10 

 15 



No. 1 



2 



Lloyd No. 1 



No. 2 



Mean, 

 No. 1 



67 

 69 



16.4 

 16.6 

 12.9 

 12.3 



67 

 69 



16.5 

 12.3 



0.980 

 1 1.000 



1.000 

 1.020 



69 



12.6 

 12.1 



The report of Major Sabine on the magnetic survey of the British Islands 

 affords ample authentic materials for putting these results to the test. Pro- 

 fessor Phillips, who observed the dip, in May, 1837, at the same place where 

 my observations were made, found it 69° 20'.2; and again, in March, 1838, 

 69° 18'.2, which, reduced to the epoch of my observations, at the rate of a dimi- 

 nution of 0'.2 per month, would give (the correction being additive) 69° 20'.4. 

 The dip observed by Major Sabine in Regent's Park, in July, 1837, reduced 

 to the same epoch, is 69° 18'.9, and by an observation in November, 1837, 

 69° 25'.0. That found by Capt. James Ross, at Westboume Green, in Au- 

 gust, 1837, similarly reduced, gives 69° 20'.8; in June and July, 1838, 69° 17'.0; 

 and in December, 1838, 69° 17'.4. That of Professor Lloyd, in 1836, also, 

 reduced, is 69° 20'.8; and of Mr. Fox, in May and June, 1838, 69° 20'.5. The 

 mean of all these determinations, omitting the dip of 69° 25 '.0, is 69° 19 '.5. I 

 am not acquainted with any series of determinations, at the same place, by dif- 

 ferent observers, and with different instruments, which agree so closely, and 

 consider it, therefore, as an important point in verifying my results, that the 

 dip observed in 1837 agrees within 1'.7 of the mean of those just referred to. 

 The second determination of 69° 12'.3, in 1838, is in defect 4'.8, supposing the 

 annual decrease of dip to be 2'.4, a difference which is admissible, since nearly 



