AT TWENTY-ONE STATIONS IN EUROPE. 81 



fessof Forbes has compared Edinburgh and Paris, and gives the intensity at 

 the former place to that at the latter as 0.8402 to unity. My direct determina- 

 tion gives almost identically the same results; namely, 0.8405. Again, com- 

 paring Edinburgh and Paris through London, I find 0.841 for the horizontal 

 intensity at the former city. Farther, using my results at Edinburgh in a 

 comparison of Dublin and London through Edinburgh, I find 0.936 for the 

 relative horizontal intensities at Dublin and London, agreeing within 0.004 of 

 the mean result of Professor Lloyd and Major Sabine's observations. All 

 these verifications go to show that my number for the relation of the horizon- 

 tal intensities at Edinburgh and London is very nearly correct. 



The dip was determined, at the same time and place with those of the fore- 

 going observations, by Professor Forbes, with a small three inch circle, to be 

 71° 47'.5. This result differs but slightly from those of Major Sabine in 

 September, 1836, and of Mr. Fox, in August, 1837, when reduced to this 

 epoch, and I have employed it in determining the total intensity. Calculating 

 this element from the mean of the horizontal intensities of the foregoing table, 

 and using the dip observed by me at London, I find 1.013 for the total intensity 

 at Edinburgh, that of London being unity. Major Sabine obtained, by the 

 statical method, 1.023. I am at a loss to explain the difference between us. 

 It does not, probably, depend upon an error in the dip used in my calculation, 

 since, taking a mean of those which Major Sabine and Mr. Fox obtained at 

 Edinburgh, and Major Sabine, Captain Ross, Professor Phillips, Mr. Fox, 

 and Professor Lloyd, at London, reduced to this same epoch, and using these 

 means with my horizontal intensity, the total intensity appears to be 1.014. If 

 my result is erroneous, the error must be in the determination of the horizon- 

 tal intensity, the numerous verifications of which render it improbable that this 

 is wrong to any considerable extent. 



LONDON AND PARIS. 



As these stations are of importance as references in connecting the magnetic 

 intensity in the United States with that in Europe, I bestowed great care upon 

 the observations, and multiplied them. They were, besides, points to which I 

 VII. — v 



