Royal Society. 529 



the moon's transit and the time of iiigh-water have been deduced* 

 But the discussion of nineteen years' observations of the tides at the 

 London Docks^ which is given in the |)resent paper, has been made 

 with reference to the moon's transit two days previousiv, and proves 

 very satisfactorily that the laws to which the pha;nomena are subject 

 accord generally with the views propounded long since by Bernouili. 

 The relations which the author points out between the height of high- 

 water and the atmospheric pressure as indicated by the barometer are 

 particularly interesting and important. The influence of the wind is 

 also considered ; and such corrections indicated as are requisite in 

 consequence of the employment by several observers of solar instead 

 of mean time. 



3. " Discussion of the Magnetical Observations made by Captain 

 Back, R.N., during his late Arctic Kxpedition." By Samuel Hunter 

 Christie, Esq., M.A., F.R.S. Part 11. 



The author proceeds, in this jiaper, which is a sequel to his former 

 communication (p. 523.), to discuss the observations made by Captain 

 Back relating to the magnetic intensity, and which were of two kinds ; 

 the first, obtained by noting the times of vibration of a needle in the 

 plane of the magnetic meridian ; the second, by noting the times of 

 vibration of three needles suspended horizontally according to the 

 method of Hansteen. The results are given in tire form of tables. 



Before deducing results from these observations, the author de- 

 scribes a series of experiments instituted with each needle, for the 

 purpose of determining the corrections necessary to be applied in 

 order to reduce the intensities which would result from observations 

 made at different temperatures, to intensities at a standard tempera- 

 ture; and he gives formulae for these corrections. He then determines 

 the relative terrestrial magnetic intensities, at the several stations 

 where observations were made, from the times of vibration of the dip- 

 ping needle in the plane of the meridian, applying the corrections 

 which he had obtained for difference of temperature ; and gives the 

 results in tables. A comparison is instituted between these results 

 and a formula derived from the hypothesis of two magnetic poles not 

 far removed from the centre of the earth. The author considers that 

 this comparison is quite conclusive against the correctness of the for- 

 mulae, and consequently of the hypothesis itself, if applied to the re- 

 sults deduced from the observations in London, in conjunction with 

 those in America; but that, in the tract of country comprised by Capt. 

 Back's observations from New York to the Arctic Sea, the phaenomena 

 of terrestrial magnetic intensity are very correctly represented by the 

 formula in question. 



The author then proceeds to determine the intensity from the ob- 

 servations with horizontal needles, applying here, likewise, to the re- 

 sults, corrections for the difierence in the temperatures at which the 

 observations were made. In these results there are great discrepan- 

 cies, which the author attributes to the inapplicability of Ilansteen'.s 

 method of determining the intensity by the times of vibration of ho- 

 rizontal needles to cases where the dip of the needle is very great, 

 rather than to errors in the observations themselves, or to a variation in 



Third Series. Vol.9. No. .17. Supplement. Dec. 1836. 3 S 



