400 



jiceord generally with the views propounded long since by BernouilH, 

 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 Expedition." By Samuel Hunter 

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



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

 communication, to discuss the observations made by Captain Back re- 

 lating to the magnetic intensity, and which were of two kinds j 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 vibra- 

 tion of three needles suspended horizontally according to the method 

 of Hansteen. The results are given in the 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 formula? 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 j 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 phenomena 

 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 difference 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 Hansteen'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 

 the magnetism of the needles employed. He concludes by a just tri- 

 bute to the zeal which Captain Back has manifested in the cause of 

 science, by availing himself of every opportunity of making these 

 tedious observations, during an unknown and perilous navigation. 



