50 Analyses of Books. [Jan. 



Isabella and Alexander, that the error increases in a much 

 greater ratio than this. Capt. Flinders conceived that this 

 increase of the error with the dip was owing to the increased 

 magnetic energy of the iron in the ship as we approach the mag- 

 netic pole ; but Capt. Sabine suggests that the cause of the 

 increase of error is the diminution of the directive power in con- 

 sequence of the dip. Now it is pretty evident that if this cause 

 is adequate to produce the effect, the rate of error might be 

 subjected to mathematical calculation. 



IX. Some Observations on the Formation of Mists in particular 

 Situations. By Sir H. Davy, Bart. F.R.S. V.P.R.I. 



When water above the temperature of 40° is cooled, it sinks 

 below the surface, and its place is supphed by hotter water from 

 below. Hence rivers in clear nights cannot be cooled down so 

 much by radiation as the land on each side of them. The 

 consequence will be that the temperature of the river during the 

 iiio-ht will be several degrees higher than that of the contiguous 

 land. The air above each will participate in this inequality of 

 temperature. Now whenever the atmosphere above the banks of 

 livers is cooled down several degrees below the air over the 

 river, and the two are so situated as to mix a little together, a 

 mist will be formed over the bed of the river. Such is the expla- 

 nation of the formation of mists in autumn above the beds of 

 rivers given by Sir H. Davy. 



X. Obseiyations on the Dip and Variation of the Magnetic 

 Needle, and on the Intensity of the Magnetic Force, made during 

 ■the late Voyage in Search of a North-icest Passage. By Capt. 

 Edward Sabine. 



The mtensity of the magnetic force increased with the lati- 

 tude ; but was not proportional to the increase of latitude. The 

 following table, indicating the time taken up by a dipping needle 

 in making 100 vibrations in different latitudes and longitudes, 

 gives the result of Capt. Sabine's observations on this subject ; 



The dip of the needle likewise increased with the latitude. 

 The following table exhibits the observation: 



