110 



Some Observations on the Format ion of Mists in particular Situalw/t.*. 

 By Sir H. Davy, Bart. F.R.S. V.P.R.L Read February 25, 1819. 

 [Phil. Trans. 1819, p. 123.] 



The author shows, in this paper, that after sunset the fall of tem- 

 perature that ensues upon the earth's surface is considerably greater 

 on land than in water, and refers to the well-known peculiarity in 

 the expansibility of water, at temperatures below 40, for the cause 

 of its superior temperature and that of the air above it. When, 

 therefore, the cold and comparatively dry air of the land mixes with 

 the warmer and damper air that rests upon the water, the diminution 

 of the temperature of the latter, occasioned by this mixture, tends to 

 separate a portion of its moisture, and consequently to produce mist. 



Observations 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-west Passage. By Captain Edward 

 Sabine, of the Royal Regiment of Artillery, F.R.S. and F.L.S. 

 Read February 25, 1819. [Phil. Trans. 1819,^. 132.] 



The dipping-needle used in these observations was similar to that 

 described by Mr. Cavendish in the 66th volume of the Philosophical 

 Transactions, and was made by the same artist. It was so adjusted 

 that no alteration took place in the indication of the dip on reversing 

 the poles, and was placed in the direction of the magnetic meridian 

 by a compass stationed at a sufficient distance, and suffered to remain 

 during the observations, for the purpose of occasional verification. 



In determining the intensity of the magnetic force, the needle was 

 drawn to a horizontal position by a magnet, and, being released at 

 an observed moment of time, was suffered to oscillate until the arcs 

 became too small to be readily distinguished : the first arc was thus 

 equal to the dip, and at every tenth vibration both the arc and time 

 were noted. The results of these observations are given in a series 

 of tables. 



The azimuth compasses used in the observations to determine the 

 variation of the needle in Davis's Strait and Baffin's Bay, and the results 

 of which are detailed in tables annexed to this paper, were constructed 

 upon Captain Kater's improved plan. They were generally employed 

 on the ice ; for as the influence of the ship's iron increased upon 

 their compasses as the directive power of magnetism diminished, 

 the observations made on board became of little or no value towards 

 a knowledge of the true variation. This remark Captain Sabine 

 illustrates by the insertion in his tables of a few azimuths taken in 

 the Isabella. They also show how essential it is to navigation, in 

 high latitudes, that the nature of the errors which the ship's attrac- 

 tion produces on her compasses should be understood. 



