On the SuhsidetKe of Aqueous Vapours. 357 



some degree of certainty, at what period the derangement 

 took place, and wliat corrections are required to be applied, 

 till the instrument was restored to its perfect state. 



I have the honour to be, my dear sir, 

 With the highest regard. 



Your most obedient humble servant. 

 Royal Observatory, Nov. 21, 1821. JoHN Pc'ND. 



L.XXIL On the Subsideyice of Aqueous Vapour ; and its Repose 

 on the Beds of Rivers. By John Murray, F.L.S. MJV.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentleimen, — X HE President of the Royal Society has in- 

 geniously ascribed the Ibrmation of mists in particular situa- 

 tions, to the superior temperature maintained by currents of 

 water from below, these supplanting the superficial wave and 

 supplying its place, when cooled down by radiation. This 

 movement and interchange v.'ould continue so long as any part 

 of the mass remained above 45°. On this principle the air 

 incumbent on the bosom of the river would, having its loss by 

 radiation thus compensated for, remain at a higher temperature 

 than that reposing on the banks of the river which can have 

 no such compensation. The cooler atmosphere of the latter 

 would therefore move towards the former, and mingling with 

 it, mist would be formed. 



Now, though the currency of a rapid river, as the Rhone 

 for instiince, might, prima facie, seem to disturb the rise and 

 fail from below and above, still it must be confessed that the 

 view sustained, is every way worthy of its distinguished au- 

 thor. 



My own experiments on the Seine and the Po seemed not to 

 harmonize with these views ; but the thermometer might be in 

 fault, and I was willing to cede the point. 



On the 3d instant a fine opportunity was afforded me for 

 ascertaining the fiuestion with exactness and precision. The 

 numerous repetitions of the experiments, and the great extent 

 of surface examined therniomelrically, with the extreme deli- 

 cacy of the instrument, enable me to submit the results to you 

 with unusual confidence. 



Rain obtained in the early part of the day, and the sky was 

 dark and lowering. The horizontality of the clouds, with their 



i)arallelisni in the distant sky, seemed to indicate the preva- 

 cnce of electric influence. Toward the afternoon, mist fbiined 



over 



