294 



March 25, 1841. 



The MARQUIS OF NORTHAMPTON, President, in the Chair. 



The following communications were read, viz. — 



1. The reading of a paper entitled, " On the Localities affected 

 by Hoar-frost, the peculiar currents of Air excited by it, and the 

 Temperature during its occurrence at High and Low Stations." By 

 James Farquharson, LL.D., F.R.S., Minister of the Parish of Alford, 

 was resumed and concluded. 



The author states that he has been accustomed, for the last forty 

 years, to make observations on the occurrence of hoar-frost, and the 

 circumstances under which it takes place, with a view of obtaining 

 a correct explanation of the causes of that phenomenon. It is well- 

 known, he observes, that the localities chiefly affected with hoar- 

 frost are the bottoms of valleys, and land-locked places of all kinds, 

 whether natural or artificial. The altitude to which its effects reach 

 on the sides of the valleys is dependent on the mean temperature of 

 the day and night at the time of its occurrence : when that tempe- 

 rature is high, the lower places only are affected by the frost ; but 

 when low, the frost extendstomuchhigher grounds. Hoar-frost occurs 

 only during a calm state of the air, and when the sky is clear ; but 

 the stillness of the air in the bottoms of the valley is invariably ac- 

 companied by downward currents of air along all the sloping sides 

 of the valley ; and it is to this fact, first noticed by the author, that 

 he wishes more particularly to direct the attention of the Society, as 

 affording a decisive proof of the correctness of the views he entertains, 

 being in accordance with the theory of Dr. Wells. He finds that 

 after sunset, in all seasons of the year, and at all mean temperatures 

 of the air, and whether or not the ground be covered with snow, 

 whenever the sky is clear, although there may be a dead calm at the 

 bottoms of the valleys, currents of air, more or less strong and steady, 

 run downwards on the inclined lands, whatever may be their aspect 

 ■with reference to the points of the compass. These currents are the 

 result of the sudden depression of temperature sustained by the sur- 

 face of the earth in consequence of rapid radiation, by which the 

 stratum of air in immediate contact with that surface, becoming spe- 

 cifically heavier by condensation, descends into the valley, and is 

 replaced by air which has not been thus cooled, and which therefore 

 prevents the formation of hoar-frost on the surface of these declivi- 

 ties. 



2. " Term- Observations of Magnetic Observations, the Variation 

 of the Magnetic Declination, Horizontal Intensity and Inclination 

 at Prague; for June, July, September, and October 1840." By 

 Prof. Kreil. Communicated by S. Hunter Christie, Esq., Sec. R.S. 



3. ''Term- Observations of the Variation of the Magnetic Decli- 

 nation, Horizontal Intensity and Inclination at Milan ; for June 

 1840." By Francesco Carlini, For. Memb. R.S., Director of the 

 Observatory. 



