110 Daniell on Meteorological Observations. 



meter so situated is not subject to fall as much as six or 

 seven degrees below the point of congelation. The effect of 

 situation is extremely curious in modifying this process. In 

 the latter end of the month of September 1821, in a garden 

 placed in a sheltered situation in a valley surrounded by low 

 hills, I observed that all the vines and Dahlias were frost- 

 bitten and turned quite black, while in another garden not 

 100 yards distant, upon the gentle acclivity of a hill, no signs 

 of frost were to be seen. At another time, early in the autumn, 

 in the same situation, the grass has been quite rigid with hoar 

 frost, while upon the hill nothing but a light dew was to be 

 perceived. The effect of this cold upon vapour is sometimes 

 very visible. In sheltered situations, and in very calm weather, 

 a low mist may often be seen rising in the meadows, when the 

 surrounding spaces are quite clear. If the ground which this 

 mist covers be examined, it will constantly be found to be 5° 

 or 6° below the heat of the clear regions. The slightest 

 breeze is sufficient to mix the air of the two, and to dissipate 

 the cloud. This will often even be effected by a person merely 

 walking through it. 



With regard to the quantity of dew deposited upon filamen- 

 tous substances, I have found that a piece of cotton exposing 

 a flat circular surface of two inches diameter, gained in weight 

 twenty grains, when the force of the vapour was .481 inch, 

 and the radiating thermometer 8° below the lowest standard, 

 that is to say when the two thermometers were respectively 

 58 and 50. At another time, when they were only 45 and 37, 

 the same piece of cotton had only gained eleven grains, the 

 force of the vapour being .316, and the difference of tempe- 

 rature the same. It is most probable, that in neither instance 

 had there been any loss from evaporation, as at the time of 

 examination the hygrometer denoted the complete saturation of 

 the atmosphere. Calculating from these experiments, and the 

 mean force of vapour and radiation, the average deposition of 

 dew for every night in the year, upon a like surface, would be 

 about six grains. 



The results of the experiments upon the direct radiation 



