upon the Minimum Temperature of the Night. 167 
This remarkable degree of cold, at that season of the year, 
seems to have been in a great measure local, and to have ex- 
tended only a few miles around Amulree. At 5 o’clock in the 
morning, the ground in the neighbourhood of that place was so 
hardened by the frost, that it could not be penetrated by an in- 
strument ; and the crops, particularly those of oats and potatoes, 
suffered severely ; the former, which were previously in a green- 
ish state, soon after assuming a pale yellowish appearance, while 
the latter were completely blackened and withered in the stem. 
The intensity of the frost was less felt in the neighbourhood of 
Crieff, which is about 10 miles from Amulree, but even there it 
sensibly affected the standing crops. Around Perth e its influ- 
ence was scarcely perceptible, the register-thermometer having 
descended only to 39J°; but in the vicinity of that place the 
air possessed a much greater degree of humidity, the point of 
deposition, by the application of my formula to Dr Gordon’s 
observations at the Manse of Kinfauns, being 38°, 
The extreme dryness of the air, which preceded the great de- 
pression of temperature, during the night, around Amulree, was 
not of long duration. On the evening of the 23d, the point of 
deposition rose to 32°, and the minimum temperature of the fol- 
lowing night did not descend below 30|°. Next day, the quan- 
tity of moisture in the atmosphere was more than doubled, so 
that the point of deposition, assigned by the formula, rose from 
32° to 47°. The elevation of the minimum temperature corre- 
sponded to this increase of the humidity of the atmosphere, and 
rose from 30i° to 46|°. These facts seem to establish, in the 
most satisfactory manner, that a coincidence exists between the 
lowest temperature of the night and the point of deposition, or 
that temperature at which the moisture dissolved in the atmo- 
sphere would return to the liquid state. The only question with 
respect to which any difference of opinion can be entertained, is* 
Whether the temperature regulates the humidity, or the humi- 
dity the temperature ? To this question I have no hesitation in 
replying, that, from the facts I have adduced, we are warranted 
in drawing the conclusion, that the temperature, in the first in- 
stance, determines the mean hygrometric condition of the atmo- 
* Perth is 18 miles from Amulree, the first range of the Grampians intervene 
