Mr. Harvey on the Formation of Mists. 57 



the conditions mentioned by Sir Humphry for the formation of 

 mist in great quantity over water is, that the excess of its tem- 

 perature above that of air should be as great as possible. 



The temperatures of the air and of the ground, at the moment 

 when the mist was first perceived, were not, however, the maxi- 

 mum depressions for the night, for at 3 A.M. the air indicated 

 39°, and the surface of the meadow 38°. These greatest de- 

 pressions of temperature were perceived just at the moment 

 when the first golden streak of the dawn had appeared, and 

 when the particles of dew which had been deposited on the 

 upper surface of a plate of glass, elevated six inches above the 

 ground, were completely frozen, the moisture on its under side 

 remaining in a fluid state. As the entire series of observations 

 may be acceptable, they are here given. 



The night ofthe 15th of May'was dedicated to similar pur- 

 suits. Observations were made from sun-set to sun-rise, every 

 quarter of an hour, and in no case was the temperature of the 

 air found below that of the ground, the nearest approach to a 

 state of equality having been at 5 A.M., when the warmth of 

 the air exceeded that of the ground 2^°. No mist, however, 

 was formed during the night on any of the neighbouring sheets 

 of water, or on the marshy lands below the meadow. The 

 greatest depression of temperature took place at four, about 

 twenty minutes before sun-rise, which same hour indicated also 

 the least temperature of the glass, of a thermometer laying on 



