56 Mr. Harvey on the Formation of Mists. 



the temperature of the atmosphere, and that of the grass of a 

 meadow in which the experiments were performed. To leave 

 no branch of the subject under consideration unexplained, 

 thermometers of a very delicate construction, and placed in 

 different situations, were successively examined every half 

 hour, from half-past nine in the evening, to nine the next morn- 

 ing. From the hour first mentioned to four the succeeding 

 morning, the temperature of the air, at an elevation of seven 

 feet above the ground, exceeded the temperature of the surface 

 of the meadow, and the upper sky and the horizon were lucid 

 and clear. After four, however, an alteration in the aspect of 

 the heavens, and also in the states of the thermometers, was 

 perceptible ; and at half-past four the air indicated 39^° F., and 

 the ground 40i° F. ; whereas, at four, the former was 41° F., 

 and the latter 40° F. At this moment a thin haze was visible 

 by the aid of the twilight, hovering over the marshy lands at 

 the foot of the meadow, and at five had considerably increased, 

 both in density and quantity, the temperature of the air at this 

 moment being 40° F., and the ground 4H° F. At halfpast 

 five A. M., the mist had very much increased, extending 

 itself into some of the adjacent fields, and having its density 

 perceptibly greater. A reference to the thermometer also indi- 

 cated a still greater difference between the temperature of the 

 air and ground than in the former instances, the air still retain- 

 ing its temperature of 40°, but the temperature ofthegrosnd 

 had increased to 43|°. At six A.M., the mist had so much 

 increased as to obscure the neighbouring town of Stonehouse, 

 and which had been visible during the former part of the night. 

 The temperature of the air at six was 41^°, and the ground 46|^° ; 

 and from this hour until nine A.M., the time when the last 

 observation was. made, the ground still continued to possess a 

 temperature greater by several degrees than the air, and during 

 the whole timeof observation the mist continued of considerable 

 density. 



From the preceding observations it appears, that the quantity 

 and density of the mist increased in proportion to the excess of 

 the temperature of the ground above that of the air. One of 



