140 M. Melloni on the Theory of Dew. 



and more, and this circumstance will induce a gradual re- 

 frigeration, and an increasing humidity in the stratum of air. 



I do not here enter into the details necessary to shew how 

 the principle of the frigorific reaction of the air serves to 

 explain all the facts which precede and accompany the ap- 

 pearance of dew in meadows, and a multitude of phenomena 

 of which a proper account could not hitherto be given. But 

 your own discernment will supply this want ; and I shall con- 

 clude by indicating the principal questions treated of in the 

 memoir, which I shall soon have the honour to present to the 

 Academy. 



I may say, then, that besides the difficulties of which I have 

 given a distinct solution in these two letters, my new experi- 

 ments on the nocturnal cooling and on dew have enabled me 

 to understand perfectly, 1st, The distribution of temperature 

 among grass, which is found to be colder in the night among 

 it than at the surface of the meadow. 2d, The inversion 

 of the ordinary temperatures of the atmosphere near the 

 earth's surface. 3d, The great humidity of the air near 

 plants, from the first instant that the dew begins to appear. 

 4th, The injurious action of the least breath of wind. 5th, 

 The formation and accumulation of dew during the whole 

 course of the night. 6th, Its successive propagation from 

 below upwards. 7th, The small quantity of dew on trees 

 when compared with grass and low field plants. 8th, The 

 disappearance of small drops of dew, which sometimes takes 

 place on the lower parts of plants, while they are forming on 

 the upper parts. 9th, The variable proportion of the meteor 

 in the different seasons of the year. 10th, Its general distri- 

 bution over the surface of the globe. 11th, The great differ- 

 ence between the diurnal and nocturnal temperatures of the 

 torrid zone. 12th, The absence of dew in the small islands 

 of Polynesia, and on vessels sailing in the midst of large seas. 

 13th, Its abundant formation when the vessels approach cer- 

 tain shores of continents. 14th, The sharp cold produced in 

 the night in the sandy plains of central Africa. 15th, The 

 natural and artificial congelation of shallow waters, when the 

 temperature of the atmosphere is from 5° to 6° above zero, 

 by taking into account the indisputable fact that water does 



