Zantedeschi.] 456 [December, 



Letters were received from Professor Zantedeschi, dated 

 Padua, September 11th and N^ovember 7th, 1864, a transla- 

 tion of one of which was read, as follows : 



I have the honor to forward to your honorable Society, a copy of 

 my letters on the origin of the dew and of the hoarfrost. I here 

 briefly sum up the principal heads of my observations. Can these, 

 my sheets, be published in the Transactions of the Society, my 

 studies may gain such notice, that physicists on the virgin soil of 

 America may repeat my experiments. Remote from the polemics, I 

 confine myself to the fundamental facts of science. 



The mode of the manifestation of the dew and the hoarfrost; its 

 sublimation from the soil, from the hour next the setting sun up to 

 the following morning, in the hour next the sunrise, which is the 

 hour of greatest cold, was the subject of my first series of experi- 

 ments; the circumstances which accompany these phenomena (me- 

 teora) were diligently examined in all the calm and serene nights, 

 in which in Italy and in the environs of Paris I made my observa- 

 tions. I find that there is a limit, beyond which dew and hoarfrost 

 is not sensible ; and that the limit varies with the variations of the 

 grade of humidity and temperature of the air. 



With comparative experiments I have determined whence proceed 

 the aqueous vapor necessary to form dew and hoarfrost. It comes 

 neither wholly from the air by refrigeration, nor wholly from the 

 earth. The greater part rises from the earth, to which is added 

 afterwards, some from the supersaturation of the air ; and, it ought 

 not to be forgotten, some from evaporation from the leaves of plants, 

 which is in least quantity. The mean relative humidity of the gar- 

 den, in which I made my observations, was 55 degrees of the 

 Hygrometer of Saussure. The Hygrometer which received the hu- 

 midity of the air alone, never rose during the hour of greatest cold 

 to 70° ; the Hygrometer which received the moisture from the air 

 and from the earth, went up to 95° ; and the Hygrometer which re- 

 ceived the moisture from the earth and from the leaves of some 

 small plants, reached the maximum of humidity, 109°. In this 

 way I discovered that the relative humidity of the air was 50°, that 

 of the earth 25° ; that of the evaporation of the leaves of plants 5°. 

 These numbers are not absolute. They will vary with the various 

 circumstances of the atmosphere; those of temperature of relative 

 humidity, in nights continually calm and serene ; but in all circum- 

 stances the abundance of aqueous vapor coming from the earth, was 



