86 On the Phenomena of the Morning and Evening Dew, 



yeaned five lambs, four of which were then living, the other appeared to be fomething of 

 an embryo foetus, though nearly perfcdi" and that "what tends to heighten aftonifli- 

 ment, is, that the fame ewe has had an equal number at each time for two preceding 

 years."— W. N. 



IX. 



On the Phenomena of the Morning and Evening Dew. By C. A. Prieub. *. 



X HE evening and morning dews {le Serein ei la rofee \) prefent themfelves fo often to 

 our obfervation, and in fo many different fituations, that it is furprifing that philofophers 

 fhould have attended fo little to thefe plienomena, or that they fhould have been contented 

 with very loofe explanations on this fubjeft. 



In the year 1788, I had occafion to ride out very frequently on horfeback in the morn- 

 ing and the evening, and being thus expofed to the impreffions of the dews. I was induced 

 to meditate particularly on thefe appearances, hitherto fo little examined. I well knew 

 that the humidity depofited on bodies in the open air at the fetting of the fun, is not the 

 fame as is afterwards obferved upon them at its rifing; and that confequently there is an 

 interruption in the appearance, namely, an evaporation of the evening dew, or humidity, 

 and a new produdtion of moifture in the morning. I was alfo aware of that partial ex- 

 planation of the evening dew, by which it is faid, that the diminution of the heat in the 

 air renders it incapable of retaining any longer the water it had taken up during the day. 

 But why is there a wind always blowing from that fide of the horizon occupied by the fun, 

 and conftantly accompanying this precipitation of water; and again, how does it happen 

 that the fame luminary a fhort time before its rife, and even after having given a flight 

 degree of warmth to the air by its prefence, appears to caufe a greater cold, a ftronger 

 wind, and a more abundant precipitation of water than in the evening. 



To remove this difficulty, I had recourfe to the fundamental principles, by the alTiflance 

 of which, Monge has fo ingenioufly accounted for the greater part of the phenomena of 

 meteorology :j; ; and I foon perceived, that they were fufficient to explain the appearances 

 which formed the fubjetH: of my enquiry. 



Thefe principles, which it will be proper to enumerate, are the three following ; i. The 

 air in like circumftances diffolves more water the greater its denfity, that is to fay, the 

 more it is mechanically comprefled. 



2. And alfo more the higher its temperature. 



• From a letter addrefTed to Cit, HalTenfrsrtz, and irifeited in the Journal of the Polytechnic School, 

 vol. ii. cahier 6, p. 409. 



f We have not diftinft words correfponding with thefe in our language N--N, 

 X Annales de Chimie, tome V. . 



3. Under 



