[ IH T 



III. Obfey-vdtions on the E'uenhig mid Morning Dczv ; in a 

 Letter addrejfcd to C. Hassenfratz, oJ the Polytechnic 

 School. By C. A. Prieur *. 



A, 



-S you requcll mc to give vou in writing the fubfiance 

 of what I faid a few days ago rcfpecling the phenomena of 

 the evening and morning dew, i fliall endeavour to gratify 

 your wiflies; being as denrous as you, arc, that feme utility 

 may refult from it to the voung citizens whom vou inllruft. 

 The evening and morning dew occur to us fo frequently, 

 and in fo many climates, that there is reafon to be aftoniihed 

 that philofophcrs fliould have hitherto paid fo little attention 

 to theie phenomena, and been contented with the vague 

 explanations given on the fubjecl. In the year 1788, I often 

 had occaiion to ride out in the morning and evening; and 

 being thus cxpofed'lo the imprcfiions of the dew, I reflefted 

 particularly on the circumllanccs of thcfe phenomena, which 

 have been as yet fo little examined. I well knew that moif- 

 turc depofited on bodies placed in the open air at the time 

 Oi funict, was not the fame as that fecn afterwards on thefe 

 bodies at the period of funrife : that confequentlv there was 

 an interruption in the phenomenon ; an evaporation of the 

 dew or water ^vhich appeared in the evening, and a new 

 produs^tion of water in the morning. I was acquainted alfo 

 v.iih this partial explanation in regard to the evening dew, 

 i'i-z. that the moifture feen in the evening arifes from the air 

 being no longer able to retain the water which it held in fo- 

 lution during the day. But w hence that wind which always 

 proceeds from the quarter where the fun is, and which al- 

 ways accompanies this precipitation of water ? And befidcs, 

 how happens it, that when the fun is about to appear above 

 the h.orizon, and even after it has been fomewhat heated by 

 his prei'ence, there fhould be occalioned a greater cold, a 

 lironger wind, and a more abundant precipitation of water 

 than in the evening? To folve tliis difficulty I had recourfe 

 to thofe fundameiital principles bv the help of which Monge 



* From Journal de I'Eeok Pol)tecb>iique, Vol, II. 



has 



