On tht Plienomena of the Morning and Evening Dew: 89 



In this manner, on the hypothefis of the immobility of the earth with regard to tlie 

 fun, there would be beneath that luminary a very extended region eternally, and uninter- 

 ruptedly fubjefted to the phenomenon of the morning and evening dew, accordingly as the 

 obferver was placed to the eaft or the weft. 



But let us now aflume the real flate of things, and rcftore to the earth its diurnal rota- 

 tion. The preceding phenomena will then take place on fuccelHve parts of its furface. 

 Thofe places from which the fun is defcending, and ready to fet beneath the horizon, will 

 foon perceive the appearance of the evening dew with a weft wind fpringing up, which 

 phenomena will increafe gradually till after the fetting of the fun, and then the efFedt will 

 diminifti and entirely ceafe. During the night the humidity will evaporate, and entirely 

 difappear, provided the air be not already too much loaded with moifture. Towards the 

 next morning, juft before day-break, the phenomenon will again prefent itfelf on the 

 eaftern fide, with the fame circumftances and gradations, the maximum of efFe£l: being 

 alfo when the fun is yet beneath the horizon ; but with this very remarkable difference, 

 that the efFe£ts will be much ftronger than thofe of the evening ; that is, there will be 

 more wind, more moifture depofited, and a more fenfible degree of cold* The reafon of 

 this is, that in the evening tlie precipitation of water, the wind, and the cold which ac- 

 accompany it, ought to be diminiftied, becaufe the whole takes place in the vicinity, and 

 by the mixture of air which the fun has heated during the dayi whereas in the morning 

 the coldnefs of the night air permits, or gives a much greater efFe£l to the phenomenon. 



We fee likewife, that in the two temperate zones, where the winter and fummer have a 

 great difference of temperature, where the length of days and nights vary much, the ef- 

 fefts of the evening and morning dews are varied and irregular. In fummer, if on the 

 one hand the air difTolves more water in the day, on the other hand, the precipitation of 

 the evening dew is made in an air very much heated, and that of the morning dew in air 

 which the fhortnefs of the night has cooled only to a certain point. In this cafe the folu- 

 tion of moifture is confiderable, and the precipitation little. In winter, on the contrary, 

 the caufe of folution is lefs, but that of precipitation more efFe£tual. Local circum- 

 ftances, more efpecially the vicinity of water, muft alfo influence the effefts. Fine 

 weather increafes, and renders them more fenfible. Clofe weather weakens or deftroys 

 them. 



Under the torrid zone the days and nights are more nearly equal, and at the equator they 

 are equal at all times. The difference between the fummer and winter temperatures is 

 lefs confiderable than in our latitude, and the fky is almoft conftantly clear. It muft fol- 

 low, that the morning and evening dews will be heavier under this burning climate, than 

 elfewhere on the earth. And this agrees with the teftlmony of travellers. In Egypt, in. 

 Afian Turkey, at the Antilles, at Mexico, in veflels failing between the Tropics, the 

 morning and evening dews fall fo abundantly, that they produce the fame efFed as (bowers 

 of rain. 



Vol. IV.— May 1800. N But 



