158 On the Dew. 



Thus Pope in his elegy to the memory of an unfortunate 



lady: 



' Yet shall thy grave with rising flowers be dres;. 

 And the green turf lie lightly on thy breast; 

 There shall the Morn her earliest tears bestow. 

 There the first roses of the year shall blow.' 



' The falling of the dew is a phrase received in all languages, 

 among all people, learned and ignorant; and all express by it their 

 opinion that those drops of water which we find in mornings and 

 evenings on the grass and herbage of the fields have descended 

 from the upper regions of the air. On the contrary, we assert, 

 not as an opinion, but a certainty, that these drops of dew never, 

 in this state, were higher above the earth than we see them, and 

 that they do not descend from on high at all, but rise out of the 

 earth, and never, as dew, fall to it again.* There is, indeed, 

 no law in nature by which dew could be formed as it has been 

 generally understood to be; but all the established doctrines of 

 philosophy and mechanics concur in the production and formation 

 of it upon this plan The earth is, to some considerable distance, 

 always more or less moist; the action of the sun heats the earth's 

 surface, and heat must raise that moisture up in vapor; the heat 

 occasioned by the sun will continue, though in a more remiss de- 

 gree, during the whole night; and while it continues, vapors will 

 also continue to be raised. It is evident, therefore, that vapors 

 are rising all day and all night from the earth. What rise in the 

 day time are dispersed and evaporated by the heat of the air as 

 soon as raised, and we see nothing of them; but what rise in the 

 absence of the sun, and in a cooler state of the air, form them- 

 selves into drops, according to the known laws of attraction. 

 Such, then, is the nature and origin of dew; it is water raised in 

 form of vapor from the earth, in consequence of its being heated 

 by the sun; it collects itself into drops on any thing proper to 

 receive and retain it; or it hangs on the lower regions of the air, 

 in form of a fog or mist, till the sun's rays evaporate and dissipate 

 it. Such are my assertions in regard to dew. The facts which 

 led to, and will be found to support them, are these. The late 

 Lord Pelre had engaged rne to spend a part of the last summer 

 of his life at his house in Essex. He was as fond as myself of 

 experiments that tended to some obvious purpose, and accompa- 

 nied my observations during that period. One of these was an 

 experiment in regard to the quantity of dew suspended in the air 



* Though the condensation of vapor into dew may take place all the 

 way to the surface of the earth, and be greatest nearest to the surface, yet 

 the dew does fall after it has been formed into sensible particles. 



