ofl rain; evaporation, &c. 163 



iftiali have 36 inches of water at a medium annually oh the **^W**^ 

 furface of the earth in England and Wales, reckoning 31 torches, 

 rain and five for dew. 



According to Guthrie, the area of England and Wales is 9 omp f uted J u t a h "; 

 46.450 fquare miles. This reduced to fquare feet, gives ^J ^^^ 

 1.S78. 586. 880.000: which, multiplied by three feet the an- cu bic miles, or 

 mial depth of rain and dew, gives 4.135.760.690.000 ^j^*jjj»j 

 feet of water = 151176. 320.0dO. cubic yards, or 28 cubic 

 miles =115 thoufand millions of tons in weight, nearly. We 

 mutt now confider how this enormous quantity of water is 

 difpofed of. 



There are two principal ways by which the water derived It Is earned off 

 from rain is carried off again : one part of it runs bffimme- fp^ngs^an/by 

 diately into rivulets, or links into the earth a fmall way, breaks evaporation. . 

 out again in lower ground in the form of fprihgs, thence 

 makes its way to fome river, by which it is conveyed into the 

 fea — another part is raifed into the atmofphere by evaporation. 

 We take no notice here of the decompofition of water by ve- 



Igetables ; becaufe it is prefumed that in the courfe of nature 

 the principles are combined and water formed again. 



3. That there exifts at all times, and in all places, a quantity of 

 aqueous vapour in the atmofphere, .variable according to circum- 

 ftances. 



4. That whatever quantity of aqueous va'poUr may exift in the 

 atmofphere at any time, a certain temperature may be found, below 

 which a portion of that vapour would unavoidably fall or be depo- 

 fited in the form of rain or dew, but above which no fuch diminu- 

 tion could take place, chemical agency apart. This point may be 

 called the extreme temperature of vapour of that denfity. 



5. And that whenever any body colder than the extreme tempe- 

 rature of the exifting vapour is fituated in the atmofphere, dew is 

 depofited upon it, the quantity of which varies as the furface of 

 the body and the degree of cold below the extreme temperature* 



N. B. The extreme temperature of vapour in the atmofphere va- 

 ries all the way from the afiual temperature of the atmofphere to 

 10, 15, 20 or more degrees below it.— The point may generally be 

 found in the hotteft months by pouring cold fpring water into a dry 

 and clean glafs, and marking what degree of Cold is fufficient to 

 produce a dew on the outfide of the glafs j at other times frigorific 

 laline folutions may be ufed. 



M 2 SECTION 



