£>f)4« PHENOMENA OF DEW. 



Difficult to mew 23. The author made feveral experiments to afecrtain whe- 



tokdiffcrl" 5 mE " ther fome of tlie metals mi g ht not be better adapted to the 

 production of thefe phenomena than others. Having, fof 

 example, expofed to the dew plates of copper gilt and filvered, 

 and plates of filver gilt only on the half of one of their faces, 

 he obferved nothing decisive. It only appeared to him, that 

 in general the drops were formed larger on the white metals. 

 "But thefe comparative experiments are difficult to make on 

 account of the readinefs with which moft of the metals become 

 oxided in moift air; which then gives more or lefs of the pro- 

 perty of glafs. 



Differences of 2 k In making thefe obfervations Cit. Prevoit almoft al- 



temperature on wavs kept account of the changes produced in the tempera- 

 the two fides. J \ ,.„, , b , . , , , *. 



ture, or the difference between the internal and external tem- 

 perature. He endeavoured to procure thermometers with 

 flat bulbs, fufficiently fenfible to point out the difference of 

 temperature of the two fides of the glafs at the fame inftant, 

 •which he thinks would have afforded inftructive refults ; but he 

 has not yet fucceeded. He only remarks that it is not necef- 

 fary that the external temperature mould, as is commonly 

 thought, be more elevated than the internal, in order that hu- 

 midity mould be formed on the outfide of the glafs, but that 

 the contrary often happens. 



25. Thefe refearches may be carried farther, for example, 

 by making fimilar experiments in vacuo, or in other gafes as 

 well as air, and by employing other liquids inftead of water. 



26. In order to reduce the principal facts contained in his 

 memoir to a fmall number of proportions, Cit. Prevoft gives 

 the name of the armour of contact to a metallic leaf applied or 

 glued, againft the glafs, and armour of diftance to that which 

 is fome millimetres off from it. This being premifed, the fol- 

 lowing are the general fa&s : 



General ftate- I. When a partition or frame of glafs, which feparates two 



mentor* the great maffes of air at unequal temperatures, is armed in con- 

 Armour of me- tad on its warm face. 



lal on the warm a. If humidity be depofited on this face, it accumulates on 

 nSftTewh" the armour, infoinuch that there is more there than elfewhere. 

 ther without or B. And if humidity be depofited on the oppofite face,, or 

 within j lhe cold y ac€ ^ j t accumu i a tes on this face in the place corre- 



sponding with the armour, fo that there is more there than 



elfewhere. 



C. The 



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