ON DEW, &c. 265 



Davy affirms this to be the case. But Mr. Wil- 

 liams, who must here be regarded as the better 

 authority, says, that it is necessary to the suc- 

 cess of the process that the straw be dry ; in 

 proof of which he mentions, that when the 

 straw becomes wet, by accident, it is replaced ; 

 and that when he purposely wetted it in some 

 of the inclosures, the formation of ice there 

 was always prevented. The reasons are clear. 

 The water, by softening the straw, renders it 

 easily compressible by the weight of the pans, 

 and at the same time fills up what would other- 

 wise be vacant spaces among its parts. The 

 straw, therefore, in this condensed state, must 

 afford a ready passage to heat from the earth to 

 the pans, the hindrance of which is allowed by 

 every person to be the use of it, in this process, 

 when dry. Again ; the moisture, which passes 

 through the straw to the earth it covers, will 

 rise afterwards in the form of vapour, having 

 the same temperature with the warm ground, 

 and will communicate heat to the pans. In the 

 last place ; a part of this vapour will be con- 

 densed into water by the pans, in consequence 

 of which heat must be extricated. 



4. It is mentioned both by Sir R. Barker and 

 Mr. Williams, in support of their opinions, that 

 the pans, when new, are so porous, that they 

 readily permit water to transude them 5 and 



