582 On Evaporation. 
the outside, and fill it with cold spring water 
fresh from the well; if dew be immediately 
formed on the outside, I pour the water out, 
let it stand a while to increase in heat, dry the 
outside of the glass well with a linen cloth, and 
then pour the water in again; this operation is 
to be continued till dew ceases to be formed, 
and then the temperature of the water must be 
observed; and opposite to it in the table (page 
559) will be found the force of vapour in the 
atmosphere. ‘This must be done in the opem 
air, or at a window; because the air within is 
generally more humid than that without. Spring 
water is generally about 50°, and will mostly 
answer the purpose the three hottest months in 
the year: in other seasons an artificial cold 
mixture is required.—The accuracy of the re- 
sult obtained this way I think scarcely needs to 
be insisted upon. Glass, and all other hard, 
smooth substances I have tried, when cooled to 
a degree below what the surrounding aqueous 
vapour can support, cause it to, be condensed 
on their surfaces into water. The degree of cold 
is usually from 1 to 10 below the mean heat of 
the 24 hours; in summer I have often observed 
the point as high as 58° or 59°, corresponding 
te $an inch of mercury in force, and once or 
