their 'TroiIucti(jn^ Suspension, and Deitructmi. 34 f 



experiments in high temperatures, I found that if it was to 

 bevcrilJed bv e.xperimcnts in low temperatures regard must 

 be had to the force of vapour actually existing in"tl)e atmo- 

 sphere at the time. For instance, if water of 59" were the 

 subject, the force of vapour of that temperature is l-60th 

 of the force at 212°, and one might expect the quantity 

 of evaporation l-6oth also; but if it should happen, as it 

 sometnnes docs in summer, that an aqueous atmosphere to 

 that amount does already exist, the evaporation, instead of 

 being l-60th of that from boiling water, would be nothing 

 at all. On the other hand, if the aqueous atmosphere were 

 less than that, suppose one-half of it, corresponding to 

 39** of heat, then the effective evaporating force would be 

 1-1 20th of that from boiling water : in short, the evaporating 

 force must be universal!}' equal to that of the temperature 

 of the water, diminished by that already existing in ihe at- 

 mosphere. In order to find the force of tlie aqueous atmo- 

 sphere I usuallv take a tall cylindrical glass jar, drv on 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 v.ell 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 obseixed ; and opposite to it in the table 

 will be found the force of vapour in the atmosphere. This 

 must be done in the open air, or at a window ; because the 

 air with.in is generally more humid than that without. 

 Spring water is gcnercdly about .50°, and will mostly an- 

 swer the purpose the three hottest months in the year ; in 

 other seasons an artificial cold mixture is required. The 

 accuracy of the result obtained this way I think scarcely 

 needs to be insisted upon. Glass, and all other hard, smooth 

 substances 1 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. TIk- degree of cold 

 is usually from 1 to 10 below the mean heat of the 21 hours; 

 in sununer I have often observed the point as high as 58" 

 or 39°, corresponding to half an inch of mercury in force j 

 and once or twice have seen it at G2"^: in changeable and 

 windy weather it is liable to considei-ablc lluctualion : but 

 tliis is not the place to enlaroe upon it. 



" For the purjxise of observing the evaporation in atmo- 

 spheric temperatures I got two light tin vessels, the one 

 SIX inches in diameter and lialf an inch deep, the other 

 •jjrht inches diurineter and three-fourths of an inch deep, 



and 



