Darnell on a new Hygrometer. 



311 



temperatures, regard must be had to- the force of vapour already 

 existing in the air. For instance, if water of 59° were the subject, 

 the force of vapour of that temperature is ^l o^ the force at 212°, 

 and one might expect the quantity of evaporation to be ^ also ; 

 but if it should happen, that an aqueous atmosphere to that 

 amount does already exist, the evaporation, instead of being 

 gijj of that from boiling water, would be nothing at all. On the 

 other hand, if the aqueous atmosphere were less than that, sup- 

 pose half of it, then the effective evaporating force would be -jlo 

 of that from boiling water; in short, the evaporating force must 

 be universally equal to that of the temperature of the water, 

 diminished by that already existing in the atmosphere. 



But the air, by its mechanical action, has another influence 

 upon the rate of evaporation. When calm and still, it merely 

 obstructs the process ; but when in motion, it increases its 

 effect in direct proportion to its velocity, by removing the 

 vapour as it forms. Mr. Dalton fixes the extremes that are 

 likely to occur in ordinary circumstances at 120 and 189grs. 

 per minute, from a vessel of six inches diameter, at a tempe- 

 rature of 212°. 



Upon these data, he has constructed the following Table: — 



Table II. — Shewing the Force of Vapour, and the full evapo- 

 rating Force of every Degree of Temperature, from 20° to 85°, 

 expressed in Grains of Water that would be raised per Minute 

 from a Vessel of six Inches in Diameter, supposing there were 

 no Vapour already in the Atmosphere. 



