OF ATMOSPHERICAL MOISTURE. 127 



" is included between the points of perfect dryness 

 and perfect moisture ; the latter, of course, being that 

 state of the atmosphere at which the dew-point coin- 

 cides with the temperature of the air. The inter- 

 mediate degrees may be ascertained by dividing the 

 elasticity of vapour at the temperature of the dew- 

 point, by the elasticity of the temperature of the air : 

 the quotient will express the proportion of moisture 

 actually existing, to the quantity which would be 

 required for saturation ; for, calling the term of satu- 

 ration 1-000, as the elasticity of vapour at the tempe- 

 rature of the air is to the elasticity of vapour at the 

 temperature of the dew-point, so is the term of satu- 

 ration to the actual degree of moisture." 



By means of this and similar contrivances,* we are 

 at all times able to ascertain exactly the quantity of 

 water that exists in an elastic state in the air. 



In this country, the changes of moisture are said to 

 extend from I'OOO, or saturation, to "389, or even 

 so low as "120, under a south wall, for a short space 

 of time ; "a state of dryness which is certainly not 

 surpassed by an African harmattan," but one which 

 produces less disastrous consequences, because it is 

 accompanied by a far lower temperature and a weaker 

 solar radiation. The mean degree of moisture of the 

 air near London has been found by Mr. Thompson to 

 be •897, on an average of ten years, while the mean 



* Other hygrometers have been invented to answer the same 

 end ; but, as Mr. DanieU's is that most eligible in this eo'untry, I 

 have thought it more ooavenient to confine my observations 

 to it 



