74 CLIMATE. 



giving a mean of 15°84. The monthly mean appears 

 hereafter. 



The ohservations on radiation with the Eethrioscope are 

 not yet in suiRcient number to afford any data from which 

 to draw accurate conclusions. 



Moisture. Next to the impressions of temperature on 

 the human body, the most sensible effects are produced by 

 the relative moisture of the air. The laws which regulate 

 this condition of the atmosphere are not yet so accurately 

 investigated as those of pressure and temperature, and it is 

 more difficult to render their operation generally intelligible, 

 from the circumstance of the phrases in common use to 

 express its variation, convejing in many instances ideas in 

 direct opposition to their philosophical meaning. For in- 

 stance, what is generally called damp or moist air, by no 

 means infers its containing more moisture than another 

 column which gives a feeling of dryness, but only that it is 

 more ready to part with its moisture, from some peculiarity, 

 either in its own constitution or that of the body with which 

 it comes in contact. "Without entering into an elaborate 

 disquisition on this somewhat complicated subject, it will be 

 sufficient to observe, that the capacity of air for moisture, 

 in other words, its dryness, depends on its relative density 

 and temperature ; rarefied air dissolving more moisture, i. e. 

 being dryer than denser air, and heated air more than cold 

 air; consequently when two columns or strata of air, of 

 different density, or (which is almost a necessary conse- 

 quence,) different temperature are mixed, the result is almost 

 uniformly a deposition of moisture in the shape of fog or 

 rain, from the capacity of the mixed column of moisture 

 being so much diminished, that it can no longer hold the 

 aggregate quantity of water in solution ; the quantity of the 



