22 DEW-POINT. 



autumnal nights and mornings. It obscures the sun until his rays 

 have raised the temperature of the air sufficiently to evaporate it, 

 when it gradually disappears and leaves a clear, blue sky. The 

 stratus deposits moisture, is called the night cloud and is most fre- 

 quent from September till January. When the temperature, from ra- 

 diation or other causes, sinks below 32°, we find it fettered with icy 

 spiculae upon trees and shrubs and sparkling in exquisite frost work 

 upon all nature." It is not known to affect the barometer much 

 either way. The nimbus is seen during showers : it is not seen with 

 the barometer at great heights. A study of vapor point would be of 

 great interest to one desiring to pursue this subject further ; and 

 there are plenty of books on Physics and Physical Geography for 

 those who are thus inclined, not to waste useless space and time 

 here upon definitions. One point further, however, before we 

 leave the subject : — 



We see, therefore, that the rising or falling of the barometric 

 needle corresponds with the pressure of the atmosphere. This 

 pressure is due to changes of temperature, moisture and wind. 

 To show that the barometer is more of an instrument than appears 

 upon first sight, and must be studied in itself without reference to 

 the words marked upon its face, a clause from one of our 

 scientific college text-books of natural philosophy tells us that "the 

 practice formerly prevailed of engraving at different points of 

 the barometric scale several words expressive of states of weather : 

 fair, rain, frost, wind, etc., etc. But such indications are worthless, 

 being as often false as true : this is evident from the fact that 

 the height of the column would be changed from one kind of 

 weather to another by simply carrying the instrument to a higher 

 or lower station." The measuring of heights by the barometer will 

 be spoken of in another place. It remains but to speak of 

 the dew-point, and the resultant forms of water manifested. The 

 dew-point is "the temperature at which vapor, in a given case, 

 is precipitated into water in some of its forms." The amount of 

 vapor in the air is expressed by its tension or elastic power, and its 

 humidity, or " its quantity present, as compared with the greatest 



