270 WELLS'S NATURAL PHILOSOPHY. 



100. The intervening space is then divided into 100 equal parts, -which 

 indicate diflerent degrees of moisture. 



Sach hygrometers are not, however, considered as altogether reliable. 



SECTION I. 



PHENOMENA AND PRODUCTION OF DEW. 



^,^ , . „ . 603. Dew is the moisture of the air con- 



What IS Dew ? 



densed by coming in contact with bodies colder 

 than itself. 



What is the 6*^4. The temperature at which the conden- 

 Dew-pomt? gatiou of moisturc in the atmosphere com- 

 mences, or the degree indicated by the thermometer at 

 which dew begins to be deposited, is called the " Dew- 

 Point." 



, ,, ^ This point is by no means constant or invariable, since dew 



Is the dew- . ^ -^ , • , 



point a con- IS only deposited when the air is saturated with vapor, and 



etant one ? j-|jg amount of moisture required to saturate air of high tem- 



perature is much greater than air of low temperature. 



If the saturation be complete, the least diminution of temperature is at- 

 tended with the formation of dew ; but if the air is dry, a body must be 

 several degrees colder before moisture is deposited on its surface ; and indeed 

 the drier the atmosphere, the greater will be the difference between the tem- 

 perature and its dew-point. 



Dew may be produced at any time by bringing a vessel of 

 production * of ^°'^^ Water into a warm room. The sides of the vessel cool 

 dew be occa- the surrounding air to such an extent that it can no longer 

 time^? * ^"^ retain all its vapor, or, in other words, the temperature of tlio 

 air is reduced below the dew-point ; dew therefore forms upon 

 the vessel. A pitcher of water tmder such circumstances is vulgarly said to 

 " sweat." 



In the same manner, moisture is deposited upon the windows of a heated 

 apartment when the temperature of the external air is low enough to suffi- 

 ciently cool the glass. 



.^ As soon as the sun has set in summer, and the earth is no 



formed in sum- longer receiving new suppUes of heat, its surface begins to 

 ™t ? "^'^"^ ^"°' throw off the heat which it has accumulated during the day 

 by radiation ; the air, however, does not radiate its heat, and, 

 in consequence, the different objects upon the earth's surface are soon cooled 

 down from 7 to 25 degrees below the temperature of the air. The warm 

 vapor of the air, coming in contact with these cool bodies, is condensed and 

 P'fX'ipitated as dew. 



In a clear summer's night, when dew is depositing, a thermometer laid 



