? 



The temperature at which dew or frost begins to be deposited is 

 called the dew -point. Wlien water is converted into vapor a definite 

 quantity of heat is alis.irljed. "When the vapor condenses into water 

 this heat reappears. Tht'Vt'furo, when dew begins to form, the tem- 

 perature at the point where the dew is forming does not fall any 

 more. Did you ever see a frozen dew-drop? Upon these principles is 

 based the method of making local 



FROST PREDKTIONS. 



There is no simple way by which the quantity of water vapor in a 

 cubic foot of air or the dew point can be directly found. But they 

 may be found through the relation between the dryness of the air 

 and the rate at which water is converted into vapor. In passing into 

 vapor, heat is absorbed, and if the heat is taken from the water the 

 temperature of the water will fall. Cooling a body by wrapping a wet 

 cloth about it is a familiar example of this principle. Place the bulb 

 of a thermometer in some water that has the same temperature as the 

 room. Remove the thermometer, leaving the bulb wet, and notice 

 the reading as the water evaporates from the bulb. If one end of a 

 piece of old, clean muslin the size of a lamp-wick be tied around the 

 bulb of the thermoineter tube, entirely co^eri^g it, and the other end 

 of the piece of muslin be placed in a cup of rain water (do not use 

 well water) the water will be renewed about the bulb, and in about 15 

 mi;nites the reading will remain constant. Such an arrangement is 

 called a "wet-bulb" thermometer. If the air contains all the vapor 

 that it can hold at a given temperature there will be no evaporation 

 from the muslin, and the wet-bulb and the dry-bulb instrument will 

 read the same. Such air is already at the dew point. But if the air 

 has less vapor in it than it can contain at the observed temperature, 

 the water will evaporate from the miislin and cool the wet bulb. The 

 temperature to which it will fall will depend upon the relative amount 

 of vapor in the air. This in turn fixes the point at which dew or frost 

 will form. 



Now let us try to foretell whether there will be a frost or not. The 

 instruments must be used at a distance from trees and buildings or if 

 in cities, they should be used on roofs and exposed to free movement 



The common thermometers sold in shops generally read too high. Most of 

 them are so bad as to be worthless. The errors range from 2 to 27 degrees. 

 Thermometers made by Henry J. Green, of Brooklyn, N. Y., are always reliable. 

 The form with a cylindrical bulb is the best. It is better to get two of them, one 

 for the "wet bulb" work and one for the "dry" or " exposed " bulb. They may 

 also serve as standiinls for checking other and cheaper thermometers. 



