Injury from Cold During Growing Period. 131 



When the dew point is above the freezing point of water 

 (32 F., C.), the precipitation is in the form of dew 

 or rain; but when it is below the freezing point of 

 water, the precipitation is in the form of hoarfrost or 

 snow. One more principle needs to be explained, and 

 we are ready to understand. 



206. How Frost may be Foretold. We know that 

 sprinkling the floor of a room cools the air, even though 

 the water used is no cooler than the air of the room. 

 This is because the air in taking up watery vapor ab- 

 sorbs heat, but this heat is set free again when the 

 watery vapor is precipitated. A steam radiator gives 

 out heat because the steam within it is condensing into 

 water. It follows that when the dew point of the at- 

 mosphere is reached, a very considerable amount of 

 latent heat is given off, which checks the fall of tem- 

 perature. The temperature of still air, therefore, rarely 

 falls much below the dew point, and since the latter at 

 any given temperature depends upon the amount 

 of the moisture in the air, if we have an instrument- 

 capable of indicating both the temperature and the 

 moisture of the air, we may compute the lowest tem- 

 perature to which the atmosphere will be likely to de- 

 scend during any given night, and thus we may fore- 

 tell frost with some degree of certainty. 



207. The Sling Psychrometer (psy-chrom'-e-ter) 

 enables us to determine both the temperature and the 

 moisture of the air. This instrument consists of two ac- 

 curately-graduated thermometers attached to a board 

 or case (Fig. 59). The bulb of one thermometer is 



