222 horticulturist's rule-book. 



3. Frosts. 



To Predict Frost (Kedzie). — 1. When the sunshine is very 

 hot and the shade very cold and the shadows very deep, " there 

 is frost in the air," because the air is very dry and radiation of 

 heat little checked. 



2. When the dew-point is more than 10° Fahrenheit above 

 frost-point (32°) , there is little danger of frost, but when it is 

 less than this, frost may be expected. To find approximately 

 the temperature of dew-point when the temperature of the air 

 is between 45° and 65° Fahrenheit, multiply the difference 

 between the wet-bulb and dry -bulb thermometers by two and 

 subtract the product from temperature of dry -bulb. If the re- 

 mainder is above 42° Fahrenheit, there is little danger of frost. 

 The nearer this remainder comes to 32°, the greater the danger 

 of frost, especially it the air is still and clouds disappear at 

 sunset. 



The dew-point is determined by the wet and dry bulb ther- 

 mometer (or psychrometer). The instrument may be made as 

 follows : For the frame take a board 18 inches long, 2 inches 

 wide, and \ inch thick, with a hole bored in one end to hang the 

 apparatus on a nail when not in use. Get two all-glass ther- 

 mometers with cylindrical bulbs, and the degrees Fahrenheit 

 engraved on the stem. Cover the bulb of one thermometer 

 with a thin piece of cotton cloth, fastening it securely by 

 a thread. When this cloth covering is wet with water and 

 exposed to evaporation in the air, it constitutes the " wet-bulb 

 thermometer"; the other thermometer has no covering on its 

 bulb, is not wet at any time, and constitutes the "dry-bulb 

 thermometer, ' ' 



The range of temperature of the open air in this table is 

 from 36° Fahrenheit to 75° Fahrenheit, and of depression of 

 temperature in the wet bulb, from 1° to 13° Fahrenheit, giving 

 a range in both directions of sufficient scope for the needs of 

 Northern farmers during the growing season. The tempera- 

 ture of the dry-bulb (or open air temperature) is found in 

 the left-hand column of the table ; the difference in degrees 



