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 



