40 POPULAR FRUIT GROWING. 



between the readings of the two thermometers to a table, the 

 dew point may be determined and if it is several degrees above 

 the freezing point, no injurious frost need be expected. 



rThe psychrometer is an instrument made up 

 of a wet and dry-bulb thermometer attached to 

 a board or frame for determining the humidity 

 of the air. One adapt-ed for this purpose may be 

 purchased at a reasonable price from instrument 

 dealers or it may be made as follows: For the 

 frame, take a board eighteen inches long, two 

 inches wide and one-half inch thick with a hole 

 bored in one end to hang the apparatus up with 

 when not in use. Get two all-glass thermometers 

 with cylinder 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 the 

 cloth covering is wet with water and exposed to 

 evaporation in the air it constiiuces 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 following extract from Bulletin No. 23 



Fig. 9. Pscyro- 



meter for fore- of the Weather Bureau, U. S. Department of 

 telling frosts. Agricillture> entitled "Frost," explains the meth- 

 od of using a psychrometer to foretell frost. 



To make an observation. The bulb of the so-called wet bulb 

 thermometer is thoroughly saturated with water by soaking it 

 in a small cup or wide mouthed bottle until the covering is 

 thoroughly wet. The thermometers are then whirled rapidly 

 for fifteen or twenty seconds, stopped and quickly read. A 

 mental note of the reading is made when they are again whirled 

 and read. Subtract the reading of the wet thermometer from 

 that of the dry. Find this difference in the column at the 

 side of the following d-ew point table. Follow the horizontal 

 line under this figure until it intersects the column under the 

 reading of the dry bulb thermometer at the top of the column 

 to the right. The number at the intersection is the dew point 



