14 THE WEATHER 



To find the dew-point. 



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

 (or psychrometer). The instrument may be made as follows : For the 

 frame find a board eighteen inches long, two inches wide, and one half 

 inch thick ; bore a hole in one end so as to hang the apparatus on a 

 nail when not in use. Get two all-glass thermometers 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 ther- 

 mometer " ; 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 the following table is 

 from 36 Fahrenheit to 75 Fahrenheit, and of depression of tempera- 

 ture 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 temperature of the dry-bulb (or open- 

 air temperature) is found in the left-hand column of the table; the 

 difference in degrees between the readings of the dry- and wet-bulb 

 is entered in the horizontal line at the top, from 1 to 13. To find 

 the temperature of dew-point at any observation, find in left-hand 

 column the temperature of dry-bulb, then follow the horizontal line 

 opposite that figure till you reach the perpendicular column under the 

 difference between dry- and wet-bulb readings, and the figures at the 

 meeting of these two columns will give the temperature of dew-point. 

 For example, suppose the dry-bulb stands at 65 and wet-bulb at 55 : 

 the difference is 10. Pass across the page in the line of 65 till you 

 intersect the vertical column under 10, and you read 47, which is dew- 

 point under these conditions. If the dew-point is 10 or more above 

 frost-point (32 Fahrenheit), there is little danger of killing frost ; but 

 if the dew-point is less than 10 above 32, danger may be apprehended. 

 If a line is drawn from the intersection of 43 1 and 67 13, 

 of the table, this may be called the danger line, and all dew-point 

 temperatures below this line indicate danger of frost, and are printed 

 in italics. This margin of 10 is taken because the temperature on a 

 still night will often sink several degrees below the first dew-point, 

 and the temperature of ;the air at five feet above the ground is 



