14 THE WEATHER 



To find the dew-point. 



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

 (or psychromctcr). The instrument may he 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 c\'lindrical 

 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 depressio.i 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-bull) 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 itnlics. 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 



