The Sling Psychrometer, 129 
and the temperature of complete saturation or dew- 
point, by measuring the reduction of temperature by 
evaporation. 
“A sling psyehrometer can easily be made, as fol- 
lows: 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 on a 
nail when not in use. Get two all-glass thermom- 
eters with cylindrical bulbs, and the degrees Fahren- 
heit 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 evapo- 
ration 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’. 
“Securely lash the thermometers on opposite edges 
of the narrow board, leaving the graduations on 
them plainly in sight, and the bulbs extending a 
short distance below the end of the board. To 
use the instrument, wet the cloth-covered bulb with 
water, leaving the other bulb dry, and then swing 
the apparatus freely through the air for three to 
five minutes, or until the wet-bulb thermometer 
ceases to fall in temperature, and then read the 
temperature of each thermometer. Unless the air 
is saturated with moisture the wet-bulb will always 
show a lower temperature than the dry-bulb. Sub- 
tract the degrees of wet-bulb from those of the 
dry-bulb, and the remainder will show the degrees 
J 
