216 



THE WEATHER 



throw moisture into the air ducts of the ventilating system 

 whenever the humidity falls too low (Chap. VI, Ventilation). 



246. Moisture and the Industries. An 

 even or uniform humidity aids some manu- 

 factures, such as cotton spinning; a high 

 humidity interferes with other operations, 

 as for instance the manufacture and storage 

 of wooden articles and food or other prod- 

 ucts likely to swell or spoil by absorbing 

 moisture. Cleopatra's Needle stood un- 

 harmed for centuries in the low humidity 

 of Egypt, but the higher humidity, with 

 the thawing and freezing in New York 

 City, soon began to crumble its surface. 



247. Measuring the Moisture ; Relative 

 Humidity. Relative humidity (Art. 187) 

 may be measured with either a PSYCHROM- 

 ETER or some other form of a HYGROMETER. 

 The psychrometer consists of two thermom- 

 eters, one with muslin around its bulb, 

 which is either kept moist or is moistened 

 shortly before an observation (Fig. 150). 

 To find the humidity the wet and dry ther- 



FIG 150 The slin m o m eters must be either whirled or fanned 

 psychrometer. The to secure a good circulation of air about 

 them. Continue whirling till the wet ther- 

 mometer ceases to fall; then record the 

 reading of each. The principles governing 

 then whirled about the use of the psychrometer were studied in 

 Arts. 12 and 13. Review Evaporation. 



If the air around the thermometers is 

 not in motion, the space close to the 

 wet bulb becomes more or less saturated 

 with water vapor. This checks evaporation and checks the 

 cooling of the wet bulb that would have been caused by further 

 evaporation. But, if the air about the thermometers is kept 



lower bulb is covered 

 with muslin and is 

 moistened before an 

 observation. The 

 thermometers are 



the hand, and read 

 every half minute or 

 so until the wet ther- 

 mometer ceases to 

 fall. 



