DUST AND ITS DANGERS 341 



the cleaning tool may be attached to the suction pipes at 

 convenient points throughout the building (Fig. 24 1) . In some 

 cities vacuum cleaners have been successfully used in street 

 cleaning (Fig. 242). 



374. Meaning of Suction. Many people use the word 

 SUCTION without knowing .its real meaning. The words 

 SUCTION and SUCKING are good, common English words, and 

 express ideas not easily expressed in other words. When we 

 use these words we ought, however, to know just what they 

 mean. Suction does not mean drawing or pulling as many peo- 



FIG. 242. A street vacuum cleaner. (By permission of the Municipal Journal.) 



pie believe. Sucking soda water up a straw does not mean 

 drawing it up the straw. Liquids and gases can not be drawn 

 or pulled; they must be moved by being pushed. 



DEFINITIONS. SUCTION is the process by which a partial 

 vacuum is produced into which a fluid, either a liquid or a gas, is 

 forced by outside gaseous pressure, usually the pressure of the 

 atmosphere. 



SUCKING is the act of producing a partial vacuum into which 

 the surrounding atmospheric pressure tends to flow. 



When we suck soda water up a straw we merely enlarge the 

 mouth cavity and thereby produce a partial vacuum into which 



