122 ELEMENTS OF GENERAL SCIENCE 



dry, hot air that the contents of the vessel are made pleas- 

 antly cool. Travelers in such regions have learned to cover 

 their canteens with layers of woolen cloth and to wet this 

 at every possible opportunity. So long as the cloth cover is 

 wet, the water in the canteen will be kept cool enough to be 

 palatable. In many parts of the South where ice is expensive 

 iceless coolers are used which work on this same principle. 

 A cage fitted with convenient shelves is covered with cloth 

 which dips into a pan of water on top of the cage. By this 

 device the entire surface of the cloth is kept wet, and the 

 evaporation keeps the air in the cage cool. 



140. Ice machines and cold storage. Cooling by evapora- 

 tion is applied in a practical way in plants for the artificial 

 production of ice and in cold-storage plants. In the opera- 

 tion of these plants advantage is taken of the fact that the 

 boiling point of ammonia at atmospheric pressure ( 28 F. 

 or 33 C.) is so little below atmospheric temperature that 

 the boiling point can be raised above the usual temperature 

 of the air by increase of pressure (see sect. 132). Thus, 

 while the boiling point is 28 F. at atmospheric pressure, 

 its boiling point is raised to 80 F. by a pressure of 155 

 pounds to the square inch. 



Figure 62 represents the essential features of an ice- 

 manufacturing plant. The condensing pump forces the gaseous 

 ammonia into the pipes to the right with a pressure of at 

 least 155 pounds to the square inch, and the gas is cooled 

 to the necessary degree by the water which flows over the 

 cooling pipes. The gas therefore condenses into a liquid. 

 This liquid ammonia is allowed to escape through the valve 

 into the pipes at the left only as rapidly as the excess is 

 removed by the pump, and the pressure in these pipes there- 

 fore never rises much above 30 pounds to the square inch. 

 The liquid which has escaped into this place of lower pressure 

 immediately begins to change into a gas and, in so doing, 

 absorbs heat from its surroundings. The pipes into which 



