THE ATMOSPHERE 123 



1. Air drills, and their uses in mines and tunnel building. Their advantages 

 over the old hand drills (a) in amount of work accomplished; (b) in the 

 health of the workmen. 



2. Pneumatic tubes, and their uses as change carriers in stores. 



3. Vacuum cleaners, and their uses for household purposes. The care 

 necessarily given them, and the causes of their failure to give satisfactory 

 service at times. 



4. Air-brakes, and their uses on railway trains. 



6. The construction and operation of an artificial ice plant. Make use of 

 a diagram, and be sure to make very clear the part played in the process 

 by heat of vaporization. 



6. The sand blast, and the uses to which it is put. 



7. Spraying outfits used by fruit growers, and for painting large structures, 

 or surfaces where hand work is too difficult or too expensive. 



8. Pneumatic hammers used in riveting together the steel frames of bridges, 

 large buildings, and other structural work. 



CURRENTS IN THE ATMOSPHERE, AND THEIR RELATION TO 

 ATMOSPHERIC PRESSURE 



It is not infrequently noted that when a great fire occurs a 

 wind begins blowing, fanning the flames and driving on the 

 fire. The great masses of air heated by the fire have a 

 lessened density as result of expansion due to the heat, and 

 a pressure less than that of the surrounding air. The result 

 of this is an inrush of the denser air from all sides, and the 

 warmed air is crowded upward as the direction of least 

 resistance. The usual statement concerning this condition 

 is that when the air is warmed it rises, and the cooler air 

 rushes in to take its place. This mistakes cause and effect. 

 The less its density the greater the ease with which the air 

 is crowded upward, and the more rapid its ascent. 



The presence of water vapor in the atmosphere lessens its 

 density. If water vapor is added to any certain amount 

 of dry air, the combined weight must of course be greater 

 than the weight of the air alone. But moisture-laden air 

 weighs less per cubic foot than dry air. If now from the 

 weight of a cubic foot of saturated air is taken the weight of 



