102 Introduction to the Study of Science 



Convection currents o the air and the ocean. The distribu- 

 tion of heat or cold by convection is familiar on a large scale 

 in the local winds, in the great trades and prevailing westerlies, 

 nailed the planetary winds (Fig. 34), as well as in the ocean cur- 

 rents. Many have experienced the hot winds that occasionally 

 sweep across the prairie region of the central United States, or 

 the cold winds that descend from high mountains. These are 

 typical cases of convection currents. Then, as you know from 



your study of geography, the Japan 

 and the Gulf streams, the Labrador 

 and other cold streams are examples 

 of convection currents in water. 



Ventilation. Heating is closely 

 related to ventilation, for both de- 

 pend upon convection and diffusion. 

 It is generally recognized that health 

 and working efficiency depend largely 

 .,, r ,., i ,. . upon the condition of the air which 



. , is breathed and which bathes the 



FIG. 34. Diagram of planet- 

 ary winds, showing belts of high body continuously. Air that dries 

 and low pressure, and upper Qut rapidly the bodily tissues, as in 

 air currents. * 7 



nose and throat, or is excessively hot, 



dead, and of disagreeable odor, is not healthful, and does not 

 permit us to make the best use of our physical and mental 

 energies. We need air of a proper temperature, with adequate 

 moisture, and moving sufficiently to bathe our bodies contin- 

 uously but without draft. The actual amount of air supplied 

 for each person, which is often stated to be about fifty cubic 

 feet per minute, is not nearly so important as that the air be 

 live or moving, and have the other qualities just given. 



Exercise : Ventilating systems. Make a study of any ventilating 

 system. How is fresh air supplied, by force fans, windows, or other 

 openings ? Are the openings arranged with reference to the heat sup- 

 ply, such as radiators ? Or is the air circulated at the temperature it 

 has outdoors? Is the air of the room live, that is, actually circulating 



