472 THE MECHANICS OF VENTILATION. 



2. — The weight of the bod}" may be less than that of the fluid displaced ; 

 in this case the body rises partly out of the fluid until the weight of the fluid 

 displaced is equal to its own weight. 



3. — The weight of the body may be equal to the weight of the fluid dis- 

 placed; in this case the two oj^posite forces being equal the body is in 

 equilibrium and remains in any position in which it may be placed. 



We will consider the application of these laws in the case of common air, 

 considering at first a single cubic foot. 



It has been shown, that at a temperature of 32°, and under a pressure of 

 29.92 inches of mercury, every cubic foot of air weighs 0.081 pound. While 

 the temperature of the single cubic foot remains at that point we have, by 

 case 3 above, a condition of equilibrium, and no movement or disturbance 

 takes place, since an equivalent volume of the surrounding air has exactly 

 the same weight. 



Suppose, however, the temperature of the air is at 70° and that b}^ com- 

 ing in contact with some cold, surface, as, for instance, a Avindow pane, the 

 temperature of the single cubic foot is reduced to 32°. The result of such 

 reduction is by the preceding discussion threefold : 1 — Volume reduced. 2 — 

 Density increased. 3— Consequently it becomes heavier than surrounding 

 air and tends to fall by a certain definite weight. * -^ * 



The cubic foot of air in passing from the temperature of 70° to that of 

 32°, increases its weight by 0.081 — 0.0752:^0.0058 pounds. Carrying this 

 calculation into grains, we have weight at 70°=7000x 0.0752=526.4 grains^ 

 weight at 32°=7000x 0.081=567.0 grains, 567.0—526.4=40.6 grains=gain 

 in weight of a cubic foot of air by reduction of temperature from 70° to 32°. 



In cold weather, when the windows and outside walls are much colder 

 than the general temperature inside, it is evident from the preceding that 

 currents of air must be continually passing downward to the floor along- 

 such cold surfaces. 



Suppose, farther, the air is at a temperature of 40° and by coming in 

 contact with a heated surface is warmed to 98°. Density at 40°=:0, 07 97 

 pounds. Density at 98°=:0.0714 pounds. Or, a cubic foot in passing from 

 temperature 40° to that of 98° loses weight by an amount equal to 0.0797 — 

 0.0714=0.0083 pounds=7000x 0.0083=58.1 grains. Our particular cubic 

 foot of air is then in the condition of case 2 of the principle of Archimedes. 

 Its tendency is to rise and it continues doing so when unconfined, until a 

 point is reached where the densities are again equal. 



The temperature of the human body is 98° while 40° is about an average- 

 temperature of the air during the whole year. It is evident, therefore, that 

 a ventilation of the body is continually taking place through the operation 

 of natural causes. We should take advantage of the hint thus obtained from 

 natiAre, and plan the ventilation of our dwellings and public buildings upon 

 a more rational basis than at present seems to prevail. 



It is evident, without sjDecial discussion, that movements similar to those 

 just ascribed to the particular cubic foot of air are, in fact, constantly taking; 



