346 
Journal of Agricultural Research 
Vol. XXI, No. 5 
even zero. On the other hand, the amounts of heat and water vapor given 
off by animals are fairly constant under like conditions and must, therefore, 
be depended upon to cause a flow of air sufficient to supply the minimum 
amount needed. Since it is this minimum air movement that concerns us 
most, only the motive power derived from the heat and water vapor 
produced by the animals will be considered here. 
ANIMAL HEAT AS A MOTIVE POWER IN VENTILATION 
The immediate cause of air movement into and out of a ventilated 
space is a difference of pressure established between the air in the space 
to be ventilated and that outside. The effect of the heat given off by 
the animals is to render the air of the stable relatively lighter than the 
air outside. This difference in density causes a difference in pressure, 
which tends to maintain a continuous flow of air into and out of the 
stable. 
The difference in pressure between the air in the bam and that outside 
resulting from a difference in temperature can be computed in the fol¬ 
lowing way: When air is warmed its volume expands 1/491 of its 
volume at 32 0 F. for each degree Fahrenheit rise in temperature. This 
expansion tends to force out 1 cubic foot of air for each 491 cubic feet 
contained in the stable, and the air remaining will consequently weigh 
less than an equal volume outside by an amount equal to the weight of 
the air thus forced out. If, for example, in a stable containing 19,640 
cubic feet, or one very nearly 40 by 40 by 12.3 feet, the temperature is 
raised to 57 0 , as compared with 32 0 outside, the air forced out by expan¬ 
sion will be ■ ° or 1,000 cubic feet. In other words, the air remain- 
491 
ing will weigh 80 pounds less than an equal volume outside. This means 
that the total pressure into the stable at the floor is 80 pounds greater 
than that exerted outward by the inside air; and, since the floor has an 
area of 40X40=1,600 square feet, the pressure tending to force air 
80 
into the stable at a floor opening and out at the ceiling is - > or 0.05 
pound per square foot. 
Based on the foregoing considerations, the magnitude of the tempera¬ 
ture effect in producing draft, according to King, is represented by the 
following equation: 
Cubic feet per hour = 60 X 60 X 8 
in which— 
60 X 60 is the number of seconds per hour, 
8 is V 2 <7> 9 being the value of gravity, 32.16 feet per second, 
T is the temperature of air inside, 
t is the temperature of air outside, 
H is the height of room or ventilator, 
1/491 is the expansion of air for i° F. 
