HUMAN LIMITS IN FLIGHT—MATTHEWS 277 
sible for a fast airplane to follow a slow one in a tight turn; both 
pilots are subjected to 5g. 
The most important stress, however, to which man is subjected in 
aircraft is that resulting from the thinness of the air at great alti- 
tudes. The air pressure at ground level is 14.7 lb./sq. in. It has 
fallen to one-half at 18,000 feet and to only one-fifth, about 234 lb./sq. 
in., at 40,000 feet. The effects of altitude on man are those resulting 
from the lowered atmospheric pressure. 
The disabilities which a man suffers at lowered pressure first came 
into prominence on the surface of the earth as “mountain sickness.” 
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Figure 8.—Showing how human tolerance of acceleration makes it impossible 
for a fast airplane to follow a slow one in a tight turn. 
Later the term “balloon sickness” was given to the troubles experi- 
enced in high balloon ascents at the beginning of the last century; 
long before airplanes had become practical flying machines, the 
problems of high altitudes had been encountered because early balloon 
ascents carried the balloonists to heights at which the air would 
hardly support life and at that time their knowledge of how to over- 
come this was lacking. 
Plate 2, figure 2, shows the first successful flight when Montgolfier’s 
balloon ascended from Versailles in 1788 carrying a sheep, a duck, and 
a cock. After the safe return of these animals to earth, Montgolfier 
