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With a constant temperature, the pressure and density both decrease as the altitude increases, a density at sea level of 0.07610 pounds per cubic foot is reduced to 0.0357 pounds per cubic foot at an altitude of 20,000 feet. During this increase in altitude, the pressure drops from 14.7 pounds per square inch to 6.87 pounds per square inch. This variation, of course, greatly affects the performance of aeroplanes flying at different altitudes, and still more affects the performance of the motor, since the latter cannot take in as much fuel per stroke at high altitudes as at low, and as a result the power is diminished as we gain in altitude. The following table gives the power variations at different heights above sea level.

This air table also gives the properties of air through the usual range of flight altitudes. The pressures corresponding to the altitudes are given both in pounds per square inch and in inches of mercury so that barometer and pressure readings can be compared. In the fourth column is the percentage of the horsepower available at different altitudes, the horsepower at sea level being taken as unity. For example, if an engine develops 100 horsepower at sea level, it will develop 100 × 0.66=66 horsepower at an altitude of 10,000 feet above sea level. The barometric pressure in pounds per square inch can be obtained by multiplying the pressure in inches of mercury by the factor 0.4905, this being the weight of a mercury column 1 inch high.