Final answer:
Atmospheric pressure decreases as altitude increases, but the rate at which this pressure decreases slows down the higher you go. This decreasing rate is reflected in the flatter slope of air density changes with altitude. An aneroid barometer can measure such changes in atmospheric pressure.
Step-by-step explanation:
The concept in discussion is atmospheric pressure, which is the force per unit area exerted by the Earth's atmosphere. An important instrument for measuring atmospheric pressure is the aneroid barometer, which measures pressure without the use of fluids and operates using a small, flexible metal box called an aneroid cell.
When considering how atmospheric pressure changes with altitude, it is known to decrease as altitude increases. This decrease is due to the thinning of the air and less air above to exert pressure downward. As you go higher, not only the air becomes less dense, but the rate at which the pressure decreases will also become slower. That is, the change in pressure per unit of altitude gain will reduce as altitude increases, fitting into the concept of a decreasing rate of change.
For instance, the density of air decreases by about 0.1 kilograms/cubic meter for every 1,000 meters in altitude between the altitudes of 4,000 meters and 6,000 meters. This gradient or 'slope' symbolizes the rate of change. If this slope gets 'flatter' (or the decrease in density per increase in altitude becomes smaller), it indicates that the rate of change is decreasing. This aligns with the concept that air pressure decreases at a decreasing rate as altitude increases.