Final answer:
A barometer measures atmospheric pressure, which decreases with altitude, hence it can be used to estimate altitude during a mountain climb. Mercury and aneroid barometers present different methods for measuring this pressure change as altitude increases.
Step-by-step explanation:
A barometer is a device used to measure atmospheric pressure, and by extension, can be utilized to measure altitude. As you ascend in the mountains, atmospheric pressure decreases, because the air becomes less dense at higher elevations. A mercury barometer indicates this change through the height of a mercury column, which falls as pressure drops. Hence, you could use the barometer to estimate your altitude by comparing the pressure indicated on the barometer to standard atmospheric pressure at various altitudes.
An aneroid barometer, which contains a small spring within an evacuated metal capsule, measures pressure through the capsule's expansion and contraction. This type of barometer is also useful for hikers and climbers, and because it doesn't rely on a liquid, it can be more portable and convenient than a mercury barometer.