On what principle does an altimeter of a Cessna 172 aircraft works?

The altimeter works by measuring minute changes in barometric pressure. Barometric pressure decreases as altitude increases, and altimeters are able to detect this change and use it to determine altitude.

Barometric pressure is also effected by weather conditions, and so pilots must frequently adjusting the altimeter setting in order to compensate.

Some new Cessna 172's are being equipped with Garmin GPS cockpits which also get altitude information from GPS receivers, but these aircraft are also equipped with a standard altimeter in the interest of redundancy.