The barometer is an instrument for "weighing" air. It works in much the same way as a set of scales: the weight of the mercury column is offset by the pressure of air on a bowl full of mercury. In 1644, Torricelli observed the level of mercury in a tube closed at one end, tipped upside down, and lowered into a bowl of mercury. He observed that the mercury in the tube did not descend all the way. This was because its weight was counter-balanced by the pressure of air on the mercury in the bowl. He also discovered that the height of the mercury column varied, at the same altitude, with changes in temperature. Torricelli's discovery paved the way for the development of the mercury thermometer. At sea level, at 0° C and at a latitude of 45°, the mercury column stays at around 76 cm, whereas at the top of a 2,500-meter mountain, it settles at only 57 cm. In the eighteenth and nineteenth centuries, the barometer underwent many design changes that made it more sensitive, easier to read, and less cumbersome. Toward 1850, the international community adopted aneroid barometers, which are compact and easy to carry.