Maximum and minimum temperatures are measured using thermometers mounted inside a wooden box with slits, called Stevensonís screen, which is generally kept facing north. The maximum thermometer is mercury-in-glass - as the temperature rises, the mercury expands and moves freely up a constriction in the thermometer. When air temperature drops, the constriction prevents the mercury from flowing back due to higher pressure, and gives the maximum reading. The minimum thermometer is alcohol-in-glass and is kept horizontally inside the box. As the temperature falls, the alcohol contracts and the index is dragged towards the bulb by the surface of the alcohol meniscus, giving the lowest, or minimum, temperature.