A barometer is a scientific instrument used to measure air pressure in a certain environment. It is used to forecast short-term changes in the weather by measuring pressure tendency. There are two main types of barometers: mercury and aneroid. In a mercury barometer, atmospheric pressure balances a column of mercury, the height of which can be precisely measured. A variation of this type of barometer can be easily made at home. In an aneroid barometer, a flexible-walled evacuated capsule deflects with changes in atmospheric pressure, and this deflection is coupled mechanically to an indicating needle. A non-liquid barometer called the aneroid barometer is widely used in portable instruments and in aircraft altimeters because of its smaller size and convenience. Barographs are barometers that mechanically record changes in barometric pressure over time. The first barometer was invented in 1643 by Evangelista Torricelli, one of Galileos assistants. Mercury barometers are also used in meteorology, mostly in barographs and as a pressure instrument in radiosondes. Home barometers are useful in planning outdoor activities, as sudden drops in pressure can indicate the approach of a low-pressure air front.