The simple barometer which Torricelli invented measures air pressure accurately. But it is not much used today. Kinds of barometers which do not require the use of mercury are mostly used instead. The most common type is the aneroid barometer. It may be as small as a watch. In an aneroid barometer the air pressure is shown by a pointer on a dial. This type of barometer is much more convenient than the mercury-column type. It is not, however, so accurate.
One type of aneroid barometer is carried on the instrument board of some airplanes. This barometer is called an altimeter. With it the aviator is able to know how high above sea level he is flying at all times, even at night or when he is above the clouds or when he is surrounded by fog. The aneroid type of altimeter is less commonly used than another type which is operated by radio waves instead of by atmospheric pressure. With this type the aviator is able to know how high he is above the land or the water over which he is flying.
A type of aneroid barometer, called a barograph, gives a permanent record of air pressure. The pointer on the barograph is in the f of a pen which is constantly supplied with ink. The point of this pen rests special record sheet on which are two scales. One scale is for reading the barometer record and the other for recording the time of day. This record sheet is fastened around a cylinder which turns around once in a day or once in a week. The pen is placed on the paper at the point representing the hour at which the cylinder is started revolving. As the cylinder slowly revolves, the pen marks a line on the paper. It moves up if the air pressure becomes greater. It moves down if the air pressure becomes less. The crooked line made by the pen is the record of the air-pressure changes for the day or week.