Barometers are used to measure atmospheric pressure, but they were created in the 17th century when an Italian mathematician named Evangelista Torricelli, a former student of Galileo’s, devised the first artificial vacuum out of a glass tube filled with mercury. Subsequently, French philosophers René Descartes and Blaise Pascal used the instrument to determine altitude, but it was only by chance that scientists noticed that weather also affected the device’s measurements.

Mercury barometers have changed little since the days of Torricelli. Cistern barometers still consist of a slender glass tube sealed at the top and connected to a larger chamber at the bottom. Like the tube, the cistern is filled with mercury. When atmospheric pressure is reduced due to, say, the presence of moisture in the air, the mercury in the barometer falls. When it increases because of heat, the barometer rises. That’s why many barometers are marked with the word “Rain” next to low readings and “Fair” next to higher ones.

The other type of mercury barometer is a siphon barometer, whose glass tube is U-shaped rather than straight, obviating the need for a cistern. In either case, glass barometers a...re fragile, so they are usually mounted to a piece of mahogany or other decorative hardwood. And because the movement of the mercury in the tube can be very small, the English naturalist and microscope enthusiast Robert Hooke invented a dial that made the liquid’s subtle movements easier to perceive.