<P> The concept that decreasing atmospheric pressure predicts stormy weather, postulated by Lucien Vidi, provides the theoretical basis for a weather prediction device called a "weather glass" or a "Goethe barometer" (named for Johann Wolfgang Von Goethe, the renowned German writer and polymath who developed a simple but effective weather ball barometer using the principles developed by Torricelli). The French name, le baromètre Liègeois, is used by some English speakers . This name reflects the origins of many early weather glasses - the glass blowers of Liège, Belgium . </P> <P> The weather ball barometer consists of a glass container with a sealed body, half filled with water . A narrow spout connects to the body below the water level and rises above the water level . The narrow spout is open to the atmosphere . When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body . A variation of this type of barometer can be easily made at home . </P> <P> A mercury barometer has a glass tube closed at one end with an open mercury - filled reservoir at the base . The weight of the mercury creates a vacuum in the top of the tube known as Torricellian vacuum . Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir . High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column . Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir . Since higher temperature levels around the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect . The tube has to be at least as long as the amount dipping in the mercury + head space + the maximum length of the column . </P> <P> Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere . He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight". </P>

What keeps mercury inside the tube in a mercury barometer