A barometer is a device for measuring atmospheric pressure and thus determining changes in the weather, the height of mountains, and other phenomena. The basic principle behind the barometer is the discovery in 1643 by Torricelli, that atmospheric pressure might be counterpoised by a column of mercury standing as high in proportion to the thirty-four feet that water in similar circumstances stands, as the specific gravity of water is to that of mercury. Pascal confirmed the conclusions of Torricelli in 1645; six years afterwards it was found by Perrier that the height of the mercury in the Torricellian tube varied with the weather; and, in 1665, Boyle proposed to use the instrument to measure the height of mountains. Various types of barometer have been invented, among the most common being the cistern barometer, Gay-Lussac's barometer and the aneroid barometer.
The common or cistern barometer, which is a modification of the Torricellian tube, consists of a glass tube 33 inches in length and about one-third of an inch in diameter, hermetically sealed at the top, and having the lower end resting in a small vessel containing mercury, or bent upwards and terminating in a glassbulb partly occupied by the mercury and open to the atmosphere. The tube is first filled with purified mercury, and then inverted, and there is affixed to it a scale to mark the height of the mercurial column, which comparatively seldom rises above 31 or sinks below 28 inches. In general the rising of the mercury presages fair weather, and its falling the contrary, a great and sudden fall being the usual presage of a storm. The weather-points on the ordinary barometric scale are as follows: - At 28 inches, stormy weather; 28.5, much rain or snow; 29, rain or snow; 29.5, changeable; 30 fair or frost; 30.5, settled fair or frost; 31. very dry weather or hard frost. Certain attendant signs, however, have also to be noted: thus, when fair or foul weather follows almost immediately upon the rise or fall of the mercury, the change is usually of short duration; while if the change of weather be delayed for some days after the variation in the mercury, it is usually of long continuance. The direction of the wind has also to be taken into account.
The siphon barometer consists of a bent tube, generally of uniform bore, having two unequal legs, the longer closed, the shorter open. A sufficient quantity of mercury having been introduced to fill the longer leg, the instrument is set upright, and the mercury takes such a position that the difference of the levels in the two legs represents the pressure of the atmosphere. In the best siphon barometers there are two scales, one for each leg, the divisions on one being reckoned upwards, and on the other downwards from an intermediate zero point, so that the sum of the two readings is the difference of levels of the mercury in the two branches.
The wheel barometer is the one that was most commonly used for domestic purposes before the computer age. It was far from being accurate, but it was often preferred for ordinary use on account of the greater range of its scale, by which small differences in the height of the column of mercury were more easily observed. It usually consisted of a siphon barometer, having a float resting on the surface of the mercury in the open branch, a thread attached to the float passing over a pulley, and having a weight as a counterpoise to the float at its extremity. As the mercury rose and fell the thread and weight turned the pulley, which again moved the index of the dial.
The mountain barometer was a portable mercurial barometer with a tripod support and a long scale for measuring the altitude of mountains. To prevent breakage, through the oscillations of such a heavy liquid as mercury, it was usually carried inverted, or it was furnished with a movable basin and a screw, by means of which the mercury could be forced up to the top of the tube. for delicate operations, such as the measurement of altitudes, the scale of the barometer was furnished with a nonius or vernier, which greatly increased the minuteness and accuracy of the scale. For the rough estimate of altitudes the following rule was sufficient: - As the sum of the heights of the mercury at the bottom and top of the mountain is to their difference, so is 52,000 to the height to be measured, in feet. In exact barometric observations two corrections require to be made, one for the depression of the mercury in the tube by capillary attraction, the other for temperature, which increases or diminishes the bulk of the mercury. In regard to the measurement of heights the general rule is to subtract the ten-thousandth part of the observed altitude for every degree of Fahrenheit above 32 degrees.
In the aneroid barometer, as its name implies (the name coming from the Greek a, not, neros, liquid), no fluid was employed, the action being dependent upon the susceptibility to atmospheric pressure shown by a flat circular metallic chamber from which the air had been partially exhausted, and which has a flexible top and bottom of corrugated metal plate. By an ingenious arrangement of springs and levers the depression or elevation of the surface of the box was registered by an index on the dial, by which means it was also greatly magnified, being given in inches to correspond with the mercurial barometer. Aneroids are, however, generally less reliable than mercurial barometers, with which they were recommended to be frequently compared. Research Barometer
Abbreviations
SciTech
Research Barometer