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72-410: A sympiesometer is a compact and lightweight type of barometer that was widely used on ships in the 19th century. The sensitivity of this barometer was also used to measure altitude. The sympiesometer consists of two parts. One is a traditional mercury thermometer that is needed to calculate the expansion or contraction of the fluid in the barometer proper. The other is the barometer, consisting of

144-409: A J-shaped tube open at the lower end and closed at the top, with small reservoirs at both ends of the tube. The lower end of the J and its associated reservoir were filled with colored almond oil, while the upper portion and its reservoir were filled with hydrogen gas. Increasing air pressure would cause the oil to be pushed out of the lower reservoir and into the tube, compressing the hydrogen gas into

216-542: A barometric scale with finer graduations "Stormy (28 inches of mercury), Much Rain (28.5), Rain (29), Change (29.5), Fair (30), Set fair (30.5), very dry(31)". Natalo Aiano is recognised as one of the finest makers of wheel barometers, an early pioneer in a wave of artisanal Italian instrument and barometer makers that were encouraged to emigrate to the UK. He listed as working in Holborn, London c.  1785 –1805. From 1770 onwards

288-619: A better way to attempt to produce a vacuum other than with a siphon. Magiotti devised such an experiment. Four accounts of the experiment exist, all written some years later. No exact date was given, but since Two New Sciences reached Rome in December 1638, and Berti died before January 2, 1644, science historian W. E. Knowles Middleton places the event to sometime between 1639 and 1643. Present were Berti, Magiotti, Jesuit polymath Athanasius Kircher , and Jesuit physicist Niccolò Zucchi . In brief, Berti's experiment consisted of filling with water

360-442: A few aneroid barometers intended for monitoring the weather are calibrated to manually adjust for altitude. In this case, knowing either the altitude or the current atmospheric pressure would be sufficient for future accurate readings. The table below shows examples for three locations in the city of San Francisco , California . Note the corrected barometer readings are identical, and based on equivalent sea-level pressure. (Assume

432-578: A large number of Italians came to England because they were accomplished glass blowers or instrument makers. By 1840 it was fair to say that the Italians dominated the industry in England. Using vacuum pump oil as the working fluid in a barometer has led to the creation of the new "World's Tallest Barometer" in February 2013. The barometer at Portland State University (PSU) uses doubly distilled vacuum pump oil and has

504-508: A liquid that was heavier than water, and from his previous association and suggestions by Galileo, he deduced that by using mercury , a shorter tube could be used. With mercury, which is about 14 times denser than water, a tube only 80 cm was now needed, not 10.5 m. In 1646, Blaise Pascal along with Pierre Petit , had repeated and perfected Torricelli's experiment after hearing about it from Marin Mersenne , who himself had been shown

576-405: A long tube that had both ends plugged, then standing the tube in a basin of water. The bottom end of the tube was opened, and water that had been inside of it poured out into the basin. However, only part of the water in the tube flowed out, and the level of the water inside the tube stayed at an exact level, which happened to be 10.3 m (34 ft), the same height limit Baliani had observed in

648-427: A manifest cause from which the resistance can be derived which is felt if we try to make a vacuum, it seems to me foolish to try to attribute to vacuum those operations which follow evidently from some other cause; and so by making some very easy calculations, I found that the cause assigned by me (that is, the weight of the atmosphere) ought by itself alone to offer a greater resistance than it does when we try to produce

720-442: A mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use typically have no compensation under the assumption that they will be used within a controlled room temperature range. As the air pressure decreases at altitudes above sea level (and increases below sea level)

792-422: A method that does not involve liquid . Invented in 1844 by French scientist Lucien Vidi , the aneroid barometer uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper . The evacuated capsule (or usually several capsules, stacked to add up their movements) is prevented from collapsing by a strong spring. Small changes in external air pressure cause

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864-505: A mountain called the Puy de Dôme , asking him to perform a crucial experiment. Perier was to take a barometer up the Puy de Dôme and make measurements along the way of the height of the column of mercury. He was then to compare it to measurements taken at the foot of the mountain to see if those measurements taken higher up were in fact smaller. In September 1648, Perier carefully and meticulously carried out

936-455: A nominal height of about 12.4 m for the oil column height; expected excursions are in the range of ±0.4 m over the course of a year. Vacuum pump oil has very low vapour pressure and it is available in a range of densities; the lowest density vacuum oil was chosen for the PSU barometer to maximize the oil column height. An aneroid barometer is an instrument used for measuring air pressure as

1008-427: A pressure instrument in radiosondes . A barograph is a recording aneroid barometer where the changes in atmospheric pressure are recorded on a paper chart. The principle of the barograph is same as that of the aneroid barometer. Whereas the barometer displays the pressure on a dial, the barograph uses the small movements of the box to transmit by a system of levers to a recording arm that has at its extreme end either

1080-696: A scribe or a pen. A scribe records on smoked foil while a pen records on paper using ink, held in a nib. The recording material is mounted on a cylindrical drum which is rotated slowly by a clock. Commonly, the drum makes one revolution per day, per week, or per month and the rotation rate can often be selected by the user. Microelectromechanical systems (or MEMS) barometers are extremely small devices between 1 and 100 micrometres in size (0.001 to 0.1 mm). They are created via photolithography or photochemical machining . Typical applications include miniaturized weather stations, electronic barometers and altimeters. A barometer can also be found in smartphones such as

1152-402: 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

1224-465: A temperature of 15 °C.) In 1787, during a scientific expedition on Mont Blanc , De Saussure undertook research and executed physical experiments on the boiling point of water at different heights. He calculated the height at each of his experiments by measuring how long it took an alcohol burner to boil an amount of water, and by these means he determined the height of the mountain to be 4775 metres. (This later turned out to be 32 metres less than

1296-407: A thumbscrew pressing on a leather diaphragm bottom (V in the diagram). This compensates for displacement of mercury in the column with varying pressure. To use a Fortin barometer, the level of mercury is set to zero by using the thumbscrew to make an ivory pointer (O in the diagram) just touch the surface of the mercury. The pressure is then read on the column by adjusting the vernier scale so that

1368-480: A vacuum, and since he was the first to view it this way, he is traditionally considered the inventor of the barometer, in the sense in which we now use the term. Because of rumors circulating in Torricelli's gossipy Italian neighbourhood, which included that he was engaged in some form of sorcery or witchcraft, Torricelli realized he had to keep his experiment secret to avoid the risk of being arrested. He needed to use

1440-533: A vacuum. It was traditionally thought, especially by the Aristotelians , that the air did not have weight; that is, that the kilometers of air above the surface of the Earth did not exert any weight on the bodies below it. Even Galileo had accepted the weightlessness of air as a simple truth. Torricelli proposed that rather than an attractive force of the vacuum sucking up water, air did indeed have weight, which pushed on

1512-399: A void would be featureless, it could neither be encountered by the senses nor could its supposition lend additional explanatory power. Hero of Alexandria challenged the theory in the first century AD, but his attempts to create an artificial vacuum failed. The theory was debated in the context of 17th-century fluid mechanics , by Thomas Hobbes and Robert Boyle , among others, and through

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1584-459: Is 1013 hPa (mbar). The word barometer is derived from the Ancient Greek βάρος ( báros ), meaning "weight", and μέτρον ( métron ), meaning "measure". Evangelista Torricelli is usually credited with inventing the barometer in 1643, although the historian W. E. Knowles Middleton suggests the more likely date is 1644 (when Torricelli first reported his experiments; the 1643 date

1656-418: Is a limit to the phenomenon. René Descartes proposed a plenic interpretation of atomism to eliminate the void, which he considered incompatible with his concept of space. The theory was rejected by later scientists, such as Galileo's pupil Evangelista Torricelli , who repeated his experiment with mercury . Blaise Pascal successfully repeated Galileo's and Torricelli's experiment and foresaw no reason why

1728-418: Is a traditional mercury thermometer that is needed to calculate the expansion or contraction of the fluid in the barometer. The other is the barometer, consisting of a J-shaped tube open at the lower end and closed at the top, with small reservoirs at both ends of the tube. A wheel barometer uses a "J" tube sealed at the top of the longer limb. The shorter limb is open to the atmosphere and floating on top of

1800-406: Is equivalent to 29.92 inches (760 mm) of mercury. Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. In 2007, a European Union directive was enacted to restrict the use of mercury in new measuring instruments intended for

1872-473: Is impossible. It is plain, then, that if there is a time in which it will move through any part of the void, this impossible result will follow: it will be found to traverse a certain distance, whether this be full or void, in an equal time; for there will be some body which is in the same ratio to the other body as the time is to the time. Plenism means "fullness", from Latin plēnum , English "plenty", cognate via Proto-Indo-European to "full". In Ancient Greek ,

1944-424: Is no evidence that he built a working barometer at that time. On 27 July 1630, Giovanni Battista Baliani wrote a letter to Galileo Galilei explaining an experiment he had made in which a siphon , led over a hill about 21 m high, failed to work. When the end of the siphon was opened in a reservoir, the water level in that limb would sink to about 10 m above the reservoir. Galileo responded with an explanation of

2016-406: Is no ratio by which it exceeds 0; for that which exceeds must be divisible into the excess + that which is exceeded, so that will be what it exceeds 0 by + 0. For this reason, too, a line does not exceed a point unless it is composed of points! Similarly the void can bear no ratio to the full, and therefore neither can movement through the one to movement through the other, but if a thing moves through

2088-409: Is only mercury vapour above this point and its pressure is very low relative to the atmospheric pressure. Therefore, one can find the atmospheric pressure using the barometer and this equation: where ρ is the density of mercury, g is the gravitational acceleration, and h is the height of the mercury column above the free surface area. The physical dimensions (length of tube and cross-sectional area of

2160-450: Is similar to resetting an analog clock that is not at the correct time. Its dial is rotated so that the current atmospheric pressure from a known accurate and nearby barometer (such as the local weather station ) is displayed. No calculation is needed, as the source barometer reading has already been converted to equivalent sea-level pressure, and this is transferred to the barometer being set—regardless of its altitude. Though somewhat rare,

2232-410: Is thinner, in time E (if the length of B is equal to D ), in proportion to the density of the hindering body. For let B be water and D air; then by so much as air is thinner and more incorporeal than water, A will move through D faster than through B . Let the speed have the same ratio to the speed, then, that air has to water. Then if air is twice as thin, the body will traverse B in twice

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2304-414: Is used to keep track of the contents of the diver's air tank. Another gauge is used to measure the hydrostatic pressure, usually expressed as a depth of sea water. Either or both gauges may be replaced with electronic variants or a dive computer. The density of mercury will change with increase or decrease in temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose

2376-524: The Samsung Galaxy Nexus , Samsung Galaxy S3-S6, Motorola Xoom, Apple iPhone 6 and newer iPhones, and Timex Expedition WS4 smartwatch , based on MEMS and piezoresistive pressure-sensing technologies. Inclusion of barometers on smartphones was originally intended to provide a faster GPS lock. However, third party researchers were unable to confirm additional GPS accuracy or lock speed due to barometric readings. The researchers suggest that

2448-532: The Shark Oil barometer, work only in a certain temperature range, achieved in warmer climates. Barometric pressure and the pressure tendency (the change of pressure over time) have been used in weather forecasting since the late 19th century. When used in combination with wind observations, reasonably accurate short-term forecasts can be made. Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were

2520-404: The actual height of 4807 metres). For these experiments De Saussure brought specific scientific equipment, such as a barometer and thermometer . His calculated boiling temperature of water at the top of the mountain was fairly accurate, only off by 0.1 kelvin. Based on his findings, the altimeter could be developed as a specific application of the barometer. In the mid-19th century, this method

2592-447: The cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. This type of barometer is common in homes and in recreational boats . It is also used in meteorology , mostly in barographs and as

2664-426: The change in pressure, especially if more than 3.5 hPa (0.1 inHg), the greater the change in weather that can be expected. If the pressure drop is rapid, a low pressure system is approaching, and there is a greater chance of rain. Rapid pressure rises , such as in the wake of a cold front , are associated with improving weather conditions, such as clearing skies. With falling air pressure, gases trapped within

2736-528: The coal in deep mines can escape more freely. Thus low pressure increases the risk of firedamp accumulating. Collieries therefore keep track of the pressure. In the case of the Trimdon Grange colliery disaster of 1882 the mines inspector drew attention to the records and in the report stated "the conditions of atmosphere and temperature may be taken to have reached a dangerous point". Aneroid barometers are used in scuba diving . A submersible pressure gauge

2808-459: The early 18th century by Sir Isaac Newton and Gottfried Leibniz . As advanced by Aristotle in Physics : In a void , no one could say why a thing once set in motion should stop anywhere; for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum , unless something more powerful gets in its way. Further, things are now thought to move into

2880-546: The experiment by Torricelli toward the end of 1644. Pascal further devised an experiment to test the Aristotelian proposition that it was vapours from the liquid that filled the space in a barometer. His experiment compared water with wine, and since the latter was considered more "spiritous", the Aristotelians expected the wine to stand lower (since more vapours would mean more pushing down on the liquid column). Pascal performed

2952-484: The experiment publicly, inviting the Aristotelians to predict the outcome beforehand. The Aristotelians predicted the wine would stand lower. It did not. However, Pascal went even further to test the mechanical theory. If, as suspected by mechanical philosophers like Torricelli and Pascal, air had weight, the pressure would be less at higher altitudes. Therefore, Pascal wrote to his brother-in-law, Florin Perier, who lived near

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3024-413: The experiment, and found that Pascal's predictions had been correct. The column of mercury stood lower as the barometer was carried to a higher altitude. 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 ,

3096-436: The first form of the modern weather map when created in the 19th century. Isobars , lines of equal pressure, when drawn on such a map, give a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface brings clouds and sometimes precipitation . The larger

3168-419: The float and turning the dial the other way. Around 1810 the wheel barometer, which could be read from a great distance, became the first practical and commercial instrument favoured by farmers and the educated classes in the UK. The face of the barometer was circular with a simple dial pointing to an easily readable scale: "Rain - Change - Dry" with the "Change" at the top centre of the dial. Later models added

3240-458: The general public, effectively ending the production of new mercury barometers in Europe. The repair and trade of antiques (produced before late 1957) remained unrestricted. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes. Fortin barometers use a variable displacement mercury cistern, usually constructed with

3312-544: The inclusion of barometers in smartphones may provide a solution for determining a user's elevation, but also suggest that several pitfalls must first be overcome. There are many other more unusual types of barometer. From variations on the storm barometer, such as the Collins Patent Table Barometer, to more traditional-looking designs such as Hooke's Otheometer and the Ross Sympiesometer. Some, such as

3384-696: The liquid. The basic concept was initially demonstrated by Robert Hooke , which he referred to as the Otheometer . However it remained unused until it was re-introduced by Alexander Adie . Barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs , pressure systems and frontal boundaries . Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially

3456-459: The mercury + head space + the maximum length of the column. 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". Inspired by Torricelli, Otto von Guericke on 5 December 1660 found that air pressure

3528-468: The mercury just touches the sightline at Z. Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport. This prevents water-hammer damage to the column in transit. A sympiesometer is a compact and lightweight barometer that was widely used on ships in the early 19th century. The sensitivity of this barometer was also used to measure altitude. Sympiesometers have two parts. One

3600-424: The mercury there is a small glass float. A fine silken thread is attached to the float which passes up over a wheel and then back down to a counterweight (usually protected in another tube). The wheel turns the point on the front of the barometer. As atmospheric pressure increases mercury moves from the short to the long limb, the float falls and the pointer moves. When pressure falls the mercury moves back, lifting

3672-418: The moving body differs from the other owing to excess of weight or of lightness. Now the medium causes a difference because it impedes the moving thing, most of all if it is moving in the opposite direction, but in a secondary degree even if it is at rest; and especially a medium that is not easily divided, i.e. a medium that is somewhat dense. A , then, will move through B in time G , and through D , which

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3744-622: The phenomenon: he proposed that it was the power of a vacuum that held the water up, and at a certain height the amount of water simply became too much and the force could not hold any more, like a cord that can support only so much weight. This was a restatement of the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle , and which Galileo restated as resistenza del vacuo . Galileo's ideas, presented in his Discorsi ( Two New Sciences ), reached Rome in December 1638. Physicists Gasparo Berti and father Raffaello Magiotti were excited by these ideas, and decided to seek

3816-486: The ratio which the time E bears to the time H . For if the body Z be as much thinner than D as E exceeds H , A , if it moves through Z , will traverse it in a time inverse to the speed of the movement, i.e. in a time equal to H . If, then, there is no body in Z , A will traverse Z still more quickly. But we supposed that its traverse of Z when Z was void occupied the time H . So that it will traverse Z in an equal time whether Z be full or void. But this

3888-401: 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 . The weather ball barometer consists of a glass container with

3960-418: 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

4032-425: The same if there are negligible changes in time, horizontal distance, and temperature. If this were not done, there would be a false indication of an approaching storm at the higher elevation. Aneroid barometers have a mechanical adjustment that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude. Setting an aneroid barometer

4104-445: The same instrument, but used for different purposes. An altimeter is intended to be used at different levels matching the corresponding atmospheric pressure to the altitude , while a barometer is kept at the same level and measures subtle pressure changes caused by weather and elements of weather. The average atmospheric pressure on the Earth's surface varies between 940 and 1040 hPa (mbar). The average atmospheric pressure at sea level

4176-407: The siphon. What was most important about this experiment was that the lowering water had left a space above it in the tube which had no intermediate contact with air to fill it up. This seemed to suggest the possibility of a vacuum existing in the space above the water. Evangelista Torricelli, a friend and student of Galileo, interpreted the results of the experiments in a novel way. He proposed that

4248-405: The standard atmospheric pressure, a water column of roughly 10.3 m (33.8 ft) would be needed. Standard atmospheric pressure as a function of elevation: Horror vacui (physics) Aristotle also argued against the void in a more abstract sense: since a void is merely nothingness , following his teacher Plato , nothingness cannot rightly be said to exist. Furthermore, insofar as

4320-472: The term for the void is τὸ κενόν ( to kenón ). The idea was restated as "Natura abhorret vacuum" by François Rabelais in his series of books titled Gargantua and Pantagruel in the 1530s. The theory was supported and restated by Galileo Galilei in the early 17th century as "Resistenza del vacuo" . Galileo was surprised by the fact that water could not rise above a certain level in an aspiration tube in his suction pump , leading him to conclude that there

4392-495: The thickest medium such and such a distance in such and such a time, it moves through the void with a speed beyond any ratio. For let Z be void, equal in magnitude to B and to D . Then if A is to traverse and move through it in a certain time, H , a time less than E , however, the void will bear this ratio to the full. But in a time equal to H , A will traverse the part O of A . And it will surely also traverse in that time any substance Z which exceeds air in thickness in

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4464-406: The time that it does D , and the time G will be twice the time E . And always, by so much as the medium is more incorporeal and less resistant and more easily divided, the faster will be the movement. Now there is no ratio in which the void is exceeded by body, as there is no ratio of 0 to a number. For if 4 exceeds 3 by 1, and 2 by more than 1, and 1 by still more than it exceeds 2, still there

4536-453: The tube) of the barometer itself have no effect on the height of the fluid column in the tube. In thermodynamic calculations, a commonly used pressure unit is the "standard atmosphere". This is the pressure resulting from a column of mercury of 760 mm in height at 0 °C. For the density of mercury, use ρ Hg = 13,595 kg/m and for gravitational acceleration use g = 9.807 m/s . If water were used (instead of mercury) to meet

4608-417: The uncorrected reading of the barometer will depend on its location. The reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305 m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be

4680-498: The upper reservoir. The pressure was indicated by the position of the top of the oil. To correct for temperature, a sliding scale was used; the operator would first use the thermometer to set the scale and then measure the pressure from it. The basic idea is similar to the common weather glass that had been in use for some time, but the use of highly compressible hydrogen in a long, thin tube allowed much more accurate measurements, as changes in pressure resulted in much more movement of

4752-424: The void because it yields; but in a void this quality is present equally everywhere, so that things should move in all directions. Further, the truth of what we assert is plain from the following considerations. We see the same weight or body moving faster than another for two reasons, either because there is a difference in what it moves through, as between water, air, and earth, or because, other things being equal,

4824-455: The water level in the body. A variation of this type of barometer can be easily made at home. A mercury barometer is an instrument used to measure atmospheric pressure in a certain location and has a vertical glass tube closed at the top sitting in an open mercury-filled basin at the bottom. Mercury in the tube adjusts until the weight of it balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on

4896-428: The water, holding up a column of it. He argued that the level that the water stayed at—c. 10.3 m above the water surface below—was reflective of the force of the air's weight pushing on the water in the basin, setting a limit for how far down the water level could sink in a tall, closed, water-filled tube. He viewed the barometer as a balance—an instrument for measurement—as opposed to merely an instrument for creating

4968-448: The weight of the atmosphere, not an attracting force of the vacuum, held the water in the tube. In a letter to Michelangelo Ricci in 1644 concerning the experiments, he wrote: Many have said that a vacuum does not exist, others that it does exist in spite of the repugnance of nature and with difficulty; I know of no one who has said that it exists without difficulty and without a resistance from nature. I argued thus: If there can be found

5040-417: Was only suggested after his death). Gasparo Berti , an Italian mathematician and astronomer, also built a rudimentary water barometer sometime between 1640 and 1644, but it was not a true barometer as it was not intended to move and record variable air pressure. French scientist and philosopher René Descartes described the design of an experiment to determine atmospheric pressure as early as 1631, but there

5112-472: Was unusually low and predicted a storm, which occurred the next day. The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters of mercury (mmHg). A torr was originally defined as 1 mmHg. The pressure is quoted as the level of the mercury's height in the vertical column. Typically, atmospheric pressure is measured between 26.5 inches (670 mm) and 31.5 inches (800 mm) of Hg. One atmosphere (1 atm)

5184-444: Was used by explorers. When atmospheric pressure is measured by a barometer, the pressure is also referred to as the "barometric pressure". Assume a barometer with a cross-sectional area A , a height h , filled with mercury from the bottom at Point B to the top at Point C. The pressure at the bottom of the barometer, Point B, is equal to the atmospheric pressure. The pressure at the very top, Point C, can be taken as zero because there

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