In the siphon barometer, the tube is bent into a J-shape, sealed at the end of the long limb, and the barometer reading is the difference between the mercury heights in the two limbs. (EB11/Barometer).
There was also a hybrid, invented by Lavoisier; essentially a "siphon barometer with the addition of some sort of reservoir of mercury by means of which the level in both limbs of the siphon can be simultaneously varied." (Middleton 228).
Efforts to improve on the mercury barometer addressed three aspects: readability, portability, and accuracy. There is likely to be little guidance in Grantville literature on how to improve these aspects, but I will briefly outline the more interesting of the post-Torricelli expedients that might be reinvented.
The world record high and low pressures differ by about five inches of mercury, and a normal range is more like three inches. For greater readability, inventors sought to increase the movement caused by a change in pressure. Hooke's wheel barometer (1664) was in use for more than two centuries; a float in the mercury was attached by a cable that ran around a pulley to a counterweight, and the axis of the pulley was attached to the pointer of a dial (Middleton 94). There was also the "diagonal barometer" that employed an obliquely bent tube; this evolved into an L-shaped instrument that could be mounted into a corner of a mirror frame. It was more stylish than accurate (112). Both are alluded to by EB11/Barometer.
With regard to portability, the concern was not so much with weight or size, but rather fragility. The major demand for more portable barometers were from those who intended to take them up into the mountains and use them as altimeters, or from mariners. Carried while climbing, they were subject to shocks and even drops. Use at sea posed the additional problem of coping with the motion of the ship as a result of the wind (or the firing of the guns) during use.
With cistern barometers, there was the further problem of keeping the mercury from spilling while still permitting air to have access. By 1688 it was known that some woods (boxwood) are permeable to air and not to mercury, and thus may be used to make a closed cistern (145). A 1695 barometer used a screw to reduce the open volume of the cistern, causing the cistern and tube to be completely filled with mercury; the filled tube was less likely to break when jostled (151).
Blondeau (1779) designed a siphon barometer for marine use with an iron (unbreakable) tube. This had an ivory float, attached by a wire to a scale pointer. The wire passed through a bushing on the top of the short limb of the tube (158). The problem of tilting was addressed by Nairne (1773); he fixed his instrument in a gimbal, a technique also used for ship lighting (163). Fitzroy (1860) shock-mounted the barometer tube in rubber (164).
Improvements in accuracy generally were achieved by improving the vacuum at the top of the closed end of the tube, increasing the bore size of the barometer (reducing the capillary depression of the meniscus at the periphery), making the level easier to read, and supplying a mechanism to maintain the level of mercury in the cistern at the zero level.
In theory, you can obtain a vacuum in a cistern barometer by turning the tube so the open end is up, pouring in some mercury, then inverting it over the cistern. The level of mercury in the tube will descend until its weight is balanced by the air pressure on the exposed mercury, leaving a vacuum in its wake. But "as early as 1649 Zucchi noted the difficulty of filling a tube with mercury without introducing bubbles." (Middleton 241). Boyle tried to clear out bubbles with iron wires, with imperfect success. Moreover, the tube would be cleaned with some solvent (ethanol) before filling, and the solvent could be entrained.
A big improvement was achieved by boiling the mercury (first described in 1723 and universal by 1772) to remove air and other gaseous impurities. The sealed end of the tube was held over a small stove, and an iron wire jiggled in the tube to expedite the bubbling of the air. Once there were no more bubbles, the tube was repositioned to heat a different part of the tube.
The reader will surely realize that this process produces dangerous mercury vapors. The Cardinal de Luynes warned colleagues to do it in a large room, with no gold or silver about (thus protecting the gilded furniture if not the artisan). In 1935, Patterson combined the purification of mercury with this outgassing process; the tubes, connected to the mercury distillation column, were placed in a heated vacuum chamber (248).
Insofar as readability is concerned, one expedient was providing a ring-shaped index that could be moved to be level with the top of the meniscus, to facilitate reading off its height relative to the external scale. Another was providing a fine scale (vernier) that could be slid up or down the tube to align with the top of the meniscus.
With regard to the cistern level, there would be a means of indicating a departure from the zero level (overflow, a fixed index, a floating index, a scale on the cistern), and a means of adjustment (pouring, putting a supply of mercury in a leather bag, a plunger in the cistern, a screw for moving the cistern relative to the tube or moving a plunger within the cistern). Or one could make the scale rather than the cistern adjustable.
The Fortin barometer (1809), later known as the "weather service" barometer, was a cistern barometer with the mercury reservoir taking the form of a leather bag (inside a boxwood or glass container) that could be raised or lowered from below by a screw. Above the bag is an ivory pointer; before taking a reading, the bag is adjusted so that the point just touches the mirror image of the point on the surface of the mercury (astarmathsandphysics.com). The glass tube is enclosed in brass tubes with a reading slit for protection. The fixed tube carries the scale on an adjustable brass plate, and the movable tube has the sighting ring and vernier for reading the mercury level (Brombacher 6). This was an extremely popular, accurate, and durable design. In 1856, Green devised a way to minimize leakage of mercury at the cistern joints, replacing cement with clamping screws, leather washers, and boxwood specially selected from the center of the wood and specially treated (saturated with shellac in a vacuum) (Middleton 346). The accuracy is dependent on the bore size (0.25-0.8 inches). Two of the ten Fortin-type barometers imported by the Signal Service in 1878 were still being used as working standards by the Weather Bureau in 1962 (Middleton 350).
There is a reasonable possibility that if an up-timer who majored in physics had to do a course lab experiment that involved measuring air pressure, that the instrument used was a Fortin-type barometer (UQ Physics Museum).
(Elastic) Barometers. An aneroid barometer, by definition, is merely one that doesn't use a liquid as a sensor. However, in practice the term signifies a particular mechanical sensor. The basic concept, proposed by Leibnitz, is that a hollow capsule with an interior vacuum has at least one flexible (elastic) wall, and this will deflect to a greater or lesser degree depending on the outside air pressure. Of course, this tiny deflection would have to be magnified by mechanical linkages so it can be read out and quantified. Elastic barometers were designed by Zeiher (1763) and Conte (1797), but the approach that caught on was that of Vidie (1844). The flexible wall was supported by internal helical springs (otherwise the capsule would be crushed) and, in the readout mechanism, there is a bimetallic strip for temperature compensation.
With the bimetal, there is perfect compensation at only one temperature. If a small amount of gas is left in the capsule, it is possible to make the compensation perfect at two temperatures (412). It also helps to keep the capsule volume very small.
But if the metal used is one in which Young's modulus has no temperature dependence (thermoelasticity) in the meteorological range, then the mechanism can be engineered to provide perfect compensation at all temperatures. NISPAN-C (patented 1955) has essentially no thermoelasticity for -50 to +150oF (Hamden). Unfortunately, I haven't found any indication that either its composition or its properties would be known in Grantville. Before NISPAN-C was available, one could make the two sides of the diaphragm out of different metals, one with positive thermoelasticity (modulvar) and the other with negative (beryllium-copper). But those alloys are also problema
tic for NTL 1630s.
The metal used to make the capsule in the nineteenth century was brass or German silver, and they were soldered together (Srivastava 56). (Nowadays, a 0.002" thick sheet of copper-beryllium alloy (a corrosion-resistant material) is stamped out to form two thin diaphragms, and their edges are electron beam-welded in a vacuum to form the capsule (McClung). Steel may be used instead, and there can be a series of capsules in a single barometer (for greater total movement). Usually, the diaphragms are corrugated for additional strength, and the supporting spring is now likely to be external. One diaphragm is clamped and the other is free to move. The vacuum is about 95% (Srivastava).
The mechanical linkages have to be accurately machined, to tolerances similar to those used in watchmaking. Friction must be minimized so that they are responsive to small changes in pressure. The encyclopedias don't provide much detail, but it is reasonable to expect that there are aneroid barometers in Grantville—it's a rural area, and the barometer has some value for short-term weather forecasting. Hence, one of these can be disassembled and studied.
Precision electronic aneroid capsule barometers feature contact-free measurement of the displacement of the aneroid capsule. There are several different means of achieving this, including capacitive and potentiometric displacement detectors, but there is probably nothing in Grantville literature about this (WMO2008 3.3.1).
Miscellaneous Electronic barometers. There are also piezoresistive (these need a monolithic silicon crystal element) and cylindrical resonator (pressure variation changes strain in a nickel alloy cylinder and thereby changes its resonant frequency) barometers (WMO2008). I think Grantville information on these approaches is minimal if not nonexistent and the required infrastructure well beyond that is achievable in the foreseeable future of the new time line.
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Deadweight Barometer. This is "weird tech." Blaise Pascal invented (~1650) a barometer which Middleton (396) describes as a sealed "concertina bellows" attached at its upper end to a roof beam and at its lower end having a chain that reaches and is partly coiled up on the floor. As air pressure changes, "various amounts of the chain will be lifted from the floor." (The chain is analogous to the drag rope used by balloonists; as the lift on the balloon changes as people get on or off, or temperatures inside and out change, the amount of drag rope lifted changes, and thus changes the weight that the balloon is lifting.) The problem with Pascal's barometer is the difficulty in making the bellows airtight.
Boyle (1660) proposed "hanging a dead weight on the piston of his air pump." The air pressure on the piston would be opposing the weight of the piston. The idea has had legs; a fancy piston barometer was described by Indrik in 1959 (397). The cylinder was evacuated by a high-vacuum pump, most of the upward force of air on the piston was balanced by weights, and the remainder by a weak spring. (The accuracy was equivalent to that of the best mercury barometer, and the temperature correction smaller by a factor of 18. Unfortunately, the design is not suitable for use as a primary (calibrating) barometer because of the difficulty in exactly measuring the piston area.)
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Mercury barographs (Middleton Chap. 11) may record the column position intermittently or continuously. Hooke adapted his float-based wheel barometer into an intermittent barograph (the mechanism moved a punch). A float may also be used to guide the movement of pens vertically across paper carried by a rotating vertical axis drum, producing a continuous record, as in the Dines barograph (1904). Barographs have also been based on the mercury movement causing a shift in balance.
A crude photographic barograph was built the very year (1839) that Talbot published his photogenic drawing method. In the years that followed, natural lighting was replaced by artificial lighting, and the simple imaging of the shadow of the column with a sharper image created by a lens system.
Wheatstone's intermittent electric barograph (1845) lowered a pair of platinum wires into the short limb of a siphon barometer, and raised them again, driven by clockwork. The immersion created a circuit and the emersion broke it. When it broke, a hammer fell on carbon paper. The printing could be at a remote location if there was suitable intermediate wiring.
Aneroid barographs. The mechanism that moves the dial on the conventional aneroid barometer is readily adapted to moving a pen across paper on a rotating drum. The drum of course is moved at a constant time rate by clockwork and in Breguet's barograph (1867) it drives a separate clock face, too, so the barograph is also a clock (Middleton 426).
As you might expect, there have also been aneroid barographs in which the mechanical movement is converted to a transmittable electrical signal.
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Correction and Calibration. All mercury barometer readings must be corrected for temperature (which can cause expansion of the glass, the mercury, and of any air above the mercury), air pressure changes with altitude, and the change of local gravity with altitude and latitude (remember, the earth isn't quite spherical).
Errors can also be introduced by individual defects, e.g., in the vacuum, the purity of the mercury (Middleton 246), the variation in the size of the bore with length, and imperfections in or misalignments of the attached scale. A cistern barometer is also susceptible to the errors of capillarity (the mercury level is convex not flat) and capacity (a change in the mercury level in the tube causes a change in the level in the cistern, which then no longer corresponds with the scale zero) (EB11/Barometer). While the siphon barometer is free of these errors (the capillary depressions in the two limbs cancel each other out, and there is no cistern), "impurities are contracted by the mercury in the lower limb, which is usually in open contact with the air . . . ." All of these errors are addressed by periodic calibration.
Aneroid (elastic) barometers have some degree of internal temperature compensation, and do not need correction for gravity, but still need the correction for direct pressure change with altitude (NPL). That may make them sound more accurate than mercury barometers, but in fact at the time of RoF they were less so because of "irregularities in the elasticity of the metal of the pressure capsule, and those introduced by mechanical linkages" (Srivastava 59). And some older aneroid barometers (certainly the household models in Grantville) do not have temperature correction graphs. Typically, mercury barometers are used to calibrate aneroid barometers, not the other way around. However, aneroid barometers are very suitable for nautical use where the fragility of the mercury barometer is a serious issue.
It is possible to design a so-called primary barometer that can be used to calibrate an ordinary barometer. It's essentially a barometer of high accuracy (and weight and cost). The field barometer is placed alongside it, and the difference in reading is applied to the field barometer as a scale correction. It is then taken back to the field and need only be corrected for gravity (to mean sea level pressure—a one-time correction, unless it is moved) and temperature. Periodic recalibration is advisable.
Required Accuracy. In 1960, primary barometers had an accuracy of 0.001-0.005 mm (Brombacher 8). WMO in 2008 expected station barometers to have an accuracy of 0.1 hPa (0.075 mm or 0.003 inches mercury). For marine barometers, 0.5 hPa was tolerated.
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Barometers in Grantville. Next to the thermometer, the barometer is the meteorological instrument that is most likely to have been found in a Grantville home at the time of RoF. The barometer that David de Vries took with him to Suriname in fall 1633 "had once hung on the roof post of a Grantville porch" (Cooper, "Beyond the Line," in 1636: Seas of Fortune).
There are certainly aneroid barometers in Grantville; I am not sure about mercury-based ones. Attempts have been made to limit mercury use since the 1970s. The amount of mercury in a liquid barometer is typically 400-620 grams. In 2001 (one year after RoF), the total mercury in barometers sold in the United States was 353 pounds (NEWMOA). With 454 grams to the pound, if we say 500 grams per instrument, that's 321 instruments sold throughout the United States, so the chance of a brand-new one being in Gra
ntville is not high. Of course, there could be barometers in Grantville that were bought earlier, and even before mercury became a concern. But some old mercury barometers would have been disposed of and replaced by aneroid ones before RoF.
Frankly, it is more helpful that we have aneroid barometers, because the down-timers will quickly understand how to build a mercury barometer, even without a sample, whereas aneroid barometers are of a more complicated and unfamiliar nature.
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There are several references to the barometer in canon. What is more difficult to ascertain is when production of new barometers commenced. In Roesch & Hidaka, "The Things We Do for Love" (Grantville Gazette 46), Logan brings Sorrento an up-time barometer which she has calibrated against the mercury barometer in the physics lab. That suggests to me that her barometer is an aneroid barometer. She proposes in turn to use it to adjust the "down-time altimeters."
These altimeters almost certainly are judging altitude by measuring air pressure; i.e., they are really barometers. (It is possible to measure altitude by bouncing a radio wave off the ground, but that's more advanced than a barometer).
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Wind
I have grouped wind with pressure because, in essence, wind is air moving down a pressure gradient from high to low pressure. For wind meteorology, see Cooper, "Untying the Wind" (Grantville Gazette 35).
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Wind Direction
Wind direction is indicated by the wind vane, which is already known to the down-timers. In essence, a vane is pivotably mounted on a vertical shaft, balanced so there is equal weight, but unequal profile area, on either side of the shaft. For outdoor readings, directional indicators marking the compass directions surround the shaft. The vane should be observed from close to directly below the vane to minimize perspective errors.
Grantville Gazette, Volume 73 Page 13
