Loading
M327-26 · Mercury
1941
Object Overview
Presented here is the Kew Marine Barometer — a high-precision marine mercury barometer of the Kew pattern, manufactured in 1941 by the renowned London company F. Darton & Co. Ltd., which specialised in the production of scientific, meteorological, and measuring instruments. The barometer is a professional instrument specifically designed for accurate atmospheric pressure measurements aboard seagoing vessels, under conditions of continuous rolling, pitching, vibration, and temperature fluctuations.
The name Kew refers to Kew Observatory in Great Britain, which played an important role in the development of methods for the standardisation and calibration of meteorological instruments. Barometers of this pattern became widely used in marine meteorology thanks to their combination of high accuracy, mechanical reliability, and special design features that reduced measurement errors under shipboard operating conditions.
brass, glass, mercury, silvered brass
Unlike ordinary station mercury barometers, which were designed for stationary installation, the Kew marine barometer constitutes a specialised measuring system. Its construction incorporates a specially designed barometric tube that prevents air bubbles from entering the vacuum space, an arrangement for reducing oscillations of the mercury column caused by the vessel's motion, a gimbal suspension, and devices for correcting readings for temperature, geographical latitude, and the height of the instrument above sea level.
The present example is constructed in the restrained, functional style characteristic of instruments of this type. Its design combines substantial brass components, a precision glass tube, a steel cistern, and carefully manufactured adjustment mechanisms. Of particular interest is the surviving Gold's Correction Slide Rule, manufactured by Short & Mason, which allows the necessary corrections to be determined directly on the instrument without consulting reference tables.
The Kew marine barometer consists of a vertical glass barometric tube and a lower mercury cistern enclosed within a robust protective metal housing. The overall arrangement of the instrument is governed by the requirements of measurement accuracy, resistance to mechanical stresses, and ease of maintenance under marine conditions.
The principal sensing element is a glass tube approximately 900 mm long, sealed at the upper end and open at the lower end. It is filled with mercury, and its lower open end is immersed in the mercury cistern. Above the upper surface of the mercury column is the Torricellian vacuum — a space containing gas at an extremely low residual pressure. The height of the mercury column is determined by the equilibrium between its weight and the atmospheric pressure acting on the free surface of the mercury in the cistern.
The tube passes through a special supporting assembly which also serves as the cistern lid and incorporates a sealing gland. The connection between the glass tube and the supporting assembly is secured by a cementing compound, ensuring reliable attachment and sealing of the corresponding components.
The cistern is manufactured from stainless steel and has an internal diameter of 50 mm and a depth of 35 mm. It screws onto the lid and is pressed against a leather washer, which performs both sealing and protective functions. Two small ventilation holes are provided in the lid to allow the space above the mercury to communicate with the surrounding atmospheric air. The air passes through the leather washer, which helps prevent dust and contaminants from entering the cistern.
The use of stainless steel for the cistern was dictated by the need to ensure chemical resistance and durability in a component permanently in contact with mercury. The compact shape of the cistern reduces the quantity of mercury required and the overall weight of the lower part of the barometer.
The construction of the barometric tube itself deserves particular attention. It is manufactured from lead glass with a carefully controlled internal diameter and consists of four functional sections joined together by glass fusion.
The enlarged upper section has an internal diameter of approximately 8 mm. This is where the upper surface of the mercury column, or meniscus, is located, and its position is used to determine atmospheric pressure. Glass blanks intended for this section were selected with particular care for the uniformity of their internal diameter, since any variations could affect the capillary correction and consequently the accuracy of the measurements.
Immediately below this section is a narrow capillary section with a uniform internal diameter of approximately 0.4 mm. Its principal purpose is to reduce the so-called pumping effect caused by the motion of the vessel. Under rapid vertical accelerations, the mercury tends to move along the tube, but the narrow capillary creates considerable hydraulic resistance and slows this movement. As a result, oscillations of the upper surface of the mercury column become less pronounced and the instrument's readings more stable.
The next component is a special air trap constructed on the principle of an inverted pipette. Its purpose is to retain air bubbles that may, under certain circumstances, enter the lower part of the barometric tube. Owing to the shape of the trap, bubbles tend to become lodged in its lateral enlargements and are prevented from reaching the upper vacuum space. This is extremely important because the entry of air into the Torricellian vacuum produces a systematic measurement error and interferes with the normal operation of the barometer.
Finally, the lower tail section has a reduced internal diameter to conserve mercury. Its external surface is cylindrical, since the displacement of the mercury levels in both the tube and the cistern is assumed to be proportional to changes in pressure. The geometry of the tube is designed with consideration for the relationship between changes in the mercury level inside the barometric tube and those in the cistern as atmospheric pressure varies.
Thus, each section of the glass tube performs a precisely defined function. Together, these features combine the high sensitivity of a mercury barometer with the stability required for operation at sea.
The barometric tube is enclosed within a long cylindrical brass housing, protecting it from accidental mechanical damage. At the same time, this protective housing serves as the supporting structure for the measuring scale and auxiliary mechanisms.
Inside the housing, the glass tube is held in place by a brass cap with a cork washer at the upper end, together with two felt rings surrounding the tube at different points along its length. These rings fit against the inner wall of the metal housing, centring the tube and preventing direct contact with the brass. In addition to securing the tube, the felt elements provide a degree of damping against minor vibrations.
The external finish of the instrument emphasises its professional purpose. The brass surfaces have a restrained finish, while the functional components are manufactured with considerable precision. The construction is almost entirely devoid of decorative elements: each component serves a specific technical purpose.
The upper part of the housing has the two previously mentioned longitudinal openings, allowing the mercury column and the movement of the vernier to be observed. The main barometric scale is engraved on the surface of this tube, to the left of the longitudinal viewing slots. It is graduated in millibars (mbar), a unit of atmospheric pressure widely used in twentieth-century meteorology.
The surface of the scale is silvered, while the graduations and numerical markings are blackened to provide the greatest possible contrast. This arrangement allows the readings to be taken confidently even in relatively poor lighting conditions.
The design also incorporates a cylindrical glass cover to provide additional protection for the scale. (On the present example, this component is missing, probably as a result of mechanical damage.) The protective glass cylinder is retained at the top by a substantial brass cap with a knurled ring and at the bottom by a brass collar.
High reading accuracy is achieved by means of a special vernier mounted on a brass cylinder that moves inside the protective housing. The position of the vernier is adjusted by a rack-and-pinion mechanism. The driving pinion is connected to a knurled adjustment knob located on the side of the housing, immediately below the protective glass cover.
When taking an observation, the operator turns the adjustment knob, moving the vernier until its reference edge is aligned with the upper surface of the mercury column. The reading is then taken from the main scale with reference to the additional graduations of the vernier.
The vernier is graduated so that ten of its divisions correspond to 39 divisions of the main millibar scale. This arrangement allows pressure to be read directly with a resolution of 0.1 mbar, while careful observation makes it possible to estimate intermediate positions to a reading precision of approximately 0.05 mbar.
Attached to the barometer housing is a brass correction plate specifying the standard conditions to which the observed readings must be reduced. The present example bears the inscription:
STANDARD CONDITIONS — 0°C — g = 980.665 cm/s²
This indicates a standard temperature of 0 °C and the standard acceleration due to gravity of 980.665 cm/s², equivalent to 9.80665 m/s². These parameters are used when reducing mercury barometer readings to the prescribed standard conditions.
The full graduated range of the scale extends from 870 to 1100 mbar. However, the effective operating range available for direct reading is limited to approximately 870–1060 mbar.
Such a wide range allows the instrument to be used under a great variety of meteorological conditions, including areas affected by intense cyclones with exceptionally low atmospheric pressure.
One of the most important features of the Kew marine barometer is its special gimbal suspension, designed to maintain the instrument in a vertical position during the rolling and pitching of a vessel.
Below the vernier adjustment mechanism is an assembly consisting of two concentric metal rings connected by pivot axes. The inner ring surrounds the cylindrical housing of the barometer and is attached to the outer ring by two screws positioned on opposite sides. The outer ring, in turn, is connected to the mounting bracket by another pair of screws, whose axis lies at right angles to that of the first pair.
This arrangement forms a two-axis gimbal suspension, allowing the barometer to move freely in two mutually perpendicular planes.
During rolling or pitching, the position of the vessel's hull and the bracket attached to it changes, but the barometer tends to maintain a vertical orientation under its own weight. This is particularly important for mercury instruments, since inclination of the barometric tube directly affects the measured height of the mercury column and consequently introduces errors into the readings.
The gimbal mechanism does not completely eliminate the effects of acceleration caused by the movement of the vessel, but it substantially reduces errors associated with its angular motion.
The mounting bracket provides two methods of attachment. It may be secured directly to a wooden mounting board by three screws, or fitted into a separate metal holder, which is then fastened to a wooden support or bulkhead by four screws.
Unlike the land-based station model, the Kew marine barometer has no upper suspension ring and is not designed to be hung from a hook attached to the upper end of the housing. Its normal installation is exclusively by means of the gimbal suspension, which provides the necessary freedom of movement.
A specially constructed, robust wooden case is provided for storing and transporting the marine barometer. It protects the delicate glass tube and the other components of the instrument against damage when the barometer is not in use.
Inside the case are two sets of annular rubber pads, together with end cushions made of soft sponge rubber. These components support the barometer in its transport position, prevent it from shifting, and reduce the transmission of mechanical shocks to the glass tube and cistern.
An additional clamp is provided in the upper part of the case to hold the barometer securely in its proper position. A metal holder is fitted in the lower part to accommodate the suspension arm.
This arrangement allows the barometer to be placed inside the case complete with its fitted gimbal suspension, without the need for dismantling.
The wooden transport case is an important component of the standard equipment supplied with the Kew marine barometer. It is intended not only for transportation but also for the safe storage of the instrument between observations.
Such protective equipment is particularly important for a mercury barometer, since damage to the glass tube may result not only in the loss of its measuring capabilities but also in a mercury spill.
The correct installation of a marine mercury barometer is of considerable importance for measurement accuracy.
On steamships, instruments of this type were generally installed, for practical reasons, in the chartroom or navigation room, where they were readily accessible to the officers responsible for navigation and meteorological observations. However, this location was not always ideal from the standpoint of measurement conditions.
The navigation room was often situated relatively high above the waterline, where the movement caused by rolling and pitching could be considerable. It could also experience noticeable temperature fluctuations due to solar heating, ventilation, or exposure to outside air.
The pumping effect of the mercury column, produced by vertical accelerations during the motion of the vessel, is minimised when the barometer is installed close to the vessel's centre of motion. On large ships, however, placing a meteorological instrument in this area was usually impractical or impossible.
Consequently, the choice of installation position required a reasonable compromise between accessibility and the stability of observation conditions.
It was recommended that the barometer be mounted in its gimbal suspension as close as practicable to the vessel's centre of buoyancy, in a position where it could swing freely without coming into contact with surrounding objects or being exposed to accidental blows from people passing nearby.
Particular attention was paid to temperature conditions. The installation location had to provide as uniform an air temperature as possible, since temperature changes affect both the density of mercury and the dimensions of the scale and glass tube.
The instrument was not to be placed near sources of strong or uneven heating, particularly near boilers, engine rooms, hot-air pipes, or other shipboard equipment capable of producing rapid temperature fluctuations.
If the barometer was kept in its protective case between observations, it had to be placed in its freely suspended operating position at least half an hour before the next reading. This allowed the mercury column to settle and the instrument to reach a condition suitable for accurate observation.
Thus, despite its specially developed marine construction, the correct use of the barometer required adherence to specific procedures and an understanding of the effects of ship movement and environmental conditions on the results of observations.
Gold's Correction Slide Rule, mounted on the lower part of the instrument's housing, is intended for the rapid determination of corrections to the observed atmospheric pressure.
Kew marine barometers issued by the India Meteorological Department (IMD), particularly its division in Pune, were commonly equipped with a special slide rule of this type.
The device was installed on the part of the housing where an attached thermometer would normally be found on an ordinary station barometer.
The purpose of the slide rule was to enable the observer to determine the necessary total correction directly on the instrument, without consulting separate tables. This was particularly convenient under marine conditions, where the speed of observation and the reduction of calculation work were of practical importance.
The present example is equipped with a correction slide rule manufactured by the well-known London company Short & Mason, which specialised in the production of meteorological instruments.
Structurally, the slide rule consists of a substantial brass base carrying a glass mercury thermometer, a movable slider with correction scales, and a separate sliding plate bearing the geographical latitude scale.
The base is manufactured from metal with carefully finished surfaces. It carries engraved scales and guides that ensure precise movement of the sliding components.
The thermometer is mounted vertically and is intended to measure the temperature required for calculating the correction. It is graduated on the Kelvin scale.
The movable slider carries the barometric correction scale and is operated by a rack-and-pinion mechanism. The movement is controlled by a small knurled knob located on the right-hand side of the slide rule.
An additional plate bearing the latitude scale moves within a longitudinal slot in the base. This scale is used in conjunction with the scale indicating the height of the instrument above the waterline.
The inscriptions LAT, FEET, HEIGHT ABOVE WATER LINE, and CORRECTION TO BAROMETER are clearly visible on the front, identifying the functions of the respective scales.
The system makes it possible to account for the principal factors involved in reducing mercury barometer readings to standard conditions: temperature, variations in gravitational acceleration with geographical latitude, and the height of the cistern above sea level.
A characteristic feature is the use of a thermometer graduated on the absolute Kelvin temperature scale rather than the familiar Celsius scale. On instruments of this kind, it was also referred to as the Absolute Scale and expressed in degrees of absolute temperature (°A). This scale has the same size of temperature interval as the Celsius scale, but its zero corresponds to absolute zero, approximately −273.15 °C. Thus, the markings 273, 285, and 293 on the thermometer correspond to approximately 0, 12, and 20 °C respectively. The use of absolute temperature was widespread in scientific meteorology because it allowed temperature values to be applied directly in physical formulas, particularly in calculations involving the state and density of air, without the need to convert positive and negative Celsius temperatures beforehand.
In the case of Gold's slide rule, the use of the absolute scale also had a particular practical significance associated with the historical system of temperature correction for Kew mercury barometers. Under the original British standard, known as the Old Convention, the scales of these barometers were calculated for a conventional standard temperature of 285 K, or approximately +12 °C. This temperature was taken into account in the construction of the correction slide rule, which combined corrections for temperature, geographical latitude, the height of the barometer above sea level, and the individual instrumental error. A new system, known as the New Convention, was subsequently introduced with a standard temperature of 273 K (0 °C), for which corresponding slide rules were produced, often graduated directly in degrees Celsius. Thus, the absolute temperature scale found on early Gold's slide rules represents not merely an unusual metrological feature but also evidence of the instrument's association with a particular historical system of standardisation in barometric measurements.
The installation of the correction slide rule increases the mass on one side of the barometer, which could disturb its balance in the gimbal suspension. To compensate for this effect, a special counterweight is mounted on the opposite side of the housing at the same level.
On the present example, the counterweight is also numbered — 2780. It takes the form of a substantial, elongated, dark-coloured component secured to the housing by metal clamps.
Thanks to this counterweight, the centre of gravity of the instrument remains in the required position, allowing the barometer to continue hanging vertically after the installation of the relatively heavy correction slide rule.
The presence of this balancing arrangement emphasises the care with which the marine barometer system was designed, including its auxiliary components.
The following markings are engraved on the lower part of the slide rule:
MB — MARK IVA — No. 822/37/51/61
The designation Mark IVa identifies a particular design variant of the correction device. In British scientific instrument making, the designations Mark I, Mark II, Mark III, Mark IV, Mark IVa, and similar forms belong to a system for classifying instrument types and modifications, which became particularly widespread within British government, military, and meteorological services. The term Mark identifies an approved design variant of an instrument rather than its accuracy class.
This system was widely used in the British armed forces and subsequently in related governmental technical organisations working to standardised specifications.
Roman numerals were traditionally employed:
A letter following the number usually indicates a design subvariant. The modification could concern the mechanism, housing, scale, materials, temperature-compensation system, or adaptation of the instrument to particular operating conditions.
There was no universal rule that Mark II had to be more accurate than Mark I. A new modification might have been introduced to reduce manufacturing costs, simplify maintenance, improve standardisation, or change the units of measurement.
It is also important to note that the numbering applied within a particular category of instruments. For example, a Mark II mercury station barometer has no direct connection with a Mark II aneroid barometer.
In Great Britain, various categories of barometric instruments existed for meteorological stations, seagoing vessels, airfields, field observations, and specialised measurements.
As the supply of government services evolved from individually manufactured laboratory instruments to serially produced equipment, the need for standardised construction became increasingly important.
Technical departments needed to know not only the manufacturer and serial number but also the precise design version of an instrument. This information determined:
Historically, the use of Mark designations was not restricted to the Meteorological Office. Similar classifications were employed by the British military and other governmental technical services, including those responsible for aviation and navigational instruments.
Another marking on the slide rule is a monogram composed of the superimposed letters M and O. This is the official monogram of the Meteorological Office, the British national meteorological service. The monogram indicates the association of the design or the particular instrument with the official meteorological service.
The study of these markings reveals an important transformation in British instrument making during the late nineteenth and first half of the twentieth centuries.
During the Victorian era, many high-precision scientific instruments were manufactured in relatively small quantities, with their construction largely determined by the individual manufacturer and the requirements of the customer.
With the development of national meteorological networks, this situation changed. Standardised instruments became necessary, designed for prolonged service, centralised maintenance, and the comparability of measurements obtained at different observing stations.
To determine the total correction to the barometer reading, the observer must first know the vessel's geographical latitude, the height of the cistern above the waterline, and the temperature indicated by the slide rule's thermometer.
The operator first adjusts the movable elements of the slide rule so that the vessel's geographical latitude, indicated on the LAT scale, is aligned with the corresponding height of the cistern above the waterline, expressed in feet on the HEIGHT ABOVE WATER LINE scale. This adjustment is made using the knurled control knob located on the right-hand side of the slide rule.
Once the required relationship between latitude and height has been established, the position of the mercury column in the thermometer is observed.
The total correction is read from the CORRECTION TO BAROMETER scale on the movable slider, opposite the upper surface of the mercury column in the thermometer.
The correction scale is divided into two sections distinguished by the colour of their markings.
If the indicated value falls on the red section of the scale, the correction is negative and must be subtracted from the observed, uncorrected barometer reading.
If the reading is taken from the black section of the scale, located below the zero mark, the correction is positive and must be added to the original reading.
The scale divisions correspond to 0.1 mbar, and the correction should be read to this precision.
The use of the correction slide rule allows the total correction to be determined with a deviation of approximately ±0.2 mbar compared with the value calculated from the corresponding reference tables.
For marine meteorological practice, this degree of accuracy was entirely adequate, particularly considering that the actual conditions of observation aboard a moving vessel imposed certain limitations on reading precision.
Gold's slide rule is a characteristic example of a mechanical calculating device designed to solve a specific practical problem. Through the predetermined relationships between its logarithmic scales, it transforms several successive mathematical operations into a simple procedure of aligning markings and reading the result.
Devices of this kind occupy an interesting place in the history of measuring technology, demonstrating methods of automating scientific calculations long before the advent of electronic calculators and digital meteorological systems.
The Kew Marine Barometer manufactured by F. Darton & Co. Ltd. in 1941 is a distinguished example of British high-precision meteorological instrument making during the Second World War. Its construction reflects many years of experience in the development of mercury barometers intended for professional observations under the demanding conditions of marine operation.
The specially designed glass tube with capillary damping and an air trap, the protected mercury cistern, the precise millibar scale with vernier, the gimbal suspension, and the system of temperature and gravitational corrections together form a complete measuring system in which every component serves the purpose of obtaining reliable atmospheric pressure readings.
The surviving Gold's Correction Slide Rule manufactured by Short & Mason, together with its original balancing counterweight, gives the present example particular historical and technical significance. These components illustrate not only the construction of the barometer itself but also the practical methods used to process meteorological observations aboard ships during the first half of the twentieth century.
The instrument is of considerable interest as a historical example of marine meteorology, precision mechanics, and British scientific instrument making.