Brass hypsometer

Classic Projects: Pressure hypsometer

Image credit: Getty Images

Date: Mid-19th century Manufacturer: Cary of London Unit cost: Pick one up on eBay for c. £40

Today, there are two types of measuring instrument called the hypsometer. More recently, there is a digital clinometer used for calculating gradients or the height of an object, using the principles of triangulation. Yet the original version – the ‘pressure hypsometer’ – was a mechanical device evolved to help 19th-century map-makers and surveyors to calculate altitude.

In the pre-digital world, the hypsometer was a breakthrough gadget, employing the principle of variation in the boiling point of water with changes in altitude. The higher you go up a mountain, the lower the boiling point due to the changing atmospheric pressure. “If you have a set of pre-defined tables,” says Eugene Rae, librarian at the Royal Geographical Society and an authority on Victorian instruments for test and measurement, “you can measure the boiling point of water with a mercury thermometer, cross-refer the temperature reading to the tables and arrive at an accurate altitude reading.”

Rae goes on to explain how 19th-century pundits (in the sense of ‘indigenous surveyor’) used the principle while secretly mapping northern India and the North-West Frontier Province by recording the temperature of their tea out in the field. These readings were converted into ‘spot heights’ and contour lines on survey maps.

This ingenuity led to a portable telescopic instrument called the hypsometer. “They were usually made of brass,” says Rae, “where the bottom of the instrument has a space where you can put a small spirit burner.” Above the burner is a water reservoir, above which are cylindrical chambers wide enough for a mercury thermometer that reads the temperature of the steam. The problem was that altitude calculations were only as good as the integrity of the thermometer, which gives the hypsometer its second function.

Reversing the principle by taking temperature readings at confirmed heights drawn from previous maps, the user was able to calibrate the thermometer and, where appropriate, rescale it. The hypsometer can be thought of as one of the early industrial calibration instruments. Before explorers went into the field to conduct surveys, their thermometers were certified at a laboratory at Kew. Despite this, Rae says, “the thermometer was always the weak point in the process”.

The reason for this weak point in the chain was that “while the hypsometer itself was virtually indestructible, thermometers are easily broken. That is why the more robust aneroid barometer came to supersede the mercury thermometer in altitude calculations.”

Back in the 19th century the stated ambition of the Royal Geographical Society was “to collect new scientific data from around the world and then disseminate it to a wider audience”. This data, and the maps that were drawn using it, became vital documents for colonial administrators. “But the explorers weren’t just mapping: they were noticing coal deposits and the potential for precious metals. Explorers such as George Hayward, who mapped the Hindu Kush, were using instruments like the hypsometer.”

Rae has delved into the collection of artefacts in the Society (including a hypsometrical apparatus and thermometer used by Dr Livingstone) and says that it’s not possible to come up with a specific name for the designer: “By and large, the hypsometers in our collection were made by a Victorian engineering company called Cary of London, known for making thermometers and compasses and all kinds of small instruments.”

Pressure hypsometer facts

‘Hypsometer’ comes from the Greek, meaning ‘measurement of height’

Originally used for mapping northern India in the 19th century

Precursor of the digital hypsometer

Superseded by the aneroid barometer

Used by Dr Livingstone in Africa

Early instrument for calibrating mercury thermometers

Diagram annotations

  1. Mercury thermometer
  2. Spirit burner
  3. Water reservoir
  4. Telescopic outer casing (brass)
  5. Steam escape vent

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