Tools Of The Trade: The Mercury Is Up

The barometer has become something of an antiquated and almost quaint instrument hung in family foyers and boat rental shops as a reminder of ye olde days of sailing. What with all the computer weather maps and so on, the humble barometer seems out of its league. In the great age of sail, things were very different. The barometer in her hey day was the weather computer.

The invention of the barometer is attributed to Italian scientist and sometime alchemist Evangelisto Torricelli around 1643. In fact a lot of people had opinions about what made the device – originally a 35 foot tube – work and figuring out what held water in the tube steady was what made the barometer possible. Great minds like Rene Descarte and Galileo Galilei got in on the act, finding out that it was not, as originally thought, a vacuum in the end of the tube that held the water up. In fact, it was the then unfathomable pressure of air.

Prior to the 1600’s, science believed that air had no weight. Not clearly understanding how high the sky really was, and the implications of changes in altitude, they simply assumed that because human’s felt nothing heavy bearing down on them from the sky, air must be weightless. In 1646, French scientist Blaise Pascall changed all that by switching the liquid in his tube – now a manageable 32 inches – to mercury and testing it’s steadiness at various heights on a climb up the mountain known as Puy de Dome in Auvergne, France. The mercury level dropped lower in the tube the higher up the mountain Pascal’s brother-in-law went.

By the 1790, Torricelli’s mercury barometer was in use at sea. It consisted of a glass tube 36 inches long. The top of the tube was open and filled with what was then known as quicksilver, refined mercury. On the lower end is a reservoir and the tube is generally mounted in a case of some kind (as with the circa 1790 examples above from the Musee des Art et Metiers in Paris). A scale is attached next to the tube for reading the mercury. Because a vacuum is formed at the top of the tube, the mercury will adjust to the atmospheric force exerted on the reservoir, thus giving a reading of air pressure. High pressure, logically, places more force on the reservoir and low pressure does the opposite.

Barometers have a reputation for being testy and there is good reason for this. Changes in temperature affect the fluidity of the mercury upon which the mechanism depends. Thus a barometer that was made in Ireland and sailed off to the Caribbean will not read the same in Clew Bay as it does in Nassau Port. The scale for reading the height of the mercury must be adjusted by the reader in such cases, so a clear knowledge of the behavior of the barometer is as important as knowing the scale. By the Napoleonic Wars, a thermometer was routinely mounted in the barometer case for ease of reference.

The scale itself is relatively simple once you get the hang of it. 1 atmosphere was equivalent to 29.9 inches and a mercury barometer would generally read between 28 and 31 inches. This gives you an idea of just how sensitive these “primitive” instruments were. For example, the average atmosphere in Seattle, Washington in 29.6, a relatively low number. As we know from watching the local weather, a “low” indicate clouds and precipitation. A high barometer may seem like a good thing but wind often slacks with what a landsman might call fine weather, and no sailor likes a dead calm.

In practice, seamen would take into account all signs that could lead to a good guess at upcoming weather. The barometer would be consulted but then adjusted to temperature and wind direction, which also affected its reading. The sky would be glanced at, the waves noted and yesterday’s weather offerings pondered. In the final analysis, nothing could match experience for accurate prediction of the weather.

Of course, like all technology, barometers improved with expert tinkering, leading to the barographs and aneroid barometers (those round kind that look rather like a clock) that we are familiar with in the modern era. It’s worth remembering, though, that the barometer was once a much coveted instrument, and even a symbol of status. Wealthier naval Captains (and ostentatiously successful pirate commanders) would have finely tuned and beautifully engraved barometers of brass or even silver hanging in their cabins. There they would be checked regularly, admired by guests dining aboard, and polished to gleaming brilliance by stewards or cabin boys.

As Torricelli put it, in poetic language with a hint of the nautical: “We live submerged at the bottom of an ocean of elementary air.” And his barometer proves him right.