Some measurement was undertaken for purely economic reasons. Nation states fighting for their positions on the globe needed revenue to support their armies and navies. By the mid-seventeenth century both England and France were forced to set up a system of indirect taxes. Customs and Excise provided nearly all the indirect tax supporting these exploits.
In England the Excise taxed drink, meat, salt, alum, ammonia, hats, silk and woollen textiles. By 1733 Excise produced 55% of all revenue and was the largest government department. Excise on alcoholic drink was always the most important, raising the most money for the national coffers.
The Excise became known as the monster with a thousand eyes. All shopkeepers and dealers had to register and were inspected. Weights and measures had to be supplied for the Excise men at the owner’s expense. Excise men became a ubiquitous presence in the eighteenth century.
While simple weighing and measuring procedures were sufficient for most produce, calculating the quantity of liquor in a barrel was not easy. Barrels were not simple mathematical shapes but had to be approximated. The problem became worse if the barrel was not full. Fortunately the slide rule could reduce these complex calculations to routine processes. Invented by William Oughtred in 1632, the slide rule was taken up by carpenters and excisemen during the 17th century. An Excise officer, Thomas Everard, proposed his sliding rule in 1684, which was followed by Charles Leadbetter's modifications in 1739. By 1700 there were over 1000 gaugers in England, all equipped with seven instruments and by 1780 there were 2800. They were the largest group of mathematically literate men in the Kingdom.
Quality as well as quantity needed to be measured as the rate of tax varied on different liquors. It was necessary to know if the spirit was ‘proof’, i.e. with the correct alcohol content, ‘below proof’, i.e. with too high a water content or ‘over proof’ with too high an alcohol content.
As the financial importance of the strength increased the older tests such as flammability or tasting were no longer regarded as sufficient. As alcohol is lighter than water, the specific gravity of the liquor became the crucial factor. The crudest method of testing specific gravity was the use of glass bubbles which would float or sink at particular specific gravities. These were particularly popular in Scotland.
In 1746 John Clarke published The Hydrometer or Brandy Prover which described his instrument for showing the strength of liquor by flotation. It eventually had full legal recognition in 1787. In the last decade of the eighteenth century the Royal Society set up a series of experiments to determine the best method of finding the alcohol content of liquids using a very accurate balance by the instrument maker Jesse Ramsden. They eventually endorsed a system by Bartholemew Sikes whose hydrometers were more accurate than Clarke’s. This method became the mainstay of alcoholometry for the next century.