Meteorologists over the centuries have devised many clever methods of measuring humidity. The simplest way, for example, is to use a material which changes its physical characteristics as it absorbs or loses moisture.
Human hair has this property. In 1783, Horace de Saussure invented the hair hygrometer, which makes use of the fact that the length of a human hair varies with the dampness of the surrounding air: it shrinks by about 2.5 per cent of its length when removed from an atmosphere of 100 per cent humidity to one of zero moisture content. If one end of a bunch of hairs is anchored, and the other harnessed to a system of levers designed to move an indicating needle, you have an instrument which registers the humidity.
Other methods make use of the phenomenon that the electrical conductivity of some hygroscopic, or water-absorbing, materials varies with the amount of moisture they contain. Lithium chloride is an example of such a substance; if a known voltage is applied across a disc with a thin coating of lithium chloride, and the resultant current measured, its magnitude allows us to calculate the humidity of the surrounding air.
One of the cleverest devices for measuring humidity, however, was dreamed up by Richard Lovell Edgeworth, the eccentric father of Maria Edgeworth, authoress of Castle Rackrent and other minor classics. The toy, for such indeed it was, would "walk" along a level surface, and its rate of progress in a given time was a measure of the moisture content of the air.
The device consisted of a length of wood, cut crosswise to the grain, and fitted with four sharp metal "legs" inclined backwards like pikes' teeth. In humid conditions, the wood expanded slightly, and the front legs slid forward across the surface while the back legs kept their grip. As the air dried out again the wood shrank, and this time the front legs dug in while the tail was dragged along. Over a period, the "creature" moved progressively forward, propelled by rises and falls in the humidity, providing, as Edgeworth put it, "a rough indication of the comparative moisture of the air".
But the most accurate humidity measurements are achieved using a psychrometer. This comprises two thermometers side by side, one of which - the "wet bulb" - has its bulb enclosed in a "glove" of muslin dampened by distilled water. The dryer the surrounding air, the more evaporation that will occur, and evaporation from the "wet bulb" results in a drop in its temperature. The difference between the readings of the two thermometers, therefore, is a measure of humidity.