RICHARD Lovell Edgeworth was a thinking sort of chap. Among the many inventions of the eccentric father of Maria Edgeworth, author of Castle Rackrent and other minor classics, was a singular device for measuring humidity. 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 pointed 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."
His, of course, was not the first device for measuring humidity. 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, then voila! - you have an instrument which registers humidity.
One could describe the effectiveness of such a device in the words of Dr Johnson: "It is like a dog walking on its hind legs: it is not done very well, but one is surprised to find that it is done at all."
The most accurate way to measure humidity is to use a psychrometer - two thermometers side by side, one of which has its bulb enclosed in a "glove" of muslin dampened by distilled water. Evaporation from the muslin results in a drop in the temperature of the "wet bulb"; the dryer the surrounding air, the greater the rate of evaporation, and the greater this drop in temperature, so that the difference between the readings of the two thermometers is a measure of the moisture content of the air.
And, finally, a slight correction. It appears that yesterday's Weather Eye cited an incorrect date for the recent tornado in Whitegate, Co Clare. Later information suggests that the whirling dervish in question arrived in town a week earlier than stated, in the early hours of Wednesday, October 16th.