IT IS a common irritant to us in Ireland to find those of our fellow countrymen who have excelled themselves, claimed as their own by the inhabitants of a neighbouring island. Thus Robert Boyle, the originator of Boyle's Law, is frequently described as an English physicist, despite the fact that he was born in Waterford. Other nations however, are more subtle they sequester, not the man, but merely his accomplishments.
Boyle's Law, you may remember, concerns the compressibility of a gas. It states that provided the temperature remains constant, the reduction in volume experienced by a gas is exactly proportional to the amount of extra pressure applied to it - or as Boyle himself put it in the context of the atmosphere: "There is a spring in the air we live in". Boyle's findings were published in France by one Edme Mariotte in 1672, with the result that the "spring in the air" is often described in French scientific literature as "Mariotte's Law".
Mariotte, however, had an interest in springs of any kind: indeed it was he who first gave a satisfactory explanation as to why, in certain places, fresh water sometimes gushes from the ground. Until this time nobody knew what caused springs, and when Mariotte noticed that they produced more water in wet weather than in dry, he suggested what seems obvious today that springs are fed by rainwater soaking through the ground.
Springs occur in areas of permeable rock. Some rocks like sandstone or limestone allow water to trickle through them: in the case of sandstone, for example, the rock has tiny gaps between the individual grains, while limestone contains a network of fine cracks through which the liquid passes. In either instance, rain water can seep down to considerable depths until it is stopped by a layer of impermeable rock below. As more and more water accumulates, it builds up inside the permeable layer until it is saturated like a sponge. Rock saturated in this way is called an aquifer, and the top of the aquifer is called the water table.
Now the water table is normally well below the ground, and its level rises and falls over a given area in a way that roughly corresponding to the ups and downs of the local landscape. Occasionally, however, a dip in the topography may cause the water table to be exposed at a particular spot - and there a spring occurs, of their nature most springs are in valleys or low places: the water is not defying gravity, but flowing upwards as a result of pressure from water within the aquifer at some higher level.