In general, from these fissures there is constantly issuing streams of gas, of a nature varying with the character of the minerals, but for the most part they are such as have been mentioned. In the mines of Great Britain, when the atmosphere above is much agitated, as by the prevalence of southerly winds, and more especially if the violence amounts to what is termed a storm, the gases pour out in prodigious quantities, making a rushing noise, and filling the pit and excavated parts. The pit then becomes so full as to interfere with the operations of the men, who are frequently, for their safety, obliged to retire. In this case the atmosphere is lightened, and the pressure it is constantly exerting on all bodies with which it comes in contact is diminished, and the consequence is, that the gases rush out, under the circumstances already mentioned. It is known, that, during the prevalence of stormy weather, the mercury in a barometer falls; it is for a like reason, the weight upon it being less. Not only the gases issue from their caverns when the air is thus lightened, but water contained in fissures in the floor or pavement of mines rises also, sometimes to the amount of an inch or two, and it is no uncommon thing to see the extrication of vapour from a little collection of water on the floor, such as takes place when water is boiling, a movement which it very much resembles. These facts shew that it is not improbable that pestilential vapours, ordinarily passing under the soil, may be extricated when fissures are present. It may happen that effluvia may be prevented from issuing even when fissures exist in the soil, from an increase in the weight of the atmosphere, and in this way may be explained the occasional disappearance of pestilence with a change of weather, not unfrequently remarked in some tropical countries. During the prevalence of strong north, north-east, and north-west winds, blowing with considerable violence the currents in mines are reversed—for, instead of gases issuing from the fissures and crannies, currents of atmospheric air pour into them. These currents may be felt with the hand, and the ear can detect the rushing sound; a flame applied to a fissure is immediately drawn in, shewing the direction of the current. These facts illustrate the influence which the state of the atmosphere has upon terrestrial vapours. As has been already observed, the exhalations from the soil obtain different names from the effects they are wont to produce. When they produce intermittent fever or ague, they are termed marsh miasms. When they produce the various forms of malignant fever, such as the yellow, the bilious fever of India, and the coast of Africa, simply pestilential effluvia—and when they induce general bad health and degeneracy of the inhabitants of a country, they are styled malaria, an Italian expression signifying bad air.