I shall make no apology for the introduction of this subject before the Geological and Polytechnic Society, as it is certain that the person who would discover any safer modes of ventilation, or even any additional precautions to those now in use, and should succeed in having them generally adopted, would be for ever conferring a lasting benefit upon the mass of our mining population. Nothing can be more simple than the principle upon which coal mines are ventilated. Atmospheric air descending by one shaft, called the downcast shaft, is made to circulate through the subterranean workings, and to ascend at another shaft, called the upcast shaft. The ascending current of air is rarefied and made specifically lighter than the descending current, whether by the men and animals in the pit, the natural temperature of the mine itself, or by a fire placed in the upcast shaft. And the efficiency of the ventilation of the mine will, ceteris paribus, be in proportion to the heat of the ascending current in the upcast pit. By the experiments of Gay Lussac, 100 parts of atmospheric air, heated from 32° to 212°, expand 137.5 parts; or 180° of heat expands it about one-third. The rate of expansion or rarefaction is not exactly equal for equal increments of heat, but on an average is 1/480 of its bulk for each degree of Fahrenheit. If heat be communicated to a particle of air, a change with respect to that particle takes place in the following ...