take in a smaller quantity by weight, though the same by measure, of oxygen, the supporter of life; but if, in addition to the air being rarefied, it be also still further distended by the vapour of water being mixed with it, it is evident that a certain number of cubic inches by measure, or the lungs full, will contain a less weight of oxygen than ever; so little, indeed, that life can barely be supported; and we need not wonder at persons lying down almost powerless in the hot and damp atmosphere, and gasping for breath.  Hence we see that any method of cooling the air for Indians, instead of adding moisture, should rather take it out of the air, so as to make oxygen predominate as much as possible in the combined draught of oxygen, azote, and a certain quantity of the vapour of water, which will always be present; and hardly any plan could be more pernicious than the favourite though dreaded one by those who have watched its results—­of the wet mats.  Cold air—­that is, air in which the thermometer actually stands at a low reading—­by reason of its density, gives us oxygen, the food of the lungs, in a compressed and concentrated form; and men can accordingly do much work upon it.  But air which is merely cold to the feelings—­air in which the thermometer stands high, but which merely gives us one of the external sensations of coolness—­on being made by a punkah, or any other mere blowing machine, to move rapidly over our skin—­or on being charged with watery vapour, or on being contrasted with previous excessive heat—­such air must, nevertheless, be rarefied to the full extent indicated by the mercurial thermometer, and give us, therefore, our supply of vital oxygen in a very diluted form, and of a meagre, unsupporting, and unsatisfying consistence....  The sine qua non, therefore, for healthy and robust life in tropical countries, is air cold and dry—­cold to the thermometer and dry to the hygrometer; or, in other words, dense, and containing little else than the necessary oxygen and azote, and this supplied to a room, fresh and fresh, in a continual current.’

He next goes on to describe the principle of his new plan of cooling:—­’The method by which I propose to accomplish this consummation, so devoutly to be desired, is chiefly by taking advantage of the well-known property of air to rise in temperature on compression, and to fall on expansion.  If air of any temperature, high or low, be compressed with a certain force, the temperature will rise above what it was before, in a degree proportioned to the compression.  If the air be allowed immediately to escape from under the pressure, it will recover its original temperature, because the fall in heat, on air expanding from a certain pressure, is equal to the rise on its being compressed to the same; but if, while the air is in its compressed state, it be robbed of its acquired heat of compression, and then be allowed to escape, it will issue at a temperature as much below the original