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СКАЧАТЬ giving judgment. Here the vicious habit of thoughtless pronouncing is doubly bad, as it involves also a kind of flattery.

      There are some novel doctrines and theories, which seem doomed to meet with prejudice and opposition, but which yet must have some vitality about them, seeing that they survive so much ill-treatment. It is curious to observe how little regard to the rules of reasoning is usually felt to be necessary in opposing these theories—how mere pronouncing comes to stand in their case in the stead of evidence and argument. Although they may have been brought forward as mere forms of possible truth—ideal points round which to rally the scattered forces of investigation—and only advanced as far as facts would go, and no further—you will find them denounced as visions, tending to the breach of the philosophic peace; while, on the other hand, those who oppose them, albeit on no sort of ground but a mere pronunciation of contrary opinion, obtain all the credit due to the genuine philosopher. Abstractly, it would be generally admitted that any doctrine for which a certain amount of evidence is shewn, can only be overthrown by a superior force of evidence on the other side. But practically this is of no avail. Doubt and denial are so important to philosophy, and confer such an air of superior wisdom, that merely to doubt and deny will be pretty sure to carry both the educated and the uneducated vulgar. To get a high character in that position is of course very easy. Little more than pronouncing is required. As to the respective positions of the affirmer and denier in some future time, when truth has attained the power of asserting her reign against prejudice, that is another thing.

      To return to the general question—If any one be impressed by our remarks with a sense of the absurdity of pronouncing without knowledge and reflection, let him endeavour to avoid it, and he will confer a sensible benefit on society. When next he is in company, and a subject occurs to tempt him into an expression of opinion, let him pause a moment, and say to himself: 'Now, do I know anything about it—or if I know something, do I know enough—to enable me to speak without fear of being contradicted? Have I ever given it any serious reflection? Am I sure that I have an opinion about it at all? Am I sure that I entertain no prejudice on the point?' Were every one of us children of British freedom to take these precautions, there would be more power amongst us to pronounce wisely. There would be a more vigorous and healthful public opinion, and the amenity, as well as instructiveness of private society would be much increased.

      COOLING THE AIR OF ROOMS IN HOT CLIMATES

      In our last number, allusion was made to a process for cooling the air of apartments in hot climates, with a view to health and comfort. The intolerable heat of the climate in India, during certain hours of the day, is well known to be the cause of much bad health among European settlers. By way of rendering the air at all endurable, the plan of agitating it with punkahs, hung to the roofs of apartments, the punkahs being moved by servants in attendance for the purpose, is adopted. Another plan of communicating a sensation of coolness, is to hang wet mats in the open windows. But by neither of these expedients is the end in view satisfactorily gained. Both are nothing else than make-shifts.

      The new process of cooling now to be described, is founded on a scientific principle, certain and satisfactory in its operation, provided it be reduced to practice in a simple manner. The discoverer is Professor Piazzi Smyth, who has presented a minute account of it in a paper in the Practical Mechanic's Journal for October 1850, and also separately in a pamphlet. We invite public attention to this curious but simple invention, of which we shall proceed to present a few principles from the pamphlet just referred to.

      Mr Smyth first speaks of the uselessness of the punkah, and the danger of the wet mats. 'The wet mats in the windows for the wind to blow through, cannot be employed but when the air is dry as well as hot, and even then are most unhealthy, for although the air may feel dry to the skin, there is generally far more moisture in it than in our own climate; but the height of the temperature increasing the capacity of the air for moisture, makes that air at 80 degrees feel very dry, which at 40 degrees would be very damp. Now, one of the reasons of the lassitude felt in warm climates is, that the air expanding with the heat, while the lungs remain of the same capacity, they must 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 quâ 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 one, as it rose above it on compression. Thus the air, being at 90 degrees, will rise, if compressed to a certain quantity, to 120 degrees; if it be kept in this compressed and confined state until all the extra 30 degrees of heat have been conveyed away by radiation and conduction, and the air be then allowed to escape, it will be found, on issuing, to be of 60 degrees of temperature. If a cooler be formed by a pipe under water, and air be forced in under a given compression at one end, and be made to pass along to the other, it may thereby, if the cooler be sufficiently extensive, be robbed of all its heat of compression; and if the apparatus is so arranged, as it easily may be, that at every stroke of the pump forcing in air at one end of the pipe, an equivalent quantity of the cooled compressed air escape from under a loaded valve at the other, there will be an intermittent stream of cooled air produced thereby, of 60 degrees Fahrenheit, in an atmosphere of 90 degrees, which may be led away in a pipe to the room desired to be cooled.'

      The only difficulty to be encountered consists in the erection and working of machinery. There can be little fear on this score. We have no doubt that any London engine-maker would hit off the whole scheme of an air-cooling machine in half an hour. What is wanted is a forcing-pump wrought by a one horse or two bullock-power. This being erected and wrought outside of a dwelling, the air will be forced into a convolution of pipe passing through a tank of water, like the worm of a still, and will issue by a check-valve at every stroke of the piston into the apartments to be cooled. Properly arranged, and with a suitable supply of water trickling through the tank, air at 90 degrees will be reduced to 60 СКАЧАТЬ