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