e 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 a