Chamberss Edinburgh Journal, No. 430 | Page 4

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