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 degrees or thereabouts, which is the temperature
of ordinary sitting-rooms in England. What, it may be asked, will be
the expense of such an apparatus for cooling the air of a
dwelling-house? We are informed that it will not be greater than that
usually paid for heating with fires in this country; and if so, the
expense cannot be considered a serious obstacle to the use of the
apparatus. In the case of barracks for soldiers, hospitals, and other
public establishments, the process will prove of such important
service, that the cost, even if greater than it is likely to be,
should present no obstacle to its application.