C. on the average. On the 40 yd. following the cooling is from 3.4° C. in the first experiment, and from 3° C. in the second; it is therefore evidently the same. In consequence of the total refrigeration which increases from the first inlet to the last, the fresh cooled air is introduced into the same hall at different temperatures, an inconvenience in itself sufficiently important; to this may be added some other disadvantages.
As the quantity of air which passes through the first portion of the aqueduct, only 15 yd. in length, is more considerable than that which passes through the second, 40 yd. in length, it is certain that the refrigeration obtained in the first section arises in a very large proportion from the evaporation of the water, which penetrates by filtration through the masonry itself. The refrigeration being quite as much in proportion as that produced by evaporation alone, shares, in the case of the Necker Hospital, the inconvenience existing in the latter method. By channels of a closer kind, such inconveniences might be avoided, and refrigeration might be augmented by using instead of brickwork, metallic tubes of greater conducting power.
On examining the refreshing action of the system, it is easy to see that it is far from being proportionate to the low temperature obtained. Supposing the conduits to be perfectly impermeable, the new air introduced always retains a relative proportion of moisture according to weight. Breathing would, therefore, remain the same, but the relative moisture would augment with the lower temperature, while perspiration would diminish, and thus would partially compensate for the direct refrigeration produced by the lower temperature. On the other hand, the conduits once established, the new air would traverse it in all seasons; the effect would be to bring up the temperature to the average level of the year, with an intensity depending upon their extent and on the varying conducting power of the sides of the conduits. From October to April, the period when the exterior temperature is less than the annual average, it would warm the new air and aid in heating the areas above; but from April to October the contrary would take place.
In all parts of France, the mean temperature is less than that maintained in the ventilated hospital wards; the effect, therefore, would be to extend the time of heating to a period when the exterior air is sufficiently warm to retain, after refrigeration, the required temperature; thus it would ever be augmenting it more or less, according to the refrigera-tive power. As a matter of economy, no doubt there would be regained in winter what is lost in autumn and -spring. Any very considerable augmentation, which would be necessary with conduits having a great refrigera-tive power, would always be of serious inconvenience. Even in the midst of summer, sometimes very cold nights succeed very hot days; the cold night air, still colder in the conduits, would penetrate into the sick wards at too low a temperature,and, following tooquickly upon the heat of the day, might cause disastrous effects. The permanent cooling of day and night produced by conduits is, therefore, objectionable. This cause, combined with increase of relative moisture in the fresh air, enters greatly into the inconveniences of the summer ventilation of the Necker Hospital. Hence, instead of endeavouring to increase the cooling power of the subterranean conduits by injection of air, it would be better to reduce them in size, and to render them perfectly watertight.
On several occasions, attempts have been made to refresh the air by these means. At the Conservatoire des Arts et Metiers, fresh air has been pumped from cellars. In Paris it has been proposed to obtain fresh air from the catacombs; it is true that the temperature of these places is about 52° F.(ll° C), but such air is unhealthy, especially on account of the exhalations from the drains and tombs. Besides, such a system is very complicated.
General Morin has sought to show that it is impossible to depend on any one of these methods. He experimented upon that system by which it was attempted to cool the air just obtained by the ventilator, by passing it through a water-spray before its introduction. According to Morin, this only produced a lowering of the temperature some 2° C.; it would thus require the use of a very considerable volume of water and a strong motive power to produce an effect most disproportionate to the cost, if specially set up for the purpose. Morin has also condemned the process of passing the air by the sides of metallic casings containing cold water. It is efficacious, but requires the use of very extensive surfaces in proportion to the volume of air cooled, even when the water has previously been cooled by a refrigerating compound, the weight of which in lb. would be twice the number of cub. yd. of air to be cooled.
Such are the difficulties which have led General Morin to propose other methods more immediately imitating the ordinary phenomena of nature, and appearing to him the only methods applicable in all cases, and sufficient for ordinary requirements.
These methods belong to the second class spoken of; they are founded upon direct refrigeration. All previous heating of the air passages is here objected to, as well as of the chambers into which the air is admitted.
One of these methods is to assure a constant draught of air by numerous proportioned air-holes, thus requiring, according to General Morin, who has taken this view into favour, only easy conditions everywhere applicable and very inexpensive. The outlets should be calculated to admit of an entire replacement of the air every half-hour, and the usual swiftness of the issue may be calculated at 0.40-0*50 m. per second. The outlets should be iron-plated outside, so that the action of the sun by heating the plates should increase the draught, their height should be 9 ft. or more above the roofs. The inlets should be as numerous as possible, and open, if feasible, on that side of the orifice not exposed to the rays of the sun; their dimensions should be determined by the condition that air should not pass through them at a greater speed than 0.30-0.50 m. per second, and that the volume of air introduced should suffice to renew the atmosphere of the area at least every half-hour. Windows exposed to the rays of the sun should be furnished with closed blinds or shielded by external blinds, unless they happen to be in the form of a verandah, in which case they should be well watered and covered with linen cloth.