The Derschau system consists, in the first place, of double roofs, painted white, with apertures between to counteract the rays of the sun. To cool the temperature of the carriages, Derschau places in them two cooling apparatus, consisting of two hollow columns in carved oak - Regnault's system. Each column communicates with the outer air by means of an air inlet in the form of a funnel placed beneath the carriage in the direction of the motion of the train, closed with very fine double gauze. As long as the exterior temperature does not exceed 71 1/2° F. (22° C), the air entering at the lower end of the column and emerging by orifices made under the capital, this apparatus offers a very useful means of ventilation by giving every hour more than three volumes of fresh air in an imperceptible manner. To render the advent of fresh air insensible, the capital is ornamented with hanging plumes. When the external heat is augmented, a metallic cylinder, of less diameter than the column, is fixed inside, and charged with ice and crystallised calcium chloride, in the proportion of 4 and 3 parts. In this case, all the windows being closed, it is possible to lower the temperature 6° below that of the exterior air.

It is possible even to attain a greater degree of coolness by modifying the composition of the refrigerating mixture and increasing the size of the apparatus; but then the moisture of the air is so much increased in the carriage as to render it disagreeable, especially when there is too great a difference between the interior and exterior temperature.

This new method of cooling railway carriages is certainly a step in advance, and superior to all the methods hitherto proposed for the purpose; but, as Der-schau himself very judiciously says, it is not to be supposed that it could be applied on a grand scale, in consequence of the expense, the difficulties attendant upon its regulation, and the constant loss of material. They have done better than this, however, in America. In that country, the railway cars are hermetically closed, there is only an opening left in the floor, beneath which is a tank of iced water; at the axles of the wheels a gearing is fixed, which puts a ventilator in motion; the air, passing through a grating, is cooled and purified by the dust being laid. As a refinement upon comfort, this ventilation takes place in winter also, the water then being heated.

During the last ten years new investigations have been made in the subject, not only in France, but in other countries, and especially in America, where contrivances are continually to be met with, the details of which would be worthy of notice. In France, favourable mention has been made of the apparatus of Piarron de Monde'sir. In 1870 this gentleman communicated to the Academy of Sciences a note in which he describes the system for which he had previously taken out a patent for 15 years. It is similar to that which he applied with some degree of success in the Champ de Mars in 1867. At the Exposition of 1867, says the author, during the days of extreme heat, the new air propelled by compressed jets of water through the wooden gratings in the central part of the Palace on the Champ de Mars, arrived clear of dust, and remarkably cooler. This method of preparing fresh air consists of the addition of a minute jet of water in the middle of the compressed motive air jet. The water is literally pulverised by the compressed air, the dust carried off falls into the trough which receives the overplus of the water jet, and in consequence of the intimate mixture of the pulverised water and the air, the latter is at once brought to a temperature as low as the outside air is high.

Monde'sir goes further, and proposes to apply his system to hospitals. In that which concerns the vitiated air of hospitals, he observes, a jet of compressed air being placed at the base of each ventilating chimney, it would be sufficient to replace the water by a disinfecting fluid in order to obtain a mechanical mixture similar to that just described for fresh air. The organic particles of the vitiated air would probably be precipitated in like manner to the air dust, and received in the trough. The intimate mixture of this disinfecting vapour, and the vitiated air carried off by the outlet chimney, would also guarantee a complete purification of that air.

The Mondesir arrangements have been applied at the establishment of La Belle Jardiniere; they have cost much money, and yield very mediocre results, as it is easy to perceive by a visit to that establishment on a hot day in June. Other applications have been tried in factories. His apparatus has not only been tested in domestic economy, but in brewing. The brewers, however, were no more satisfied than the authorities of La Belle Jardiniere. It must be admitted, however, that the system was ingeniously conceived. It has done good service, and will do so again.

Tellier discovered, by combining the known laws of nature with the properties of ammonia, a plan, the advantages of which were soon recognised. It is known that in vacuo liquids instantaneously give off vapours, reaching their maximum of tension at once. On the other hand, it is also known that if two intercommunicating receivers are maintained at unequal temperatures, there is always vaporisation going on in the colder receiver. The following application is the result of a combination of these two laws with the properties of ammonia. The apparatus represented in Fig. 14 is intended to produce a cooling of the air. It is composed of a chimney a, the height of which is variable; at the top of that chimney is vertically placed the tubular generator b, containing a solution of liquefied ammonia to the line c. This perfectly isolated receiver is in direct communication with the serpentine condenser d by the two pipes e and f - the receiver d is also perfectly isolated. Around the serpentine circulates well-water. No matter what the temperature may be outside the apparatus, it is evident that the interior pressure would be superior to that of the atmosphere; the ammonia would therefore vaporise as well in the chamber cc gg as in the tube f.