3. By Expansion of Gases. - The atmosphere may be used as the medium by which freezing is effected. This depends on the following natural laws: - When air is compressed, considerable increase of temperature is made sensible, exactly proportioned to the work done in compressing. If, now, this heat be extracted when sensible, upon reduction of pressure and increase to normal volume, the air will be minus the amount of heat which has been abstracted from it by the water. In this way, by compression, cooling, and after re-expansion, intense cold is produced, quite accidentally, by the use of compressed air operating mining engines, the cold of the exhaust air being intense. This production of cold in the one machine is effected by a pump, alternately compressing and again allowing to expand a given quantity of air. When the air is compressed, and its heat is sensibly raised, its position in the machine is determined by a second non-conducting piston, which causes the air when hot and under compression to be always on the one side, and when cold and expanded to be always on the other. Upon thai side at which the heated air is always collected is a hollow cover, through which a constant stream of cold water is running in order to abstract the heat as it is rendered sensible.

On the other side to which the expanded and cold air is driven is another hollow chamber with large surface, through which is driven the ferine or other solution whose temperature it is required to reduce below freeziug-point. The compressed air - always the same quantity, but rising in density as the cold increases - thus acts as a carrier of the heat from the liquid to be frozen to the constant stream of cold water which carries it away. (Iron.)

By Gifiard's system on this principle, a machine using 18-horse power and burning 792 lb. coal in 10 hours, produces 1 ton of ice in this time. The ice thus costs 1/20d. per 2 1/4 lb. with coal at 23s. a ton. This price may be reduced to 1/2, since there are steam engines that do not burn more than 21/4 lb. of coal per horse per hour. The same size machine furnishes 2145 cub. ft. per hour of cold air at 32° F. (0° C.). (Chronique In-dustrielle.)

Bell and Coleman's apparatus is designed to overcome the difficulty encountered in the formation of particles of ice during the re-expansion. This is avoided by a more effectual cooling of the compressed air, and by subsequently treating the air so as to separate moisture from it, by subjecting it, before re-expansion, to an atmosphere cool enough to ensure the deposition of any remaining moisture that would be liable to freeze. (See Spons' Encyclopaedia, pp. 1019-20.)

4. By Mechanical Means, - It often happens in towns and where manufactories are crowded together, that the supply of water for condensing purposes is very small, and consequently that it attains an inconveniently high temperature under unfavourable conditions of weather, resulting in the deterioration of the vacuum and a consequent increase in the consumption of fuel. To remedy or to diminish this difficulty, Boase & Miller have brought out a water cooler, which consists of a revolving basket of wire gauze surrounding an inner stationary vessel pierced with numerous small holes, through which the heated water discharged by the air-pump finds its way into the basket, to be thrown out in the form of fine spray to a distance of 20 ft. at each side. The drops are received in a tank, and in their rapid passage through the air are sufficiently cooled to be again injected into the condenser. A cooler having a basket 3 ft. in diameter, making 300 revolutions per minute, and discharging into a tank 40 ft. square, requires 3 to 4 indicated horse-power to drive it, and will cool 300 gal. per minute.

The following decrease of temperature has been observed in actual practice: - Water entering at 95° F. fell 20° in temperature; water entering at 100°-110° fell 25°; and water entering 110°-120° fell 30°. The machine with which these trials were made was so placed that the top of the basket was 4 ft. from the surface of the water in the tank. With a greater elevation, better results can be obtained. The advantages claimed for the cooler are that by its means the temperature of the injection water can be reduced, the cost and size of cooling ponds can be diminished, and condensing engines can be employed where hitherto they have not been possible. The apparatus has been for 2 years in operation at several large factories, and there is every reason to believe that its use will extend, as it supplies a real want in a very simple and ingenious manner. Duncan Bros., of Dundee and 32 Queen Victoria Street, London, are the manufacturers.