Page:Popular Science Monthly Volume 39.djvu/39

Rh supply to the consumers in our large cities. The provisions, when brought into these buildings, have the temperature prevailing outside, and warm the air that comes in contact with them. This air rises into a loft, where it comes in contact with pipes containing cold brine, becomes chilled, and descends through flues to the room below, entering it near the floor. This circulation goes on until the provisions have been cooled down to the temperature of the room. The air may be cooled, also, without the use of brine, by letting it come in contact with the coils in which the ammonia expands. Air has also been used direct for the production of cold by compressing it. Like condensed ammonia, it takes up much heat in expanding to its ordinary volume, but this system is not economical. In Fig. 5 a somewhat different arrangement is represented. Where there is not space for the loft, the expansion coils may be placed in the same room with the provisions. Before refrigerating machines came into use, refrigeration on the large scale had been tried with ice, and had failed. This was owing to the dampness imparted to the air by the melting ice. The brine or ammonia coils not only do not add any moisture to the air, but even withdraw a great deal that it naturally contains. This moisture becomes condensed on the pipes as the air circulates around them, and makes itself visible as a gleaming white coating of hoar-frost. On board steamers, machines are employed both to preserve dressed meat and to prevent live cattle transported through tropical regions from dying of the heat in their confined quarters. Machines of moderate size also find application in hotels—two of the recently built houses in New York have them—in dairies, chocolate factories, and they are used also in making stearin and margarin, in rectifying alcohol, extracting paraffin from petroleum, etc. A machine of the size represented in Fig. 2 will produce a refrigerating effect equal to that obtained by the consumption of two hundred and twenty tons of ice a day, or it will make one hundred and thirty tons of solid ice daily. The company that makes this style of machine is now building one of three hundred tons refrigerating capacity, which will be the largest in the world. But that is soon to be exceeded, as the contract is already made for a five-hundred-ton refrigerating machine.

Artificial refrigeration has also been applied to sinking shafts and driving tunnels through quicksand and loose wet gravel. These materials wash into an excavation as fast as they are removed, and in many cases progress through them is next to impossible by ordinary methods. The difficulty is overcome by freezing the loose soil around or in front of the work. This process was first used by a German mining engineer in 1883. In sinking a shaft, pipes of about eight inches diameter are driven