The condensers used are similar to those already described in connection with the engines, and need no further comment. About the evaporators, it may be said that they are so constructed that they can be cleaned out easily as soon as the accumulation of salt and other matter renders the operation necessary. Usually one side is hinged, and provided with a number of bolts all round the edges which are quickly removed and replaced.
The United States Navy includes a ship, the Iris, whose sole duty is to supply the fleet she attends with plenty of fresh water. She was built in 1885 by Messrs. R. and W. Hawthorn, of Newcastle-on-Tyne, and measures 310 feet in length, 38 1 / 2 feet beam. For her size she has remarkable bunker capacity, and can accommodate nearly 2,500 tons of coal. Fore and aft are huge storage tanks to hold between them about 170,000 gallons of fresh water. Her stills can produce a maximum of 60,000 gallons a day. It has been reckoned that each ton of water distilled costs only 18 cents; or, stated otherwise, that 40 gallons cost one penny. At many ports fresh water costs three or four times this figure; and even when procured is of doubtful purity. During the Spanish-American War the Iris and a sister ship, the Rainbow, proved most useful.
REFRIGERATORS
Of late years the frozen-meat trade has increased by leaps and bounds. Australia, New Zealand, Argentina, Canada, and the United States send millions of pounds' worth of mutton and beef across the water every year to help feed the populations of England and Europe.
In past times the live animals were sent, to be either killed when disembarked or fatted up for the market. This practice was expensive, and attended by much suffering of the unfortunate creatures if bad weather knocked the vessel about.
Refrigerating machinery has altered the traffic most fundamentally. Not only can more meat be sent at lower rates, but the variety is increased; and many other substances than flesh are often found in the cold stores of a ship—butter and fruit being important items.
Certain steamship lines, such as the Shaw, Savill, and Albion—plying between England and Australasia—include vessels specially built for the transport of vast numbers of carcases. Upwards of a million carcases have been packed into the hull of a single ship and kept perfectly fresh during the long six weeks' voyage across the Equator.
Every passenger-carrying steamer is provided with refrigerating rooms for the storage of perishable provisions; and as the comfort of the passengers, not to say their luxury, is bound up with these compartments, it will be interesting to glance at the method employed for creating local frost amid surrounding heat.
The big principle underlying the refrigerator is this—that a liquid when turned into gas absorbs heat (thus, to convert water into steam you must feed it with heat from a fire), and that as soon as the gas loses a certain amount of its heat it reverts to liquid form.