It is a simple matter to make a machine which will alternately compress, cool, and expand air in such a way as to produce a lowered temperature. Such a machine is indicated diagrammatically in Figure 79. There are two cylinders, _A_ and _B_, and a condenser at _C_. When the piston _a_ in cylinder _A_ descends it compresses the air in the cylinder; this air flows into the condenser _C_. There is a coil of pipe in this condenser through which water circulates. This carries off the heat of compression and then a valve is opened which permits the cooler air to pass off into cylinder _B_. As the air expands in this cylinder it becomes chilled. This chilled air is then forced out of cylinder _B_ by means of piston _b_ and flows into the refrigerator or cold storage room _D_. As the air is liable to take up moisture and to introduce objectionable vapors from oil used to lubricate the pumps, it is usually confined in pipes in the refrigerator and then returned to the cylinder _A_. [Illustration: COMPOSITOR AT WORK ON A LINOTYPE MACHINE] [Illustration: THE OPTOPHONE An instrument which enables the blind to read common print] [Illustration: A BLIND MAN READING WITH HIS EARS] This is a type of a refrigerating machine that is used very extensively on ships for chilling perishable foods. However, air has only a very low capacity for heat, and in order to obtain an appreciable amount of refrigeration very large volumes of air must be handled. This means that cold-air machines must be very large and bulky. The efficiency of such machines is low, but they find favor on shipboard because there are no inflammable or poisonous gases to be dealt with. In the standard machine of the United States Navy air is compressed to 260 pounds; and in the expansion is raised to 60 pounds pressure, which is enough to reduce the temperature to between 70 to 90 degrees below zero. LATENT HEAT