1935 Mack Model BM Statotherm Refrigerated Truck, "Dairylea Milk," September 1935

Mack Trucks introduced its Statotherm refrigeration system about 1933. It placed a refrigerant -- usually dry ice -- in a special compartment, while electric blowers controlled by a highly sensitive thermostat circulated cool air throughout the truck body. In a test run from Florida to New York, the Statotherm system kept a truck's interior at a near-constant 33 degrees Fahrenheit.