Spacing wind machine-heater units close enough together to allow an overlapping effect offers more efficient frost protection than is possible with single isolated installations. A spacing interval of 300 feet is probably the optimum for most conditions, according to last year's tests with nine ground-level revolving units, producing 60 pounds thrust and burning 20 gallons of diesel fuel per hour.

Ralph Parks, Robert Parsons, and Herbert Schultz, Department of Agricultural Engineering, Davis, assisted in these studies.

The wind machine-heater units and fuel were provided by Robinson Blower and Engineering Corporation; a Ford tractor used in the experiments was loaned by Pacific Tractor Co.; and the fork lift attachment was loaned by Blackwelder Manufacturing Co.