TEMPERATURE FACTS 209 



than soil, and keeps the air above it too warm for frost. 

 Covering the marsh soil with an inch or two of sand also 

 keeps the ground warmer and prevents some frosts (Fig. 143). 

 The cost of these protective measures in orchards, fields, 

 marshes, etc., varies with circumstances but is often a tenth 

 or less of the value of the crop saved. The frost warnings 



FIG. 144. Frost fighting in a lemon grove. Firepots along path near 

 middle. In foreground an alarm thermometer, arranged to ring a bell in the 

 watchman's headquarters when temperature falls to danger point. 

 (F. A. Carpenter, in Monthly Weather Review.) 



issued by the Weather Bureau 12 to 20 hours, or more, in 

 advance are of great value (sometimes amounting to $100,000 

 for one frost in a single state) in enabling preparations to be 

 made before the cold arrives. Large orchards are often 

 equipped with a system of alarm thermometers (Fig. 144) 

 which are arranged to ring a bell in the watchman's quarters 

 when the temperature drops to the danger point. This en- 



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