FROST PROTECTION. 339 



Two kinds of thermometers are in use, one depending 

 upon the contraction of a column of mercury, the other 

 depending upon the contraction of a piece of metal, a ther- 

 mostat in short. In fact, an ordinary thermostat, cost- 

 ing about two dollars and a half, makes an excellent sub- 

 stitute for a thermometer. When the mercury falls 

 or the metal contracts the circuit is closed and the 

 bell rings, warning the grower of approaching danger. 



Fig-. 75. Bolton alarm thermometers, showing thermometer, 

 wires, bell and battery. 



The thermometer must be adjusted so as to give sufficient 

 time to light fires or for covering the trees. Ordinarily, 

 if set or fixed at 30 degrees Fahr., sufficient warning will 

 be given. 



INJURIOUS TEMPERATURES. 



It is difficult to determine exactly the degree of cold 

 at which the different parts, trunk, branches, leaves and 

 fruit will be injured. The danger point is determined by 

 many factors, principally, however, by the state of dor- 

 mancy of the tree, the variety, the duration of the cold, 

 and by the weather conditions immediately preceding the 

 cold snap. If the tree has already been partially or wholly 



