268 The Principles of Fruit-growing 



cooling, it becomes very hard, and reduces the capacity of 

 the pot for future fiUings. The best oil for orchard-heat- 

 ing purposes is one of approximately 20° Baume test, con- 

 sidering the matter of cost as an item. A lighter oil up 

 to 32° Baum^ may be used, but being hghter, it will bum 

 more rapidly and reduce the effective burning time of the 

 orchard-heater." 



The equipment for orchard-heating comprises appara- 

 tus for determining weather conditions as well as devices 

 for making and regulating the fires. For a large plantation 

 in which heating is likely to be practised, there should be 

 thermometers and frost-alarm thermometers (psychrom- 

 eters). the heaters themselves, devices for igniting the 

 heaters, reservoirs for storing the oil (if oil is used), and 

 a wagon-tank for distributing the oil. 



Many types of oil-heaters are now available, some of 

 them with reservoirs for holding an extra quantity of oil. 

 O'Gara reports, however, that extensive experiments 

 have shown "that the plain sheet-iron pot, holding approx- 

 imately five quarts of fuel-oil is just as effective as the more 

 complicated and expensive types. This pot is inexpensive, 

 and, when made of No. 29 or 26 sheet-iron, will cost from 

 5 to 8 cents. In using a heater of such small size, the 

 number to the acre should not be less than 100, and 

 preferably more in districts in which periods of low tem- 

 perature continue more than four or five hours. Owing to 

 the cheapness of such pots, a very large number may be 

 used in preference to using a few expensive heaters of 

 the reservoir type. Many small fires give better results 

 than a few large ones, as the heat is better distributed 

 and convective air-currents are not produced. It must be 

 imderstood that any orchard-heating device which is in 

 any way complicated by dampers, cocks, valves and the 



