SPRING FROSTS 131 



perature raised above the danger point, but not raised sufficiently 

 to cause it to ascend to a high level. 



The relative effect of one large source of heat and of numerous 

 small sources in heating air at the lower levels, may be easily 

 established by burning in a room the same amount of oil, in 

 one case in one large lamp, in another in a number of small 

 ones: the relative efficiency of the latter in warming the air 

 throughout, instead of causing an accumulation of hot air at 

 the ceiling, is very marked. 



The practical application of such a principle appears to be 

 possible. The burning of 6J quarts of paraffin is capable of 

 raising the temperature of a layer of air 20 feet in depth over 

 an acre by 10 F. ; and that amount of oil would be burnt in 

 six hours in about 200 small lamps, each burning 6 c.c. of paraffin 

 per hour. 



There are, of course, many circumstances which render it 

 impossible to calculate what the heating effect of such flames 

 on the air would be in actual practice, and any calculations 

 must simply be taken as indicating possible results. The initial 

 cost of small tin lamps for such a purpose would not be large, 

 but the use of them might involve more labour than would be 

 practicable : a more feasible plan would be to have recourse to 

 coal gas, distributed through small pipes pricked with holes at 

 intervals. In some parts of the country electricity might be 

 available for the purpose. 



Investigations on the subject are being conducted at Woburn, 

 by making a survey of the relative temperature conditions 

 obtaining in different parts of the ground when radiation is 

 active, after which it will be possible to ascertain how these 

 conditions may be modified by various heating arrangements. 



The temperature of the air may sometimes be raised by natural 

 conditions, and the beneficial effect is then very apparent. It 

 has been shown that in the vicinity of a river, there is a zone 

 on either side of the water where damage from frost rarely 

 occurs, the relatively warm water heating the air above the 

 danger point ; similarly, in the neighbourhood of the sea, trees 

 hardly ever suffer from spring frosts. Artificial heating of the 

 air by means of irrigation, when a natural source of compara- 

 tively warm water is available, has also been adopted in Cali- 

 fornia for the protection of orange groves, and has been found 

 much more effectual than smudge fires all of which instances, 

 it may be remarked, afford evidence that moisture in the air 

 does not always increase the damage done by frost (cf. p. 127). 



