338 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY 
and nearly always saved it from frost. The theory is simply this, 
that any cover intercepts the re-emitted heat waves going from the 
ground to space. These are not exactly the same heat waves which 
were received during the day, being much longer, having approxi- 
mately wave-lengths of 0*012 millimetre. These longer waves are more 
easily trapped or intercepted. Any medium— cloth, paper, smoke, or 
floating ashes— that interferes with free radiation stops the outflow 
and loss of energy in the form of heat waves and is in this sense a 
protective frost cover. 
The best type of cover is a cloud ; and clouds, whether high or low, 
are good frost protectors. When we can make artificial clouds we 
can eliminate frost. 
It is an advantage to cover early ; indeed, an hour or two before 
rather than after sunset. Newspapers, cloth screens, fibre screens can 
be used. If soft material is employed, especially paper, there may be 
wetting, and so in the long run it will be found cheaper to employ 
weather-proof material. It is also important that the material used 
be properly fastened and kept in place. 
The method of direct heating has met with much success in the 
orange groves of California and elsewhere. Modern heating methods 
date from experiments begun by the writer in that State in 1895. 
The fuel originally used was coal ; later wood, straw, and brush were 
used, now however supplanted by crude oil or distillate. With modern 
orchard heaters, about thirty to the acre, the temperature can be main- 
tained 3 0 C. above freezing, thus preventing refrigeration of plant 
tissues. 
The method of utilizing the heat of higher levels by ventilation has 
not been commercially developed. The method of applying water 
either in spraying or by running ditches or flooding has proven entirely 
successful. The method of sanding, cleaning, and draining over bog- 
lands is effective. 
One other important consideration remains, and that is injur} 7 of 
plant tissues due to rapid rise in temperature after the frost. De- 
frosting if too quick may result in damage where no damage was effected 
by the low temperature. In this connexion water may be used to ad- 
vantage, for both water and water vapour have high specific heat as 
compared with air ; roughly, water has four times the capacity for 
heat that air has. It is essential that the restoration of plant juices 
and tissues to their normal state be accomplished gradually, neither 
too rapidly nor yet too slowly. There is probably an optimum 
temperature for thawing or defrosting frozen fruits and vegetables. 
Finally, temperature records need careful interpretation. The freezing- 
point of liquids under pressure in the plant cells or exposed at the 
stomata may be different from free air values. Air gains and loses 
heat chiefly by convection or translation. Losses by conduction are 
small. Plants gain or lose heat by radiation, convection, and conduc- 
tion of an internal character. Soil gains and loses heat by radiation 
and conduction. The loss of heat in the air stratum near the ground 
