PRESERVATION OF CROPS FROM INJURY BY FROST. 337 
circulation is the slow drift of overspreading warm air toward the 
slope. 
The forecaster's great aids are cloudiness and wind. If the observa- 
tions show conditions favouring cloud-building, which means strong 
convectional currents, there is little danger of frost. The cloud cover 
is Nature's own way of preventing frost. Even a fine veil of upper 
cloud will suffice. 
In the main, frosts occur when an anti-cyclone stagnates, if we may 
so express it. The circulation at the surface is certainly sluggish for a 
period of forty-eight or seventy- two hours. This is why frosts recur, 
with the minimum temperature getting lower on successive nights. 
The rapid dying out of convectional currents before sunset, a low 
percentage of saturation, and a tendency of the dew-point to go lower 
are all favourable for frost. 
II. 
The problem of protection, from the agriculturist's point of view, 
will now be considered, it being assumed that forecasts of frost have 
been received. Forewarned is forearmed ! But not in this case, 
unless the grower has prepared a supply of fuel, covering material, 
water and sand, or fine ash, preferably wood-ash. It will not do to 
wait until frost comes. 
First, how shall we prevent cooling near the ground ? The easiest 
way is to conserve the earth's heat by covering the plants with cloth, 
paper, straw, or by a suspended cover of dense smoke, generally called 
a smudge, and most easily produced by sprinkling water on small 
fires made of brushwood. This causes a thick heavy smoke, and all 
things considered is the cheapest and most effective protection against 
frost. 
Another method is to apply heat directly in the shape of open small 
fires or fuel in fire-baskets, or by the so-called orchard heaters, which 
are essentially metal containers holding less than a gallon of oil and 
arranged for slow burning. Large open fires are not effective, because 
they warm the higher levels where the heat is not needed. Indeed, 
the problem really consists in displacing or heating a comparatively 
shallow stratum of air close to the ground. There is therefore no gain 
in burning bonfires unless in some way a circulation can be established 
and the warmer upper air brought down. Unfortunately in most 
instances this is not the case, and the surface cold air is simply re- 
placed by other cold air. The slight gain due to mixing and motion 
is incommensurate with the fuel used. 
Another method would be to mix the air, establishing a circulation 
by blowers or windmills ; and still another method would be the use 
of substances with high specific heat, which had previously been heated. 
Such agencies would be water, sand, and wood-ash. 
The first method, that of covering, was practised on a small scale by 
our grandmothers, who covered at sundown their favourite rose-bush 
