II SITUATION AND SOIL 35 
Frost is seldom very severe in England at the sea- 
side, but the salt spray and violent winds would 
render such a place generally undesirable, though 
good Roses are grown in some seaside localities. 
The old-fashioned saying is that ‘‘ frost falls.’ This 
is of course not true in itself, but it is true in effect. 
Heated air, being lighter, ascends; colder air, being 
heavier, descends; and it is found that frost is 
always most severe and dangerous in low-lying 
places, and that a covering overhead is a_ better 
protection than one at the side, because evaporation 
upwards towards the sky produces cold. My neigh- 
bour, a quarter of a mile off on a little hill, has 
always from three to five degrees less of frost than I 
have; and even if it were not so I believe that the 
same amount of frost would be more destructive to 
vegetation to me in a river valley than it would be 
to him on the upland. Valleys or low-lying ground, 
especially if near water, should therefore be avoided, 
and the uplands in all cases be preferred. 
Mere height above the sea-level would not, in 
most cases, be a matter of much moment; though 
on the one hand the top of a mountain would not of 
course be a desirable spot, and on the other a very 
flat plain with little height above the sea would 
probably be subject to severe frost; thus the flats of 
Cambridgeshire, which have such a slight fall to the 
sea, are well known as registering very low degrees 
of temperature. Rather high ground, not neces- 
sarily the top of a hill, with valleys in the neigh- 
bourhood for the cold air to fall to, would probably 
be a good situation as to comparative immunity 
from frost. In such a place the heavier or colder 
air literally drains away to the valleys, which thus 
D 2 
