132 
of heat which a wet clay soil can never acquire, and this heat 
shows itself not only in the abundant and healthy growth of all 
plants grown in it, but it shows itself also in the power of 
resisting frost. People are apt to talk of a frost as if it was 
something that came down from the skies and caught hold of the 
earth, whereas the simple explanation of frozen ground is that the 
radiation of heat from the ground is not equal to the cold 
atmosphere above it ; and the depth to which frost penetrates is 
largely ‘determined by the amount of heat in the earth; and so 
wherever means can be found to keep in the heat which exists in 
the earth, and wherever the earth is so warm that it can retain its. 
heat in spite of all the cold air above, there frost does little or no. 
damage. This explains the difficulty which people who have been 
in the tropics often speak of, when they have seen beautiful 
tropical flowers growing in places where there are often sharp 
frosts ; the earth has been so roasted by the tropical sun that a 
few degrees of frost lasting a very short time have no effect on 
the plants. It explains, too, how the possibility of growing 
tender plants does not so much depend on latitude, for there are 
parts in the north where many things can be grown which we. 
cannot think of. At Castlewellan, in County Down, nearly four 
degrees north of us, Lord Annesly grows a marvellous collection 
of plants which we should consider too tender to attempt here, 
including a large number from Australia, New Zealand, and the 
Cape. When I tell you that his catalogue of hardy plants 
contains eleven species of New Zealand acacias and nineteen 
species of eucalyptus you will get some idea of what can be grown 
there. The garden is on a granite soil, and within the influence 
of the Gulf Stream, there very narrowed. It explains also why 
many things cannot be grown in England. There are parts of 
Sussex and Cornwall which are in the same latitude as parts of 
the champagne district of France, but their soil is not warmed by 
the sun of France, and so they cannot produce champagne. And 
this explains also the value of mulching of all sorts; any kind of 
sy 
o (ties 
