Chimatal Influences. 567 
The northern isothermal line of a mean annual temperature 
of 50° entcrs England on the eastern side in 51° 51’ N. lat., 
and proceeding to the north-west, leaves it in about 53° N. lat., 
passing through the centre of Ireland, and onwards nearly in the 
same latitude, across the Atlantic, to about 40° W. long. From 
this point it forms a gradual curve southwards, until it reaches 
the Ainerican coast, in about 42° or 43° N, lat. ; thence onwards, 
leaving the lake district to the north, in about 41° 50’ N. lat., 
and then again taking a northerly direction until it reaches 
50° on the western side of the continent. In the Pacific it 
stretches still farther to the north, reaching 55° N. lat. in 160° 
W. long. Returning to England, and following the line east- 
ward, we find it touches the continent of Europe on the coast 
of the Netherlands, turning gradually to the south as we 
travel into the interior, along the north shore of the Black Sea, 
across the Caspian in about 45° N. lat., through Tartary, Mon- 
golia, Mantchouria, and the Japanese Islands, in about 43° N. 
lat. In the centre of the American and Asiatic continents, 
the high summer temperature brings the mean annual of 50° 
much farther north in proportion to the cold of winter as com- 
pared with the climate of Britain, But although plants grow- 
ing a considerable distance south of the mean annual of 50° 
in these countries are hardy with us, they do not ripen their 
seeds, and some shrubs and trees rarely flower. The two most 
interesting regions on the 50° line to the gardener are the 
western coast of America and the eastern coast of Asia in 
North China and Japan, for from these countries we get a large 
number of plants chiefly hardy in the south-west, but requiring 
protection more or less in other parts of the British Isles. 
The course of the southern isothermal line of 56° is much 
inore uniform, as it passes through no broad expanses of kand. 
Roughly speaking, we may put the latitude at 45° 8. It in- 
cludes the southern part of Patagonia in South America, and 
the extreme south of the middle island of New Zealand, a very 
small tract of country indeed, in comparison with that tra- 
versed by the northern line of the same annual temperature. 
And as these countries, from the same causes as our own, have 
a high winter mean, they offer few plants that can be suc- 
cessfully cultivated in the open air in Britain. 
The mean of 40° for January passes through the centre of 
Britain, and in both Asia and America it deflects southward to 
39°N. lat., or about 3° farther than the mean annual of 50° Fabr. 
