Climata I Influences. 567 



The northern isothermal line of a mean annual temperature 

 of 50 enters 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 American 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 50 is much 

 more uniform, as it passes through no broad expanses of land. 

 Roughly speaking, we may put the latitude at 45 S. 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 Fahr. 



