mean temperature falls as we ascend the hills at the rate of 1° for every 100 yards. For the interior of the country, from London northward to Edinburgh, we may take from 47° to 49° as a fair average, 47° for the South of Scotland, 48° for the North of England, and 49° for the Midland counties: the difference between summer and winter being usually from 18° to 20°. On the sea coast the difference between summer and winter is smaller than in the interior, and the mean temperature generally a shade higher; but in our table this last character, as between By well and Shields, does not hold good. We will take then the annual average of our Lower zone at from 48° to 45°, of the Middle zone at from 45° to 42°, and of the Upper one at 40° or 41°.
The great characteristic of the climate of Britain, as compared with that of other parts of the North Temperate zone, is its equability, the absence of extreme degrees of either heat or cold, and the consequent smallness of the difference there is between winter and summer. We shall see shortly what an important influence this has upon the distribution of plants. Even within the compass of England we can trace palpably the influence of the sea in cutting off the extremes in both directions. Of course, it is on the west side of the island that this influence operates most powerfully, and we see from the table just given, that although Helston is 6° warmer in winter than Greenwich or York, it is rather colder than the former and only 1½° warmer than the latter in summer; at Shields and Alnwick that the difference between summer and winter is only 16° and 14°. The winter is not materially colder at Newcastle than at London; but all the other seasons are, though the difference, when expressed in figures, is probably much smaller than any one who has not studied it as a question of figures will be likely to suppose. The summer is warmer in the interior of the south-east of England than anywhere else in Britain, rising in some places to an average of a little over 60°. The autumn, both in the north and south, comes very near to the average of the whole year, being generally a little above it. But if we turn to the Continent, we shall see how much colder the winters and warmer the summers are in proportion, and how the divergence increases as we pass into the interior.
