566 Cultivation of Plants. 



inches, and in some localities even this large amount is ex- 

 ceeded, especially in the western Highlands of Scotland and 

 in Cumberland. In less elevated parts of the west, it ranges 

 from 30 to 40 inches, and in the east and south-east from 20 

 to 28 inches annually. During a period of forty years, the 

 average rainfall at Chiswick, near London, has been about 23'5 

 inches. But these figures, by themselves, are of little use to 

 the gardener. It is only when they are compared with those 

 furnished by the countries whence we obtain our hardy exotic 

 plants, and with purely local conditions, that they become really 

 interesting and serviceable. As we have already stated, the 

 insular position of Britain, and especially its exposure to the 

 softening influence of the Atlantic Ocean, raises its mean 

 annual temperature considerably above that of continental 

 countries in the same latitude. The most important point 

 in this increased mean annual temperature for latitude, is 

 the fact that it is principally due to the high winter tem- 

 perature of those parts most favourably situated for receiving 

 the full effect of the neighbouring ocean. The mean annual 

 temperature of Britain in round numbers is 50. Now, if 

 we follow the lines denoting this mean temperature through 

 the different countries of the world, both in the northern and 

 southern hemispheres, we may form an approximate idea as to 

 what countries will furnish us with hardy plants. In con- 

 tinental countries where this mean is raised considerably by a 

 higher summer temperature, we may safely conclude that plants 

 growing several degrees south or north of the line will prove 

 hardy in the warmer parts of Britain. Again, insular countries 

 on the same line, owing their mildness to the same causes as 

 Britain, would naturally furnish us with plants that would 

 flourish with us in some parts, whilst in others they would 

 succumb to the rigours of winter. Another circumstance to be 

 considered in conjunction with those already mentioned, in 

 judging of the hardiness of a plant, is the elevation above the 

 sea at which it grows in its native country. The rule for cal- 

 culating for elevation is to deduct one degree of mean annual 

 temperature for each hundred yards of height. As a rough guide, 

 we may indicate the course of the north and south isothermal 

 lines through those countries having a mean annual tempera- 

 ture of 50 Fahrenheit. But we must caution the reader 

 against taking them as absolute and definite in the information 

 they convey. 



