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 fur 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. 
