ii8 THE MOVEMENTS OF SOIL WATER [chap. 



local variations, there is a steady fall of average tempera- 

 ture with increasing elevation, so that nearly all the soils 

 in Britain usually known as early and used for the 

 productions of the first vegetable crops, are comparatively 

 near the sea-level. At the same time, soils which lie 

 at the bottom of valleys are subject to late frosts in the 

 spring, and early frosts in the autumn ; the frosts always 

 come more severely and more frequently in the lowest 

 parts of the valley, and often do not extend many feet 

 up the sides of the valley. Such frosts arise in still 

 weather and cloudless nights, and are due to the cooling 

 down of the earth's surface by radiation into space, when 

 there are no clouds to act as a blanket. If the atmo- 

 sphere is also still in such radiation weather, the air in 

 contact with the ground becomes chilled, whereupon it 

 contracts and grows denser. Owing to its increased 

 density it then begins to roll downhill like so much 

 water and accumulates at the bottom of the valley until 

 it is displaced by the still colder air which occurs later 

 in the night. Stillness is necessary, otherwise the cooled 

 air would not lie in the valleys, but soon become 

 disseminated. The proximity of the sea or other large 

 body of water, is also a considerable factor in the pro- 

 duction of an early soil ; the water, because of its large 

 specific heat tends to keep temperatures equable, and 

 does much to mitigate the untimely spring or early 

 autumn frosts which are so much dreaded by the fruit 

 and vegetable grower. Nearly all the districts in 

 which early potato-growing is practised in the United 

 Kingdom are near the sea, e.g. Jersey, the neighbour- 

 hood of Penzance, Ayshire, Kerry, parts of West 

 Lancashire, and a few places on the seaboard of Kent 

 and Essex, etc. 



A soil which is thus early because of its coarse 

 texture, its dryness, the readiness with which it can be 



