REMARKS ON SURFACE TEMPERATURE. 121 



It may then be stated as a general result from the observations here 

 recorded that, in the climate of Cornwall, the soil covered with vegetation 

 will be from 8 to 10 degrees warmer, in sharp frosts, if screened from the sky by 

 straw matting, or other moderately thick material, spread over it, 3 or 4 feet 

 from the ground, than if left unprotected. This is a difference of the utmost 

 importance in those j)arts of the county in which early vegetables are grown, 

 which lie chiefly along the southern coast, where the minimum on the stand 

 rarely falls below 25°. That degree of cold would seldom be destructive if the 

 air is at all still ; but a depression of temperature to 15° or sometimes lower, by 

 radiation, might per sc seriously imperil the crop ; and it is usually a night 

 or two of this degree of depression united with wind that causes the blight 

 by which the hopes of the season and many thousands of pounds are lost. 

 A further benefit resulting from shelter is the prevention of the deposit of 

 dew, and at times of hail and sleet, the evaporation from which further 

 chills the plant, and abstracts its heat in a way more pernicious than the 

 action of the same degree of dry cold. Snow a few inches thick constitutes 

 in itself, while it lasts, an effectual shelter ; but frost supervening on its 

 thaw renders it often a delusive one. 



Another element of great moment in relation to the effect of tempera- 

 ture is the movement of the air. The blight caused by a local draught of 

 icy wind may be frequently traced in well defined lines from a gap in a 

 north eastern hedge along the rows of early plants. In a still frost, the 

 conditions are similar to those illustrated by Mr. Whitley's brocoli, the leaves 

 preventing radiation from the soil, and the specific heat of the vegetable 

 being retained around it ; but a high wind robs it of both protecting influ- 

 ences with every blast. So much is this the case that the placing of wattled 

 hurdles or other effective screens, about 4 feet high, on the north and east 

 sides of a tender crop, in rows 20 or 30 feet apart, must be recommended as 

 a preservative measure even more important than the provision of horizontal 

 covering. It would also entail no very heavy charge ; whilst it would furnish 

 a frame-work from which straw matting, or other sheltering material, might 

 be readily spread over the whole or any more delicate portion of the plants. 

 It is not, however, for me to point out in what precise manner, or with what 

 materials, the object in view may be most readily and cheaply attained ; par- 

 ticulars varying in different localities, and on which the practical knowledge 

 of the grower and market gardener is greatly superior to mine. My purpose 

 has been to state some of the meteorological facts and principles which may 

 guide the cultivator, leaving their application in his hands. 



I may add, as the result of observations continued through a long series 

 of years, that in the more intense frosts associated with a settled wind from 

 the east, the temperature of the night is at Truro about 10° lower than at 

 Penzance, and at Penzance 10° lower than at Scilly. It has not come to my 

 knowledge that the early potatoes or brocoli have ever been materially 

 blighted in the Islands by the light frosts to which alone they are exposed ; 

 and shelter such as has been suggested will just prevent the surface tem- 

 perature at Penzance from falling below what it is naturally and without 

 shelter at Scilly. This rule may be probably extended to various sheltered 



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