B. TERRESTRIAL PHYSICS AND METEOROLOGY. ^3 



the fact is patent to every one that, when the sky is clear, the 

 temperature to which vegetables and man and animals out 

 of doors are exposed is from 5 to 10 lower than that re- 

 corded by a sheltered thermometer; and it is just these ad- 

 ditional degrees of cold, when the ordinary sheltered ther- 

 mometer hardly sinks to the freezing-point, that destroy 

 tender plants, and more or less seriously injure others. The 

 difference between the temperature of a sheltered thermome- 

 ter and that indicated by one partly exposed to radiation 

 toward the sky depends, as is well known, on the cloudiness 

 of the sky ; and, if the sky be clear, on the quantity of invisi- 

 ble moisture in the air. A series of observations made for 

 ten years at Helston, by Mr, Moyle, shows that in each year 

 differences of 6 to 12 have been observed, the exposed ther- 

 mometer being invariably the lower. At Pengerrick, near 

 Falmouth, the difference between the minimum thermometer 

 lying out on the grass and the minimum sheltered in an ordi- 

 nary thermometer-screen sometimes amounts to 15 or 17. 

 The thermometers exposed to radiation, and lying not upon 

 grass, but upon other substances, such as garden-mould, snow, 

 sand, stone, and gravel, generally show slightly higher tem- 

 peratures than those on grass. Their variations among them- 

 selves are, however, due to a somewhat complicated relation 

 between the conduction of heat from the interior strata of 

 the earth and the radiative power of the surface stratum it- 

 self. The radiative power of straw, whether flax, wheat, or 

 rye, is one of the largest known. A bedding of straw is used, 

 as is well known, in India in the process of forming ice dur- 

 ing clear nights. From a series of observations made for a 

 period of eighteen months on garden-loam, sand, clay, and 

 peat, Mr. Whitley found that garden-loam showed a power 

 of receiving and retaining heat superior to that of any other 

 naked soil. The temperature of the peat was nearly equal to 

 the garden-loam, although naturally wet and cold. Silicious 

 sand did not come up to the expectations formed of it. Clay 

 maintained a bad pre-eminence for coldness, but improved by 

 good drainage in the second year's observations. The effect 

 of a thin screen in preventing the radiation of heat at night 

 was shown by some observations which prove that a screen of 

 muslin does not protect the thermometer placed beneath it 

 so well as straw matting. The matting is surpassed in its 



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