660 



DEW DE WITT. 



temperature of the air, and of the surface of the 

 earth, always accompanies the falling of dew, the sur- 

 face on which it is deposited being, however, colder 

 than the air above. 



These phenomena admit of an easy and elegant 

 explanation from the well known effect of the radia- 

 tion of caloric from bodies. This radiation constant- 

 ly taking place in all bodies, it is obvious that, the 

 temperature of any body can remain the same only 

 by its receiving from another source as many rays as 

 it emits. In the case of the earth's surface, so long 

 as the sun remains above the horizon, it continues to 

 receive as well as to emit heat ; but when the sun 

 sinks below the horizon, no object is present in the 

 atmosphere to exchange rays with the earth, which, 

 still emitting heat into free space, must consequently, 

 experience a diminution in its temperature. It thus 

 becomes not only many degrees cooler than in the 

 daytime, but also cooler than the superincumbent 

 air : and, as the atmosphere always contains watery 

 vapour, this vapour becomes condensed on the cold 

 surface ; hence the origin of dew, and, if the tempe- 

 rature of the earth is below 32, of hoar frost. And 

 since the projection of heat into free space takes 

 place most readily in a clear atmosphere, and is 

 impeded by a cloudy atmosphere, it is under the 

 former condition that dew and hoar frost are 

 formed ; for if the radiant caloric, proceeding from 

 the earth, is intercepted by the clouds, an inter- 

 change is established, and the ground retains nearly, 

 if not quite, the same temperature as the adjacent 

 portions of air. 



Whatever circumstances favour radiation, favour 

 also the production of dew ; and, accordingly, under 

 the same exposure, dew is much more copiously de- 

 posited on some surfaces than on others. Gravel 

 walks and pavements project heat and acquire dew 

 less readily than a grassy surface. Rough and 

 porous surfaces, as shavings of wood, take more dew 

 than smooth and solid wood. Glass projects heat 

 rapidly, and is as rapidly coated with dew. But 

 bright metals attract dew much less powerfully than 

 other bodies. Water, which stands at the head of 

 radiating substances,- is seen to condense the vapour 

 of the superincumbent air in such a manner as to 

 create thick mists and fogs over its surface. The 

 unusual abundance of precipitated moisture over 

 ponds and streams is attributable, however, not 

 merely to the inferior temperature of their waters to 

 the air, arising from radiation, but to the circum- 

 stance that more moisture is ordinarily contained in 

 such air, since the sheltered situation it enjoys pre- 

 vents its being borne away by those aerial currents 

 prevailing elsewhere. 



An acquaintance with the cause which produces 

 dew and hoar-frost enables us to understand the ra- 

 tionale of the process resorted to by gardeners to 

 protect tender plants from cold, which consists 

 simply in spreading over them a thin mat or some 

 flimsy substance. In this way, the radiation of their 

 heat to the heavens is prevented, or, rather, the 

 heat which they emit is returned to them from the 

 awning above, and they are preserved at a tempera- 

 ture considerably higher than that of the surround- 

 ing atmosphere. To ensure the full advantage of 

 this kind of protection from the chill of the air, the 

 coverings should not touch the bodies they are in- 

 tended to defend. Garden walls, operate, in part, 

 upon the same principle. In warm climates the de- 

 position of dewy moisture on animal substances 

 hastens their putrefaction. As this usually happens 

 only in clear nights, it was anciently supposed that 

 bright moonshine favoured animal corruption. 



This rapid emission of heat from the surface of 

 the ground enables us to explain the artificial for- 



mation of ice, during the night, in Bengal, while the 

 temperature of the air is above 32. The nights 

 most favourable for this effect are those which are 

 the calmest and most serene, and in which the air is 

 so dry as to deposit little dew after midnight. C louds 

 and frequent changes of wind never faifto interrupt 

 the congelation. Three hundred persons are employ- 

 ed in this operation at one place. The enclosures 

 formed on the ground are four or five feet wide, and 

 have walls only four inches high. In these enclo- 

 sures, previously bedded with dry straw, broad, 

 shallow, unglazed pans are set, containing water. 

 Wind, which so greatly promotes evaporation, pre- 

 vents the freezing altogether ; and dew forms, in a 

 greater or less degree, during the whole of tin; 

 nights most productive of ice. The straw is care- 

 fully preserved dry, since if, by accident, it becomes 

 moistened by the spilling of water, it conducts heat, 

 and raises vapour from the ground, so as greatly to 

 impede the congelation. 



The radiation from the earth's surface is one of 

 those happy provisions for the necessities of living 

 beings, with which nature everywhere abounds. 

 The neavy dews which fall in tropical regions are, 

 in the highest degree, beneficial to vegetation, which, 

 but for mis supply of moisture, would, in countries 

 where scarcely any rain falls for months, be soon 

 scorched and withered. But, after the high tempera- 

 ture of the day, the ground radiates under these 

 clear skies with great rapidity ; the surface is quick- 

 ly cooled, even to a great extent, and, as soon as 

 this refreshing cold is produced, the watery vapour, 

 which, from the great daily evaporation, exists in 

 large quantities in the atmosphere, is deposited 

 abundantly. This deposition is more plentiful, also, 

 on plants, from their greater radiating power ; 

 while, on hard, bare ground and stones, where it is 

 less wanted, it is comparatively trifling. In cold 

 climates, the earth, being cold and sufficiently moist, 

 requires little dew ; accordingly the clouds, which 

 are so common in damp and chilly regions, prevent 

 the radiation of heat : the surface is thus preserved 

 warm, and the deposition of dew is, in a great mea- 

 sure, prevented. 



DE WETTE. See IVette. 



DE WITT, JOHN, grand-pensioner of Holland, 

 celebrated as a statesman and for his tragical end, 

 was the son of Jacob De Witt, burgomaster of Dort, 

 and was born in 1625. His father was imprisoned 

 for some time on account of his opposition to prince 

 William II. of Orange. John De Witt inherited 

 from his father republican principles and a hatred to 

 the house of Orange. After having carefully culti- 

 vated his talents, he entered into the service of his 

 country, and was one of the deputies sent by the 

 states of Holland to Zealand, in 1652, to dissuade 

 this province from conferring the office of captain- 

 general on the young prince of Orange, William III., 

 who was but two years old. His eloquence procured 

 him universal confidence ; but to preserve this was 

 almost impossible during the dissensions which 

 raged in the states-general. One party was anxious, 

 during the war between England and Holland, to 

 have all power and honours conferred on prince 

 William III. ; the other, with De Witt at its head, 

 endeavoured to withdraw all authority from this 

 prince, and entirely to abolish the stadtholdership. 

 The war with England, sometimes fortunate, some- 

 times adverse, was injurious to commerce, and ex- 

 cited the displeasure of the nation against the latter 

 party, of which excitement the Orange party took 

 advantage to effect then* purposes, until, in 1654, the 

 former concluded a peace with Cromwell, with the 

 secret condition that the house of Orange should be 

 excluded from all situations of authority. Thus Uie 



