1140 



METEOROLOGY. 



[DEW FROST roan. 



formation, directly follow* from a consideration .f tho 

 law* of radiant matter , itn<l it may be readily illustrated 



it. 



by a diagram. Let us suppose that a screen of glass 

 be supported over the radiating surface of the earth in 

 one case horizontally, in the other case at an angular 

 inclination as represented by the accompanying dia- 

 grams (Figs. 45, 40). 



. 4i. Fig. 48. 



It will be evident that the horizontal glass in Fig. 45 

 will radiate more heat than the diagonal glass in Fig. 40. 



l>it' PHMMofion of tlie Amount of Dew. Although the 

 laws which regulate the formation of dew are perfectly 

 well known, and a rough method of determining the 

 amount of dew deposition not difficult, yet no correct 

 means of estimating its actual amount has yet been de- 

 vised. Instruments for determining the amount of dew 

 are called drosometers. A drosometer is a balance, sus- 

 pended to one arm of which is a plate, to the other a pan 

 containing weights, exactly proportionate to the weight 

 of the plate, so that both may be in equilibrio. Supposing 

 dew to be deposited on the plate, the latter will evidently 

 increase in weight by the amount of such deposition, and 

 a deviation of the beam from the horizontal will ensue. 

 The principle of the instrument is unimpeachable, but in 

 practice it is im]>erfect. Instead of the plate as described, 

 recourse may bo had to a lock of wool or eider-down, or one 

 of a large choice of hygrometric materials. Holies of this 

 kind were employed by Wells. Wilson and Flange! 

 had recourse to a plate ; but it seems that the materials 

 employed by Dr. Wells should have the preference. 



A ^c^y instructive experiment relating to a dew for- 

 mation, and one which may be regarded as presenting a 

 summary of the whole matter, is as follows : 



If, on a clear, cloudless night, when dew is being de- 

 posited, a glass ball (Fig. 47) bo suspended in the open 

 air, Home height from the ground, dew-drops will form on 

 the ball ; not, however, equally on all portions of its sur- 

 face. Its upper aspect will be bedewed, then its sides ; but 

 only randy, and in extreme cases, the inferior aspect of the 

 glass globe becomes covered with dew-drops ; and when 

 they they are very small ; indeed, a complete 



tiou of size is manifest, the dew-drops decreasing in 

 sue as the upper aspect of tho globe is departed from. 



Oetu < -The following generalisations relating 



to the phenomena of dew may now be appended ; they 

 will, for the miwt part, be seen to be directly dedueible 

 fr..m n <-.. Moderation ,,f tho laws of radiant heat: 

 ('loud* und briKk winds are both inimicil to the forma- 

 tion of dew ; the foi : their nun ladiating 

 power; the latter, because of the removal of cool air, 



H*. 47. 



^-~~^\ 



its place being supplied by air already warmed. If the 



night In- cloudy, and the 

 wind still, very litt!. 

 results. If clouds and wind 

 together, dew i.s to- 

 tally absent. Screen-like 

 objects, interposed bf 

 tin: sky and the radiating 

 surface, produce an 

 identical with clouds ; hence 

 bodies freely exposed to the 

 atmosphere, ctittrii p.i 

 are most fiecly ln-u 

 Morning and evening are 

 not the times, as commonly 

 supposed, when dew is form- 

 ed most copiously. It is 

 deposited at all hours of 

 the night, but most co- 

 piously rather after mid- 

 night. It sometimes occurs 

 even before night, late in 

 the afternoon. Dew is not 

 deposited with equal readi- 

 m->s in all parts of the world, 

 but attains its maximum in 

 warm lands near the margin 

 of the sea, rivers, or lakes; 

 as, for example, near the Red 

 Sea, the Persian Gulf, the coast of Coromandel, at Alex- 

 andria, and in Chili. It is quite absent in very arid 

 regions, in tho interior of continents ; such, for example, 

 as central Brazil, the Sahara, and Nubia; neither does it 

 frequently occur at sea, because of the bad radiating 

 quality of a surface of water. 



The imperfect radiation of a surface of water is well 

 illustrated by tho following striking experiment : (I lass 

 is a good radiating surface ; whence a piece of glass 

 freely exposed in an atmosphere when dew is forming, 

 soon becomes covered with dew. If, however, tin- 

 have its surface wetted previously to exposure, instead 

 of becoming more wet it becomes dry, simply because 

 radiation is impeded, and evaporation takes place 

 unchecked. Cdti-ris paribm, the amount of dew pro- 

 duced, will be proportionate to the amount of aqueous 

 vapour present in the atmosphere ; and this readih 

 plains the fact, that a copious production, of dew is fre- 

 quently the precursor of rain. 



II<iney-Deic. Occasionally a sweet, damp, sticky 

 moisture attaches itself to leaves during the night, and 

 does U ot disappear throughout the day. The term 

 honey-dew has commonly been applied to it, though, in 

 point of fact, it is not dew at all, being merely an excre- 

 tion from certain insects termed iijitn- 



Soar-Frost. Hoar-frost differs only from dew in tho 

 circumstance of temperature. One is deposited, and 

 remains uncongealed ; the other, becoming consolidated 

 by the agency of freezing cold, is converted into ice. 

 Every meteorological observer knows that the existence of 

 hoar-frost is held to be indicative of coming raiu, and in 

 most instances the opinion is verified. The phenomena 

 of hoar-frost are even more beautiful than the correspond- 

 ing ones of unfrozen dew. Upon leaves and vegetable 

 stems, the deposit ion of hoar-frost is particularly beautiful. 

 If hoar-frost be examined microscopically, or sometimes 

 even attentively by the naked eye, a crystalline structure 

 will bo evident. The crystals belong to the same crys- 

 tallographic system (the rhombohedric) as those of snow, 

 but their general appearance is somewhat different. 



Foos. Fogs may be regarded as clouds, which form 

 close to tho earth's surface ; hence we might discuss 

 their peculiarities under the general head of Clouds. 

 They' are characterised by some peculiarities, chiefly 

 dependent, however, upon the vesicular aqueous vapour 

 of which they are c'iiupo>ed. embracing and retaining 

 volatile particles evolved naturally, or from the opera- 

 tions of man. In this way it i.s well known that London 

 fog is anything but pure aqueous matter. One of its 

 very important cunstitucaU is the coudousible part of 



