July i, 1882.] 



THE TROPICAL AGRICULTURIST. 



81 



DEW AND HOAR FROST. 

 All pliiits require moisture in order to live and 

 grow, and Nature ba<! provided in the abundant sup- 

 ply of this element one o{ the principal factors which 

 determine the greater Or less vigour of the growth 

 of plants upon the earth. 



It is to the copious dews that we have in a great 

 measure to .attribute the productiveness of the mead- 

 ows bordered by rivers. 



Water covers about three-fourths of the surface of 

 the globe as ocean, partly in a solid condition, as at 

 the exlreme north ; paitl.v fluid, as in all warmer 

 regions ; it flows through Mie land in all directions 

 in the form of rivers ; it ascends into the air as 

 v;>pour, and falls again to the earth as r.ain, snow, 

 or dew. This continuous circulation nf " water" pro- 

 duced by "heat," is the especial agent which penetrate? 

 the earth, filling it with life and prosperity. 



Where water exists the sandy desert changes into 

 a luxuriant oasis; where it is absent, even the greatest 

 richness of soil remains a "waste," and unproductive. 

 Dew, which forms our particular subject, is the 

 humidity whicli the air tinder certain circumstances 

 deposits in the form of minute watery globules on 

 the surface.*! of the bodie-i in contact with it. For 

 instance, on the withdrawal of the sun's raya the 

 atmospheric air becomes colder, consequently the 

 blades of grass, and leaves of pl.auts become chilled 

 from ex|)osure to the influence of the cold, and in 

 their turn cool the damp air which touches them, 

 and cause it to drop its moi-ture upon their exposed 

 surface. When the condensation of vapours occurs 

 by contact with cold solid bodies it is called " dew ;" 

 when, on the contrary, the whole body of air is 

 cooled "mists," "clouds," or "rain" is formed. It 

 is probable that if it were not for the effect of dew 

 the rapid escape of heat from the earth by radiatioi), 

 the temperature of the soil would become so de- 

 pressed during the sun's .absence, tliat the extremes 

 of heat and cold in the course of twentj-four hours 

 might be so great as to destroy vegetable life in the 

 summer season : and it is generally after nights 

 of copious depositions of dew that the mornings are 

 the brightest, and the sun's heat, the most powerful. 

 Dew is only formed beneath a calm and cloudless 

 sUy, and never in windy weather, for under the in- 

 fluence of wind the moisture nf the earth is carried 

 ofif with extreme rapidity. A north wind also checks 

 its production, but a gentle southerly breeze, charged 

 with humi.iity, will occasion a copious deposit. When 

 the atmosphere has a temperature below tlic point 

 of congelation, the dew, which might adhere to the 

 substances exposed, passes into the form of hoar 

 frost. It is also found that if the temper.nture of 

 the earth's surface sinks during the night down to 

 freezing point, the particles of water deposited be- 

 come solid anil form hoar frost. 



A deposit exactly similar to dew is formed when- 

 ever the air becomes suddenly chilled, by touohin'^ 

 any surface colder than itself. Thus the walls of long 

 passages, vaults, or massive buildings gener;dly, drip 

 with wet during the early part of the summer, before 

 the external warmth has sufiiciently penetrated. 



Leslie says:— "In fine, calm weather, after the 

 ravs of the declining sun have ceased to warm the 

 surface of tlie gnuiud, the descent of the higher mass 

 of air gradually chills the undermost stratum, and 

 disposes it to dampness, till their coniinued inter- 

 mixture produces a fog or low cloud. Such fogs are, 

 towards evening, often observed gathering in narrow- 

 vales or along the course of rivers, and generally- 

 hovering within a few inches of the surface." 



In clear and warm weather the air is always drier 

 near the earth's surface during the dny than at a 

 certain height above the ground, but it becomes 

 damper on the approach o£ cveuiug j hence, dew i» 



always more dense on grass and low-groning plants 

 than on the leaves of shrubs which stand up some 

 feet from the ground. 



iMany substances are endowed with the property 

 of radiating their heat, and of thus becoming cool 

 with different, degrees of rapidity, and it is found 

 that those substances which in the air become cool 

 hrst, also attract tlie most quickly and abundantly 

 the particles of falling dew. 



Dr. Wells in his observations on the deposition of 

 dew found that grass lieoame 12° colder than ordinai-y 

 garden mould, and 16'-5 colder than a Jiard gravel- 

 walk ; hence we find that dew deposits the most 

 readily on living vegetation, and that well-pulverised 

 soils draw much more dew than those which have a 

 hard surface, and are close and compact. Wherever 

 the atmospheric air can freely penetrate there the 

 depositions of dew under favourable circum-^tanc-s 

 take place. Sandy soils are powerful attraotors of 

 moisture, and on the sandy plains of Chili, whe.e 

 rain is scarcely ever known to fall, vegetation depends 

 almost entirely on the nightly depositions of dew for 

 its support. 



It has been estimated by Dalton that the amount 

 of dew which is annually deposited upon the soil 

 amounts to about 5 inches, equal to more than 501) tons 

 of water per acre. But the rueteurologioal records 

 inform us that the yearly rainfall on the eastern side 

 of our island amounts to about 23.^ inches, while 4!) 

 inches, or nearly double the quantfty. is precipitated 

 on the western side; and, as we find the rainfall 

 varying considerably at different sea.sons and in different 

 localities, so wo know it is in the case of dew. 



Steinmetz remarks that "the quantity of aqueous 

 vapour in the atmosphere is greatest in equal lat- 

 itudes on the sea-coast ; it diminishes in proportion 

 as we advance into the interior of a continent." In 

 Algeria, at the stations on the coast, after the driest 

 and hottest days, immediately the sun has set the 

 soldiere' uniforms become wet with dew, and in a 

 single night the blades of knives in the pocket be- 

 come rusted, but at 30 miles from the sea night 

 exposure is not attended with any inconvenience from 

 the humidity of the air. 



In countries near the equator tho depositions of 

 dew are much more abundant than in Ensland, owing 

 to tlie grc Iter amount of he it absorbed from the snu 

 during the burning heat of the day. 



The quantity of vapour which the air is capable 

 of holding in suspension is dependent upon the 

 temperature, and wo always find the atmo.^ohere in 

 the vicinity of currents of water more highly "charged 

 wiih moisture than it is in the uplands. In India 

 the deposition of dew ne.ar the rivulets when all 

 around is perfectly dry is said to be exceedingly 

 marked. Colonel Sykes observes that "When in 

 Poena in September and October, 1850, if there was 

 no dew anywhere else it was found on the banks of 

 rivulets aud the Mota Mnlu river, but 15 to 20 feet 

 from the water were tho limiti of the dcp isitimi." 



Turning now to 'ha composition of i-.tki, dev, and 

 hoar-frost, Boiissingauli has g-ven th.3 following 

 analyses of .■sample? collected at Paris an I Liel-.franen" 

 berg in the year ISo" ; — 



Tabic I. — 'ihowiiig the Composition of Rai ', Dew, and 

 Fotj, in Gruim per Imperial Oalloi. 



Raiu 



( Paris 



1[ Licbfivnuenbary 



.Vmmoiiia. 



0-2100 

 C-0:i5U 



Silrio Tot.al 



Ai i(l. Xitrogeu. 



C-1'708 0-380.S 



0-uiio 0-0-iao 



Dew Liel.rrLU.cnber^ fMa.Muium 0-131U 



0-.17S5 

 C-.Miso 



0-512.5 

 0-OT-il 



-p J Paris 



" \ I,iebfriuieiibei-K 



... 0-l!i)0O 0-:il!>2 1C-:M!I2 

 „ - .. 0-17ft» 0-..71.-i 0-iS08 



It thus appears that in Paris the quai-t 'y of nitro. 

 gcnous matter brought down in the r.iin.w:tor is 



