Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/595

 DEW

muft wet a body placed lower fooner than one placed higher, and mult wet its under fide fooner than its upper, and of a number of bodies placed in this manner, the lower muff be thus gradually wetted all the way before the upper. It ap- peared certain to him, that the Dew arofe in vapour from the earth after it had been heated by the fun, and that not lefs of it arofe in the middle of the day than afterwards, but that then it was difperfed and evaporated as fbon as raifed. On this foun- dation he determined to make his trials, and for this purpofe he plac'd two ladders againft one another, meeting at their tops, fpreading wide afunder at their bottoms, and fo tail as to reach thirty-two foot high. To the feveral fteps of thefe be faftened large fquares of glafs like the panes of a window, and let them in fuch a manner, that they fhould not fhade or overhang one another. It was plain that, on the whole, if the Dew descended without firft arifing, the top fquares muft be firft wetted, and that on their upper furfaces; but, on the contrary, if the Dew firft afcended from the earth, the bottom furfaces of the lower panes muft firft receive it. On the trial it appeared exactly as expected in the latter man- ner. The lower furface of the loweft piece of glafs was firft wetted, then its upper furface ; then the lower furface of the pane next above it, and fo on gradually till the whole were wetted to the top of the ladders.

It was happy that this Experiment fucceeded in fuch perfect re- gularity ; but this is not always to be expected, for a thoufimd accidents may prevent it, and the lighted: wind deftroy it: thus, fuppofing the Deiv to have afcended in this exact order up to a certain height, it may there meet with a gult of wind, which fhall diffipate it in fuch a manner, that it {hall no more affect the higher planes, till that wind ceding, gives it leave, which perhaps may not be till it has rifen to the height of the whole ladder ; and then at an inftant all the upper planes {hall after this interruption be wetted at once.

This gentleman afterwards tried the fame experiment with pieces of cloth tied to the fteps of the ladder inftead of glafles, as he could be able, by weighing thefe, to know of certainty, how, and in what order they received the dew ; in this trial foine accident prevented the fame perfect regularity from appearing in the afcent of the Dew, but the quantity of moifture imbibed by the higher pieces^ was fo much lels than that imbibed by the lower, as abundantly to prove that the moifture they received was from below not from above. Mr. Mofchenbroek, who is no friend to this opinion, thought he had invalidated all thefe proofs, by trying the fame experi- ments, and finding the fame fuccefs on a plane covered with fheet lead, as had before appeared in the garden of the firft experiments, where the ladders were placed on the ground. Mr. Mufchenbrock, who eafily faw that the vapour could not afcend, at leaft not in fuch quantity through lead, thought he had proved, that however it appeared on the cloaths, it muft have defcended from above; but Mr. du Fay very pro- perly overthrew all thefe arguments, by obferving, that the vapour need not rife through the lead, nor from that very fpot, but that as it arofe from the adjoining open ground, the conti- nual fluctuation of the air, could not but fpread it abroad, and

arry

it in its afcent thither.

The order and manner of the afcent of Dew is thus afcertain- ed, and its defcent again cannot appear at all wonder- ful when we confider, that at fuch times, as the heat of the fun is not fo great as to diffipate it, as it rifes, it will, by its natu- ral attraction form itfelf, into drops fo large, and of fuch gravi- ty, that the air cannot fuftain them. From the fame fource and origin, we muft alfo fometimes have thick fogs, and thefe, properly fpeaking, are only Dezu, in a certain ftate. Mr. du Fay has proved alfo, that the vapour which is to fcll in Dew, continues to mount in a direct courfe upwards the whole night. For having hung up a piece of woolen cloth, after he had carefully weighed it, he examined it, at times, through a whole night ; and weighing it every time, he found it fenfibly increafed in weight ; that is, its quantity of moifture fenfibly increafed every time. Thefe are the moft obvious qualities in regard to the Dezu, but there are others more nice and curious, which owe their firft notice to Mr. Mufchcn- broek, and have been afterwards verified by Mr. du Fay. Several different fubftances expofed to the iame Dew, receive and charge themfelves with it in a very different manner, fome more, others lefs, and fome even not at all. The drops feem to make a fort of choice of what bodies they {hall affix themfelves to ; glafs and cryftals are thofe which they affix themfelves to, in the moft ready manner, and in the Iar°-eft quantity ; and metals, of all kinds, do not receive them at all, nor do the drops ever affix on them. The two extremes are thus fixed ; the intermediate bodies are to be judged of by repeated experiment. The extremes are indeed lb well afcer- tained, that they are eafily proved ; for, if a cryftal, or glafs veffel be fet out in an evening on a filver plate, the glafs will be found covered all over with Dew drops, and the filver per- fectly dry. China ware is a fort of glafs ; fix pounds of mer- cury being expofed to the air in a china plate, the Dew was found running in ftreams on the edges of the plate, while not the leaft trace of a fingle drop appeared upon the mercury. Supfl. Vol. I.

DEW

The plain way of accounting for this would feem, not that Dew falls upon fome bodies without touching others, but that it readily evaporates from the furfaces or" fome, while it re- mains and collects itfelf into drops upon others: and it is hence that we find fome wet and others dry, which have had the fame expofurc ; but this, though the moft obvious, and feemingly apparent reafon, is not the true one. Mr. du Fay hasobferved, that if this were the cafe, in thofe bodies, which we find dry after expofure, the evaporation muft be more Hid- den than there are powers in nature, to 1 effect ; or, in other words, than it is polfible for it to be ; and the truth, in regard to the obfervation, is not this ; but that, in reality, the parti- cles of this afcending Dew really do not affix themfelves at all to fome bodies, while they very readily do it to others ; as we fee in water itfelf, that in a common river, a rough dog, matt be miferably wetted all over, while the feathers of a fwan {hall not receive a fingle drop of moifture. There is alfo another fingle obfervation, which mould feem' to prove that this vapour only afcends, and does not at all defcend in form of Dew again ; but that what we fee collected into drops on grafs,and plants,col!ects itfelf" in that manner in the beginning of its afcent. For if a cup of any metal be expofed a whole night, it will never be found to contain any Dew, which if that liquid defcended muft have been the cafe ; fince, though it did not naturally ftick to that metal, it muft fall into it, and remain there ; but this docs not happen. What can be collected from the whole, feems, that Dew is a vapour, which arifing into the air, and increafmg in quantity, through the courfe of a whole night, expands itfelf every way in it, and moiftens or wets fome fubftances, and does not moiften others, and this merely according to the differences of their furfaces.

Thefe however are but probable fyftems, not abfolutely known facts : ideas of the manner of the actions and ope: ations of bo- dies, readily flow into the mind, when we have formed a ratio, be it whatever, in our own thoughts for them. Mr. du Pay has many nice opinions in regard to this vapour or liquid, which it were much to be wifhed were carefully exa- mined by experiments. He fuppofes fome diftant alliance be- tween the phenomena he has obferved in Dew, and thofe which appear in electrical bodies, and in thofe bodies which afford phofphori. He has obferved, that all bodies, which are capable of a fufficient degree of rubbing, become electrical, ex- cepting only the metals ; and that all bodies, except only the metals, may alfo be made phofphori. The metals are the only bodies alfo which wholly refufe to admit the Dew ; here are certain properties all appearing in metals, and in metals only ; and it feems hence there may be fome affinity in thofe things, which do, or do not affect the metals only. It is certain, that there are connections in natural bodies, which are too nice for hafty elucidation, yet are not unworthy all the care that can difcover them. Mem. acad. Scien. 1736. It is very certain that fubftances of a very different kind from the ufual and natural matter of the Dew, have fometimes fallen in that form. Our Philofophical Tranfactions give an account, that in the year 1695, there fell in Ireland, in feve- ral parts of the provinces of" Leinfter and Munffer, for a confiderable part of the winter and ipring, a fatty fubfta'nee, refembling butter inftead of the common Dew ; it was of a clammy texture and dark yellow colour ; and was, from its great refemblance, generally called Dew-butter by the country people. It always fell in the night, and chiefly in the moorifh low grounds, and was found hanging on the tops of the rrafs, and on the thatch of the houfes of the poor people. It was feldom obferved to fall twice in the fame place, and ulually wherever it fell it lay a fortnight upon the ground before it changed colour, but after that it gradually dried up, and be- came black. The cattle fed in the fields where it lay, as well as in others, and received no hurt from it : it fell in pieces of the bignefs of ones finger end, but they were difperfed fcatter- ingly about, and it had an offenfive fmell, like that of a church- yard. There were in the fame places very ftinking fogs during the winter, and fome people fuppofed this no other than a fediment or connection of the heavier matter of thofe fogs. It would not keep very long, but it never bred worms. The country people willing to have fome good of it, tried it on their childrens fore-heads, and it always cured them. Philof. Tranf. n° 220. p. 222. Cryjidline Dews, a term ufed by fome modern writers, to ex- prefs certain Dews, or fteams ; which, in their fall upon the earth, become, as they fay, cryftal. This greatly favours the opinion of the antients, that all cryftal was only water frozen into a kind of ice harder than ordinary, but it is errone- ous. Mr. Beaumont gives an account in the Philofophical Tranfactions of thefe cryftal Dews on Mendip hills ; in which he tells us, that the miners of that place find fometimes in the roads where the earth is bare, triangular cryftals, about two inches in length, and an inch over, not with fharp angles, but roundifh and blunted ones, and carried up round at the ends like a cocoa nut i and that none of thele are ever found under ground in digging, but only lying on the furface. He tells us alfo, that he had feen fome taken up in Gloucefter- 9 D (hire,