description of a new hygrometer: by mr. john smeaton, f. r. s

Description of a New Hygrometer: By Mr. John Smeaton, F. R. S.Author(s): John SmeatonSource: Philosophical Transactions (1683-1775), Vol. 61 (1771), pp. 198-211Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/106093 .

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XXIV. Defcription of a rew Hygrozter-* e AIr John Smeaton^, F.R.S

;Read March 21,HAV I N G fonze years ago attempted ^ to make an accurate and Senfible

hygrometer, byNmeans of a hempen cord) of a very conf1derabie lellgth; I quicldly-fbund, that, though it was more than fufficiently fuSceptible of eve-y change in the lt}azidity of the atmofphere, yet the cord was, upon the whole, in a continual Itate of lengthening Tlhough this change was the greate{t at firRv yet it did not appear probable that any given time would bring it to a certainty; and futhermore it Seemed, that, as the-cord grew more determ-- nate in nican length, the alteration by certain differences of moiRure grew leSs. Now as, on confi- derilag wood, paper, catgut, &c. there did not ap- eear to be a Iikelihood of finding alqy fubRance fiufliciently Senfible of differences of moiRure, that nvould be unalterable u-nder tlze fame degrees there of; this led me to confitler of a confirudion whiclu would readily admit of an aSuRtnent; ro thatZ thougll; the cord whereby the inkrument is adtuated may be variable in ltWel£ -both as to abSolute l-ength, and differeruce: of lxngth under given degrees of moiS ;ture, yet thats on fiupp-ofition of a material depar taxre from its original: feale, it might be readily re_

4 Ilored

[ T98



[ l99 ] Rored therete, sand in confiequence that any numbers of hygrometers, fimilarly conftruEted, migllt, like thermonweters, be capable of fpeaking the fame lsgtl,3ge.

The -two- points -of heat, the more readiloy deter minable in a tlzermorneter, are the points of freesing and boiling water. In tike manner, to conAruEt hygrometers whie;;h {hall be capable of agreements it is nxeSary to eftabtilh two different degrees of a moiure which llwall be as fixed in thetnIblves, and to which mfC can as readily and as often have re_ courfe as pofflble. One point is given by moking the fubilance perfeEtly wet, which feems fllfficiently determinable; the othcr is that of perfeEt dry; but which I do not apprehend to be attainable with the [ame precifson. A readlnefs to imbiobe wet, fa that the lubIRa-nce may be- foon and fully faturated, and alWo a facilay of parting with its moifRure, on being expofed to the fire to dry; at the fame time ia£

neither merfioe in water, n0r a moderate ex- poftion to the svarmth of the fire, {hall injure its texture, are properties requ-Efiee to the firll mover of fuch arz hygrometer, that in a mannqer exclude all fobRances at [ am acquainted with, befides hempen arxd flaxen threads or cords, and sfirhat are compounded thereof.

Upon thefe ideas, in thesyear X 758 I con{iruEted two hygrometers, as near alike as poilble, in order that I might have the means of examining their agreement or difagreement on fimilar or ditlimilar treatmerst! The interval or fcale between dry and wet, I divided into IOO equal parts, which Icall ffie dEgrecs of this hygrometer. The point of o

denotes



[ noo ]

denotes perfeA dry, and the numbers increafe with the degrees of moiRure tO IOO, which denotes per- fedc wee^-

On comparin,g them for fome time, when hung up near together in a pa(Iage or Sair-cafe where they would be uery little affeEted bt fire, and where they would be expoSed to as free an air as poilble tn the xnfide of the houSe, -I -found that they generally were withia one degree, and very rarely diered tzro de- glees but, as thefe compariSons neceffiarily took up fotne time, and were frequently interrupted by long- avocations from home, it was fome years before I could form a tolerable judgement upon them. One thing I foon ob£crved, not altogether to my liking; which was, that the flaxen cords, which I made uSe of, feemed to make fo much refillance tv the entry of ft-all degrees of moiRure (fuch as is commonly experienced within doors in the fituation above_ mentioned) that all the changes were comprized within the firA 3o d-egrees of the fcale; but yet, on expofing them to- the warm IReam of a wa{h-houSe, the index quickly mounted to IOO I was therefore defirous of impregnating the cords with fome- thing of a faline nature, which Ihould difipoNe them more forcibly to aetrs6t moilture; -in order, thae the index might, with the ordinaty changes of an-oiRure in the atmofphere, travel over a greater part

of the Scale of loo: how to do this in a regular and fixed quantity, was the fubjedr of marly ex- periments, and feveralyears interrupted enguiry. At lafl:, I tried the one here_ after defcribed, which feemed t-o anfwer my intentions in a great meaufure and though, upon ehe whole, it does not ap

pear



< Zza Fo{S.IX . Tab.M/Z. J.

FX

^8fi@ J- ^e: 2*

F8.3 b

F8fi. k 5

G c

t Fcr.4.

-



[ 20I ]

pear likely tllat this inR3rument will ever be made capable of So accurate an agreeement, as mercurial thermometers are made to be, yet, if we can reX duce all the difigreelnents of an bygrometer within .4x<,th part of the whole Scale it will probably be -of uSe in fotne philofophical enquiries, in lieu of in- Ilrunzents which have ngt as yet been reduced to any common Scale at all.

Defription of the Eygro^&fer, :Fig. 1 and , A B C is an orthographitE delinta

tion of the whole inIlnxment leen in front ia its true proportion.

D S: is that of the proSle, or the inRrament feen edgeways.

F Gf in both, repreSents a flaxen cord, about 35 inches long, fuEpended by a turning pet 1Fs and attached to a loop of braEs wire at A, which goes down into the box cover Hs which defends: the index, 8zcW from injury, and by a glaSs expofes the Scale to view.

}?ig 3. ffiews the in0amen-t to a larger fcale the upright part being {hortened, and the box cover re moved; in which the fame letters repreSent the fame parts as in the preceeding figures; C; I are two loops or long links of braSs wire) which lay hold of the index K L moreable upon a fmall Itudd or center Kb The cord F G i8 kept nioderately fRrain ed by a weight M, of about half a peatnd avoir- dupoizeJ

It is obviou6, that as the cord lengthens and ffiortens, the extreme end of the index rifes and fillsy and fuccefl lvely paR*es over

Vo. IJI. D d N sJ



[ 2O2 a

N 2 the fcale, diEpofed in tlle arctz of a circler and containing loo equal divifions. This Scale is attached to tlle braSs Riding ruler Q P, which moures upOn tlle direEting piece R R, fixed by ficrews to the boad, which makes the frafme or bare Of the whole; and the fcale and ruler, N Q P, is retained in any place, nearer to or further from the center K, as may bW re{uired, by the fcrew 5"

Fig. 4. repreSents in profile, the Riding pieceX and Rudd I (fig. 3.), which traverSes upon that part ofthe index nPxt the center K; and wh;ich-can, by t-he two ficrews of t-lle Itudd, be retairled upom any part of the index that is made parallel; and vhich is doe for 3 or 4 irlches frorn the center, for that purpoi. I>he Rudd is filed to the edgesj lilte the lcrum of & Scale bearn, one being formed c)n the underfide, the other upon tlle upper, and as Izear as may be to one another. An hesk formed at the lower end of the -wire loops C t retains the index by -ehe IvwermoR edge of the Rudd, while tlle weight M hangs by a fmall hook alpon the up per ed-d: by thefie means the index is kept Readys and tlle cords Itrained - by the weight, witl] verr I;ittle friecion or burtllen uporl) the central ltudd K.

Fig. 5. is a parallelogram of plate braSs, to keep out duR, which is- attached- to the upper edge of th-e oox cover Hj and Serves to iEut tIze part of the bow cover necefl4arily cut away, to give leave fc)rt the-wire G I to traverie w-ith-the niding Rudd (fig. 4.) nearer to, or furtlaer from, the center of the iredex K; and whe-re in (fiF. 5.) a is an hole about QF of a-n inchr diaretel, for the wire- G I to paSs through, in theX riing and iallseg of the indexF

* Sec1y



[ 203 ] freely without touchillg; b is a nit of a lefl*er fize, fiufficient to paEs the wire, alld admit the cover to come off without deranging the cord:-or in-dex, s-¢ are two finall fcrews applied to two {lits, by which the plate nides lengthways, in order toXadapt the hole a to the wire G I} at any place of the Rudd I upon the index K L.

Re>7warfis ota the preceding Conf ruAio 2*

s*. In this conArudion the Index K IF being z inches long, + inches from the extreme end are filed fo narrow in the diredlion in which it is Seen by the eye, that any part of theSe 4 inches, Iyxng over the diviflons of the fcale becomes an index there- to. The fcale itSelf nides 4 inches, fo as to be brought under any part-of the + inches of the indexs attenuated as before mentioned.

jdly lehe pofition of the direAing piece R R is lgo determined, as to be parallel to a right line dravun through the point o upon the Scale, and the center K of the index; confequentlys as the attenuated part of the index forms a part of a radius, or right line from the fame center, -it follows, that when- ever the index points to a upon ehe ficale, thollgh: the fcale is moved nearer to or further from the center of the index, yet it produces no change in the place to which the index points.

3dly. When the divided arch wof the fcale is at lo inches from the center (that is,- at its- mean dlf- tance) then the cen.ter of the arch and the center of the index are coincident. At other di{}ances, the extremes of which are 8 or I a inches, tbe center of

D d 2 the



t a°4J the divifions and center of the lndex, poxnting thereto, not being colncidenta ti index cannot move over fpaces riaally proportionable to one another xn a}l figuat1ons ofsthe Scale; yet} the whole fcale not exceeding 30 degrBes of a circle) st will be found on computationt that the error can never be fio great aS wtX pare of the fcale, or X degree of the hygrometer; which in this inkrument betng conf1dered as an indivifeables the mechanical crror will not be Senflble.

- Chon've and Preparatian of the Cord

The cord lzere made ulRe of is of Rax, and be- tWiXt th and th Of an inch in dianleter; which can readily & 7w1kertained by meaEuring a nllmber of tmrns made tound a pencal or fm-all llsk. It is a fort of mrd efed in Lonin ir making netsy and Ds of that Darticular kind called bs net m-akers * three threa lwi d. I do not imagine that the fabrick of ehe cordv is of the mok material confequence; but yet I fuppofes when cords can be had of fimilar fabricks and nearly of the lime flse7 that Some Emall fources of variatsns will be avoided. In general I look uzporl it- that cords, the more they are twRedt the more {hey vary by di£erent degrees of moillure,+ and the lefs we are certain of their abSolute length r

therefore thofe moderately twiVed, I fuppoSe; ar: likely to anfiwer beRc.

A competent quantity of tllis cordx was beiled in: one pound avoirdupoife of water, in which was pv}t two pennywaghts troy of common falt, the wile was reduced by boiling to 6 § av-oirdupoiSe which

Q was



f 205 ] was done tn about half an hotlr. As this aScertains a given Rrength of brine on- taking out the cord, it nway be fuppofed that every fibre of the cord is equally impreFated s^,ith falt. The cord being dried, it will be proper to Ilretch it; which may be done fo as toprevent it from unewiRings by tying three or four yards to twcx nails, againil a wall, in an horisontal pofition, arld hanging a weiffit of a pound or two to the middle, fO as to make it form an obtuSe angle. T}wis dotie for a week or more ln a ro, will lay the fibres of the cord cloSe togetlxere and prevent its Rretching fo faR after being applied eo the iruRrument, as it otherwiSe would bc apt to d-o.

I have mentioned the flzes arud principal dimen- f1Qns that Itave uSed; as the inficruments may a& well be fim-ilarly conftruEted as otherways; but I do not apprehend it to be very material eo agree in any thing but the flcrength of the brine on taking ie cord out of i£. If the cord is adapted to the infirument fome-days before its firA adiuPEment, t appsehend it will be the more fettledr

. \ .

Adi1wJhntnt of the Hygrometer.

The box cover being taken off, to prevent its being fpoiled by fire, and chufing a day naturally dry, fet the inIlrumen$ nearly upright, about a ya-rd from a moderate fire; fo that the cord may become dry, and the inRrument varmX but tzot fo ntar as would fpoil the finek linen by too much heat, and yet illy evaporate the moiRure; there let the irstiru- ment Ray, till the indes is got as low as it will go}

now



w - -

4 * aoo J now and then fltroking the cord betwixt the thumb and finger downwards, in order to lay the fibres ellereof clofe together, and thereby cauting it to lengthen as much as poilble: when the index is tllus become llationary, which witl generally hapZ pen in aboutan hour (more or leIi as the air ss naturally more or lefs dry), by means of the peg at top raife or depreii tise index, till it lays over the point o; this done, remove tbe inltrument from the fire, and having ready fome warm-water in a teacup, take a middling camel's hair pencil; tand dipping it in the srater, gently anolnt the cord, till it will dtinlQ up no more, and till the index becomes ilationary, and watcr will no more have effed upon it; which will alib generally happen in about an IQour. If in this Rate the index lay over- the degree marked loo, all i$ right: if not, Rack thee icrew S) and Ride the fcale nearer to or further from the center, till the point tOO comes under ffie index, and then the inArument is adjuRed for uSe: but, if the compafs of the llide is not futicient to efit this, as may probably happen on the iR adjut:lment, nack the proper rcrews, ar)d move the Iliding Itudd I nearer to or further fFpm wthe center of the index, according as the angle f6rmed by the index, between the points of dry and weta happeneth to be too fmall or too large fbr the kale; the quan tity can eafily be judged of; fo as tlize next time to come witllin the compaEs of the nide of the ikale; the quantity of llide being T of the length of the index, and coniquently its compaXof adjllk- mtot I_ of the whole variableXquantity* Now fi fsldiog tle Itudd I will srary the pOfltion of the int

dex 7



[ 2<57 ] dex tetpeEting the point of o, tllis nlovement is only to be conf1dered as a rough or preparatory ad- ju{tment, to bring it w!ithit, the corzlpa& of the llide of tlae fcale; which will not often hayspen to be neceSary after the: firR time; but in this caSe, the adjuttment tnuR be repeated ill the fame man er, by drying alld wefting as before deScribed. It is to be remarked that, as the cord is fuppoSeci

nnpregated in a given degree witll common faltfi a-nd this not liable to esraporate, care mu{} be taken in wetting, tilat no drops of wet be fiffered to fill from the cord: for, by the obServance hereof} the original quantity is preServed i--n the cord.

ObJertaZions made ton e)o orzgznal Hygrof.refers

Thtfe hygrorneters were firll adjuRed, after the impregnation of the cords with cohmmon falt, in Febreary 1770; they were kept together in a Rair- caSe tlll the fumnzer following; they were freqtlently obServedt and rarely found to diEer more than one degree

1h fummer, one of them remaintng in the fiornler place, the other was retnoved illtO a paSage througl a building; which having no doors, and the in{tru- ment being hung fo that neit}3er rain nor the direA rays of the fun could fall upon it) thereby it became expoSed to the winds and the f-ree paflage of the open air. In thefe fltuations tle two hygtometers notonlydifferedrver)? greatlyin quanoty, buteven frequently were moving differellt srays. They w7ere thus continued till Januarar Is7Iv in wiz-ich fpace of iime I obServed, that tlle invS- ordinary place of the

inds



[ 208 ] i-ndes was between IS° and zS° in the open air; that at 40° the atmofphere felt very fbnfibly nzeiti ) but yet it 5^tas freqvlently- above 60°; alzd more than once at :7o° or very near X have therefore tnarked the point of o dry; 204 the s.rean, 40 tnoifl, yo° very Xefl, sooo wet. I do not, hc)viever, mean theSe words (that of dry and wet excepted) as of any otller inteIlt, than that of general direEion, in llke nanner as thofe upon the barometer; leaYing the

lelatiere degrees of moidure to be judged of by the fcale.

In the month of January lal}, I reIlored the exo pofed hygrometer to its former place in the Raircafes rhen both inPrruments were again compared together; and they rarely differed more than I degree, and never fO much as 2°* After this, they were both removed together to the out paSage; and there -they agreed nearly in the fame manner, -the utmoR difference not exceeding 2 degrees. Afiter fiome tryal herc, one of them was- readjuRed, leaving the other han&ng in its place. On reRoring the new adjuRed intiru ment to the otherS they now differed about 5°, the new adjuRed one Randing fo mllch higher The day following the other was readjuflced alfo, and afterwards reflcored to its place with the formers which had been left in the out pafl4age; and aftcr this readjuRment they both agreed to 1°. Thisbe- ing obServed for fome days, one of them was taken down, in order to be packed up for London; this 1 have now the honour of exhibiting to the Royal Society; axld I beg eo leave it in the Society's houSe1 that in cafie any one ihould be

X defarous



[ 209 1 defirous of harring an inRrument made on thefame plan, they may have recourSe thereto.

It appears from the foregoing obServations} that, in the conzpaSs of 1: I months, the cords had Ilretch- efd the value of 5°: and I alro obServed that they both had contraded their compati about 10°. I would, therefole recommend, that an hygrometer {hould from its firIt adjuIRment, be readjuSed at the end of three months, and again, at the end of fix tnonths from the firPc; after that, at the interval of about f1x months, to the end of two years from the beginning; and after that, I apprehend that once a year will fuffice; the beSt time of adjuflcment, being

in the dry and warm weather of July or Augu0: and by theSe means, I apprehend the inIlrument will be always kept within 2° of its proper point.

ReEpeEting the fenfibility of this inkrument, it has that i-n a greater degree than its conRancy to its fcale can be depended upon, which; was all that I intended; where greater degrees of fenfibility are required, to make comparifons at fmall intervals of time, the beard of a wild oat, and other con- llruflcions may be uSed, with advantage; this inflruv ment being confldered as a cheque upom them as t0 mor-e dsRant periods.

Geaeral ConclgJvon.

I am aware that an hygrometer aAuated by any principle of the kind here nzade uSe of may not be a meafurer of the quantity of moi(lure, adcLally dif folved in, and intimatelymixedswiththe air; but only indicates the dirpofition of the air to part withs

VQL, LXI. - E e or



[ 2IO 3

or precipitate the water contained inits fubRance3 or, on the contrary, to diiSolve and imbibe a greater quantity: but as it is by feparating the effieEts of na- tural cauSes, that we are enabled to judge of thefe cauSes, and from thence their effedrs when aga;n compounded; every attempt to aScertain the op atic3ns of a f1mple cauCe will have its value in the fearch into nature: nor can we a praori determin-e t}we value of any new infirument; for, if it Mould lead to a fingle diScoverv, or everl to aScertain a fingle fadt, this may again lead to others of great importance, of which we might have, either none, or an im- perfeE idea of before. For my own part, I have always looked on a thick fog, and the fweating, or condenEation of the water's vapours upon the walls in the infide of buildings, to be the greateR nzarks of a moiR atmofphere: whereas 1 have not always found the hygrometer affeAed at thefe times in the higheR degree. On the contrary at the cloSe of -a fine day, and the fall of the dew on the fud- den approach of a froIt, I have found the hygro meter more affeEted by moitiure than -n fome of the preceeding cafes; and Rill more by a falling dew in the time of an hard froIl. I jull mentlon theSe nlatters of hints for the enquiry of others; not having had length of time, flnce I brought tlle inllrument to anfwer my intention, to make any abSolute con- clufions.

I am forry I llave been obliged to take fo much compaSs, to defcribe and explain a very f1mple in- Ilrunlent; but as I meant at the fame time to give fome idea of what is to be expeEted from it, I thollght

*t



[ 2IX ]

lt more excufable to lze prolix than not Sediciently explicit.

March 2 Ix I77 rw J. Smeaton.

P. S. It is to bDe noted, that, after each readjull- ment, thougll the lzygrometers would generally within a few hours come llear their point, yet it was not till the next day that they could be depended ont as having come to their neareflc agreementr

Ee s XXV. Let^t-



description of a new hygrometer: by mr. john smeaton, f. r. s

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