PLA

305

PLA

This

a ° n 'Which the sphere is, as it were, flattened.

A db a the plane of the projection.

Ven.T*^ 6 plane is only a plane of projection placedHich *h * e e ^ e an( l the object, so as to contain all the pointslrt) Ptesa fi Sever al rays, drawn from the object to the eye,Ane 0 f * er eon. But in planispheres or astrolabes, the■>'e spti e >e P ro jection is placed beyond the object, which iscircl® G * Pbe plane of the projection is always one of.. A-rtionr, 8 °f tb ® sphere.

i er ent S i 6 ^ n fi n ite number of planispheres which thele ey e P anes of projection, and the different positions of^ferrea , furnish, there are two or three that have beenp . e of o °. tbe r est. Such are that of Ptolemy , where the* r >siu Si P ^?j ection > is parallel to the equator ; that of Gemmau' st 'tial tn B1 - e . tbe pl ane projection is the colure, ori^' a t of er |dian, and the eye the pole of the meridian;

at

* ut anv* mo eye me yuic vi me rncnumu ,

a I11 erid;! n Hoyas, a Spaniard, whose plane of projection

■r. a n inft'" 11 ’ an< l Ae eye placed in the axis of that meridian,con? mte distance. ~

’ st °rt an? 01 ? defect of

This last is called the analemma.all these projections is, that they

ui mi mese projections is, mat mey

d° l 6as y t a ' ter tbe dgure of the constellations, so that it ispiS r ee a j n ° Com pare them with the heavens ; and that the0p ^Per a t; 0 80nrle places are so small, that they afford no room

|?‘jecti 0 ^" e fau, ‘s M. de la Hire provided against in a new? 30 Plac ? r P' an ' s pltere ; where it is proposed the eye shallB a H be go •v*' 8 tbat the divisions of the circles projected .1 ! ? e of i,j nSl x equal in every part of the instrument. Thea]] Projection is that of a meridian.

1 t'lobej. ’ ' l0 m the French , planche) a general name fora /° ar in ^ Xce P t ’ n o fir, which is from one inch and a halfp ttr d. C les *h'ck: if of less dimensions, it is called

Hic h A *K-Ho,

°cca

- nav? 00K ’ a P 0 ^ with an iron hook at its end, withw si °n s hift their runs, or wlieeling-planks, as

,, P, -ANK-q"' reS -

f 0 P ,° u Sht i ln t '. KElts > or, Plan-Sheers, the pieces of plankcj^tle, riZ0n tally over the heads of the timbers of thecoi? r ' n 8 't( 1 Uai ter ‘deck, and round-house, for the purpose ofp£V6oar d t0 P tb e s 'de ; hence, sometimes called

^NG, in architecture, denotes the laying the first° a11 the °? e ° n tbe P oun dation, according to the measures,

■■m alt on the foundatio

P LAq-in. exactness possible.Hie, s -R, (from vXaito) in

Wc„ 80na 'ttim., X ■ -

son, ::**> V u- om trXa^ii)) in building, a composition of^get tln >es with hair, sometimes with sand, &c. to• p t.As T J“° Ver the nudities of a building.

? Nldin! ° F hhtRig, a fossile stone, serving many purposes? a k e at,** ; a "d used likewise in sculpture, to mould andlecture® 8 ’ basso - relievos, and other decorations in

'-Vs °V t of quarries, in several parts of the neighbour-

! When- J.-- m - . . '

The finest is that of

f d

p, l,tl artr e ''^hence its name.

\ c kf Of p* <W»I.

or jl'' 18 ,’ am ongst our workmen, is of two kinds,f l'iai. r ° C ’ u de j s t | e ®tone, and burnt , or beaten.t '° n >L i i' 11 Win'll t Native plaster, as it comes out of theot„*_ j t j a use< j as ghards in the founda-

the

—

?*tto p h like P j‘ lste ^ is a preparation of the former, by cal-sq ^der, and'a-i' 1 '. a b >ln or furnace, and then beating itt|,j 8 Mortar r . 1 uting and working it. In this state it is^h en ’ W pi, Cei ? ent > in building.

18 Used i S1 ^ ef i> and reduced into an impalpablegt>. fe > and ; a v° ?° m! >ke figures, and other works ofVot 8e> &c. i n S ’ “Oxides, of seme use in taking out spotst -H. m stuffs and silks

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The method of representing a face truly in plaster of Parisis this : the person, whose figure is designed to be taken, islaid on his back, with any convenient thing to keep off thehair. Into each nostril is conveyed a conical piece of stiffpaper, open at both ends, to allow respiration. These tubes,being anointed with oil, are supported by the hand of anassistant; then the face is lightly oiled over, and the eyesbeing kept shut, alabaster fresh calcined, and tempered to athinnish consistence with water, is, by spoonfuls, nimblythrown all over the face, till it lies near the thickness of aninch. This matter grows sensibly hot, and, in about a quarterof an hour, hardens into a kind of stony concretion ; which,being gently taken off, represents, on its concave surface, theminutest parts of the original face. In this a head of goodclay may be moulded, and therein the eyes are to be opened,and other necessary amendments made. Tiiis second facebeing anointed with oil, a second mould of calcined alabasteris made, consisting of two parts joined lengthwise along theridge of the nose ; and herein may be cast, with the samematter, a face extremely like the original.

If finely-powdered alabaster, or plaster of Paris, be putinto a bason over a fire, it will, when hot, assume the appear-ance of a fluid, by rolling in waves, yielding to the touch,steaming, &c., all which properties it again loses on thedeparture of the heat ; and being thrown upon paper, willnot at all wet it, but immediately discover itself to be asmotionless as before it was set over the fire; whereby itappears, that a heap of such little bodies as are neitherspherical, nor otherwise regularly shaped, nor small enoughto be below the discernment of the eye, may, without fusion,be made fluid, barely by a sufficiently strong and variousagitation of the particles which compose it ; and, moreover,lose its fluidity immediately upon the cessation thereof.

Two or three spoonfuls of burnt alabaster, mixed up thinwith water, in a short time coagulate, at the bottom of avessel full of water, into a hard lump, notwithstanding thewater that surrounds it. Artificers observe, that thecoagulating property of burnt alabaster will be very muchimpaired, or lost, if the powder be kept too long, especiallyif in the open air, before it is made use of; and when it hathbeen once tempered with water, and suffered to grow hard,they cannot, by any powdering of it again, make it service-able for their purpose as before.

This matter, when wrought into vessels, &c., is still of soloose and spungy a texture, that the air has easy passagethrough it. Mr. Boyle gives an account, among his experi-ments with the air-pump, of his preparing a tube of thisplaster, closed at one end and open at the othe>, and, onapplying the open end to the cement, a3 is usually done withthe receivers, it was found utterly impossible to exhaust allthe air out of it ; for fresh air, from without, pressed in asfast as the other, or internal air, was exhausted, though thesides of the tube were of considerable thickness. A tubeof iron was then put on the machine; so that, being filledwith water, the tube of plaster of Paris was covered with it;and, on using the pump, it was immediately seen, that thewater passed through into it as easily as the air had done,when that was the ambient fluid. After this, trying it withVenice turpentine instead of water, the thing succeeded verywell; and the tube might be perfectly exhausted, and wouldremain in that state several hours. After this, on pouringsome hot oil upon the turpentine, the case was much altered;for, the turpentine melting, it became a thinner fluid, and,ir. this state, capable of passing like water into the pores ofthe plaster. On taking away the tube after this, it wasremarked that the turpentine,-which had pervaded and filledits pores, rendered it transparent, in the manner that water

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