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• Ä Lirmcn Cloth, bcing first wet ia fair Water, and then laid onbolick Fi»«; a ConcaveCylindcr, as the Vcrge of a Seivc, Keeler, or the like, its CenaiiemftcA by tral Parts will delcend so as to form a very regulär Concave Superficies. AndCixay. 1 ^ CU 11 Thrcad, bcing first v.'et in common Water, and theil fulpended with its

N. V2«. f. 542. two ends, or any two Points nearer than thcir utmost extern, so as it might

O. 235. f- 787. touch the Center of the ftlsoertded Cloth, and its two opposite Points on the

Ring, was found to hä've the fame Curvature. My Business was then to ex-amiiie the Figure of the Thread thus fulpended, which 1 did in mannet föl-lowing j On the side ol a Wall I defcribed Parabola s of leveral Species,whole Ax'cs were Perpendicular, and Perimeter Horizontal, to which the Linebcing applied, so as it might touch the Vertex, past very nearly throngh allthe intermediäre Points ot the Parabola , much nearer than the Portion of aCircle, which past through the Extremity of the Perimeter and Latus Rectum,wotiId do.

Front hence I conclude, That a ponderous and pliable Substance, bcingfulpended on a Ring or hollow Cylinder, so as that its Central Parts may• delcend, will form it seif into a Figure that is more commodioris for Burning-Glastes than the Spherical, of which they are now made, bcing much nearerthcir most absolute Figure the Parabola.

Now if there may be a way found to give to Cloth or Leather a Metal-line Surfacc, or a Varnish that may bear a good Polisti .; or if this be foundImpracticable, nerhaps Plates 61 Metal may be beat oüt so thin, as bcingfulpended on a large Ring, will by theil - own Gravity veceive theil - true Fi-gure, one nray make Speculums of wliat largenels he plealeth.

Upon this Consideration, 1 deviled the following Experiment. There wastaken a sufficient epiantity of Pottess Clay, of which there was fornted aplain circular Platc, by help of an Iron Ring about 13 lnches Diameter.This was laid on a lesser Ring, which was fupported by four Feet, and itintmediately became of a very regulär Concave on its upper, and Convex ohits under Superficies : but notwkltstanding ’twas fet to dry in the Shade, yetbefore it was dry enough, its Central Parts extended so as to beconte alntostPlain, not without lorne Defects ; if it had continued in its Regulavity, I de-ligned to Itave burned and gkzed it in a Potter's Furnace.

To miethe XXIX. Take Quick-silver, Marcbasite of Silver, each tltree Ounces ;Tin«;/<■!«• Loobing- 2itd Lead, each half an Ounce ; titele two first throw on the Marchasite,r ' sJuthwd! and last of all the Quick-silver: stir thent well together, but they must betaken front the Fire, and be towards Cooling before the Quick-filver be ad-ded j let your Gkls be well warmed, then pour in the Mixture, and roll itfront side to hdc. _

Narr, This will do also when Gold, but ’tis best when the Glas 13 heatea

and very dry.

Sotc also , That if at the Glaß-Houfc, your Ball be of yellow Glusi, thenall will ffiine like Gold.

XXX. Paper;