C 789 ) 
from the other Points of the Parabola than the fufpen- 
ded Line. 
From hence I conclude, that a ponderous and pliable 
5ubftance being fufpcnded on a Ring or hollow Cylin- 
der, fo as that its Central Parts my defceod^ will form 
it felf into a Figure that is more commodius for Burning* 
Glalles than the Sphserical, of which they are now made, 
being much nearer their moft abfolute Figure the Pa- 
rabola. 
Now if there may be a way found to give to Cloth or 
Leather a Metalline Surface, or a Varniil that may bear 
a good Poliiih • or if this be found impradicabk, perhaps 
Plates of Metal may be beat out fo thin, as being fuf- 
pended on a large Ring, will by their own Gravity re- 
ceive their true Figure, one may make Speculums of 
what Largenefs he pleafeth, and there will be another 
Property in them, which makes it very defirable, that 
one and the fame Speculum will be mutable into all De- 
grees of Concavity, and fo have its focal Length in- 
creafed or dimimihed, according to the Purpofe 'tis de- 
figoed for. 
Not long after I had made ihefe Obfervations, Idevi- 
fed the following Experiment. 
There was taken a fufficient Quantity of Pptters-Clay, 
of which there was formed a plain Circular Plate, by 
help of an Iron Ring about 13 Inches Diameter. This 
was laid on a lelTer Ring, which was fupportedby Four 
Feet, and it immediately became of a very regular Con- 
cave on its upper, and Convex on its under Superficies : 
but notwithftanding 'twas fet to drie in the Shade, yet 
before it was dry enough, its Central Parts extended fo 
as to become almoft plain, not without fome Defcdis; 
if it had connnued in its Regularity, I defigned to have 
burned and glazed it in a Potters Furnace. But I have 
fince had a Concave Plate of Clay, which I formed by 
B b b b b b hand 
