240 M E M O I R S of the 
was found to have the fame curvature 5 in order to difcover the 
figure of the thread, thus fufpended, Mr. Gray defcribcd 
feveral fpccies of parabola's, whofe axes were perpendicular, 
and perimeters horizontal 5 to which applying the line, fo as 
to touch the vertex, it paffed very nearly thro' all the inter- 
mediate points of the parabola, much nearer than the portion 
of a circle, which pafled thro' tho extremity of the perimeter, 
and Zatus Return would do. 
From hence he concludes, that a ponderous and pliable 
fubflance, being fufpended on a ring, or hollow cylinder, fo as 
that its central parts may defcend, will form itfelf into a figure 
that is more commodious for burning-glafles than the fpherical, 
of which they are now made, coming much nearer their moft 
abfolute figure, the parabola : !Now, if a way could be found 
to give cloth, or leather, a metalline furface, or a varnifli that 
might bear a good polifli 5 or if this be thought impra6^icable, 
if plates of metal were beat out fo thin, as that being fufpend- 
ed on a large ring, they would by their own gravity receive 
their due figure, Specula might be made of any largenefs: 
Upon this confideration, Mr. Gray devifed the following expe- 
riment 5 he took a fufficient quantity of potter's clay, of which 
he formed a plane circular plate, by means of an iron ring 
about i; inches diameter; this was laid on a lefler ring, fup- 
ported by four feet, and it immediately became a very regular 
concave on its upper, and convex on its under furface ^ but 
notwithftanding it was fet to dry in the fhade, yet before it 
was dry enough, its central parts werefo extended as to become 
almoft plane> not without fome defects; if it had continued in 
its regularity, Mr. Gray defigned to have burned and glazed it 
in a potter's furnace. 
77:7^ Height of the Mercury at the ^op of the Monument ; 
find a portable Barometer; hy Mr. Will. Dcrham. Phil. 
Tranf N° 29^. p. 2. 
IN September, 1696, Mr. ^erham obfcrved the variation of 
the mercury on the Momwient, and found, by one of 
Mr. §hmre's belt portable barometets, that it defcended |o of 
an inch at the height of 80 foot, and -f^r at \6o foot; bur fince 
that, finding his o'blervations a lirtle different from Mr. Ilalley's 
on Sno^ivdofi Hilly he tried it again more nicely, in November^ 
1(^97, in this manner; he provided a pretty large glafs tube, 
well cleaned, which he cafei^ in wire, and filled it with well 
ftrain;.d mercury, which being cleared of air, he then plunged 
the 
