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mil be worn or grownd away,fo as that a Ring of an ineh broad 
may be made to touch the^'/'^m^^/tS'^^r/^^^ of the Glafs ; nayj 
if it be neceffary^without much trouble,efpecially in the grind- 
ing of longer Glafses ) the whole Concave Surface of the Teol 
may be made to touch a Glafs..Befides,that as to the keeping a 
quantity of the fame fand and Powders of fevera! finefses , ac- 
cording as the glafs wears^ the fame is poffible to be dongas with 
the fame Sand wrought finer by working in the Ordinary way. 
The giving the Inclination to the Mandrils^ is not at all diffi-: 
cult; though perhaps to determine the length exadly which 
the Glafs fo made lhall draw, is not fo eafie ; But 'tis no matter, 
what length the Glafs be off, foicbe made good, whether 60 
or 8 o f ootj or the like. Nor is it fo very difficult, to lay them 
both in the fame Flain, And to keep ihtmfleddj^ when once 
fix'd^ is mofl eafie. 
As to the Calculation of the propriety of a Glafs of a thou« 
fand foot, perhaps for that particular Length, I had not, nor 
have as yet calculated, that the Convexity of one of eighteen 
inches broad^ will not be above a feventh part of a Line, But 
it does not thence follow 3 that I had not confidered the diffi- 
culties, that would be in making of it. For, I muft tell him^ that 
I can make a Piano convex Glafs though its convexity be a fmal- 
er fphere than is ufual for fuch a length to be an ObjeB-Glaf^ of a=. 
bout 1 5 o foot in Lengthg nay of 300 foot, and either longer or 
%iQvttr^.withmitzi all altering the convexity. So that^ if he will 
by any Contrivaoce he hath^give mQ^Plano-cenvex Glafs of 20^ 
or 40 foot Dinmeter ^without FeinT, and truly wrought of that F/- 
gure^l will prefently make a Tekfeope with ir^that with a fingle Ey- 
glafs fliall draw a thoufand foot : Which Invention, I fliall Ihortly 
difcov^er, there being, I think, nothing more eafie and certain. 
And if a Plano-convex Glafs can be made of mj Spher§ between 
twenty and fonrty foot radius^ fo as that both the Convex and 
PteV^ fide of the Glafs be exadtly polifli'd of a truef/Va^r^'^I will 
ftortlyfliewj how therewith may be made a T^elefcope of any 
Length, fuppofing the Glafs free from all kind of Feins, or ine- 
quality of T^fraBion, 
As for the fliding of the Glafs upon the Cemmt^ I fee no rea- 
fon at all for it, at leaft in Cement^ I make ufe of^ having ne- 
ver obferved any fuch accident in hard Cement. And 
