Pe to 
The Preface. 
means,the lenger the Glafs be, the bigger aperture pill it bear.if the Glaffes’ 
be of an equal. goodnefs in their kind. Therefore a fix will indure a 
much larger Aperturethen a three foot Glaf : and a fixty foot Glafs will’ 
proportionably bear a greater Aperture tken a thirty,and will as much ex- 
celit allo as a. fix foot does athree foot, as] have experimentally obferv’d 
in one of that length mate by Mr. Richard Reives: here at’ London,’ ° 
which will. bear an Aperture above. three inches over, and. yet make. 
the Objet eer big and di iftintt ; ‘mbereas there are'very 
nail So that for Telefcopes, - fuppof ing we vba avery jh way 
= making” their Objeét a of exaétly fpherical Surfaces, we might, by 
- inereafing the length of the be Glaf?, magnifie the Obje6i to any affignable big. . 
nebo And for performing both thefe,  Icannot imagine any way more ea. 
fie,and more exati, then by this following Engine,by means of which, any | | 
Glaffes,of what length foever snay be _fpeedily made. It feemsthe moft eafie, 
becauife with one and the fame Tool may be with care ground an Object 
Glof, of any length or- breadth requifite, and that with very dittle or no 
trouble in fitting ‘the Engine’, ‘and without much skill in the Grinder. 
| f aes tobe the. moft exatt, for to,the very laft flroke the Glafs does, 4 
3 and reff the Tol tits exadt Figure 5) cand the longer‘or more. 
ie he Tol tall Go aré wrought together, the sore exa€t will: both of them 
be of the defir'd Figure... Further, the motions of the Glaf3. and Zool do’ 
fo crofs each other that there's not one’ point of eithers Surface,put bas 
thoufands of crof motions thwarting it, fo. that there can, dh ng aos q 
Rings or: Gutters made either in the Tool o Glas.» mae? om 
» ‘The contrivance of the Bnigitia! ig: rob to sealers sk ends bth two. wihidge q 
Mandrils{o't0 move ,. that the Centers of them'may’ be at any‘corvenient’ 
diftanceafunder , andthat the Axis of the eae: | lying both in'the fame 
plain et d, may meet each other in any affignable Angle's: both which 
requilites may be ~ well perform’d by the Engine deferib'd inthe third’ — 
Figurevof the firft Scheme : where AB fi ignifies the Beam of a Lath fixt per- 
pendicularly: eas C.Dythe two. Poppet heads, fixt at abouttwo 
foot diftance, an Iron Mandeil whole Fapering neck F'runs.in amada 
Siperiie brafs Collar: the other and Eyuns- on the: point of a Screw 
ina Convenient pie le faftned H a pully Wheel, and into'the end, of 
it,that comes throug ae Poppet head C, is. ferewed a Ring of ahollow — 
/ Cynder K; or fome other conveniently thap'd Tool, of what wideneds fhalf 
be 
a Re ee ee Se ae ee BO eso ae i i. 
