(59) 
Imlimtm, that would be neceffary for great GlafleSj to 
make the two Mandrils to have one and the fame Plains as is ne- 
cejQTary. Andjhaving done all thiSj he perfuades hiralelf, that it is 
exceeding difficult, if not irapoflible, for two contrary motions^ 
where fo many pieces are, to reft for a long tiinefteddy and 
firm, as is requifite for the not fwarving from it a hair's thick- 
nefsj fince lels than that can change ali. 
He goes on, and, feeing that this Inventor fpeaks of GlafTes of a 
thouland,&ten thoufand foot^which he fuppofed not impoflible 
to be made by this Engine, difcourfes of what is neceifary for 
the making Glaffes of fuch bignefs 5 which he believes this hu 
vrntorm^^ perhaps not have thought of. Wherefore he affirms, 
that if theT^/^/r^made by himfeh for the Apertures of Glaffes 
(which is that, that is above delivered) be continued unto a 
thoufand feet, by taking always the Subduplicate proportion o£ 
Lengths^ it will be found^that for pretty good ones, the Aperture 
muft be of I 5. Inches- for good ones, more than 18. and for 
fuch as are excellent, more than 21. Inches: whence itmaybe 
judged, what piece of Glafs, and of what thicknefs it muft be,to 
refiit the workings But he proceeds to fpeak of the Inclination^ 
which xki^Mandnt muft have upon \\\z?lain of theK/wg,when the 
i^^>2^ fliouldhave lo.or 12 lnches>and finds,that it would make 
but 6 <or. 7. minutes of inclination, and that a Glafs would have 
Icfs Convexity y^xa^ Gonfequentlyjlefs difference from a Glafs per- 
fed:lyplain,than the 7. or 8. part of a Line. And then he leaveth it 
to be judgedjwhether a Glafs of fuch a Length being found, we 
ought to hope, that a Turn can be firm enough to keep fuch a 
piece of Glafs in the fame Inch'nation, fo that a Mandril do nor 
recede fome Minutes from it : and, though even the Glafs cou!d 
be faftned perfecSlly perpendicular to xh^Mandrtl^ that thefe 
two Mandrils could beput'in one and the fame place,& that that 
little Inclination, which is requifite^ could be given, and the 
i1</«?2^n7 be continued to be prefTed mth.^x.fhmQ Inelinaiion, ac- 
cording as the Glafs is worn. All which particulars, he conceives 
to be very hard in the pradice ^ not to mention, that the weight 
of the Glafs,that fliould be inclined to the Horizon, as 'tis repre- 
lented by Mr. HgoI<^^ would make it Aide upon the ^>;7fi?;2^3and fo 
chance 
