Inclination , that would be neceffary for great Glaffes, and to 
make the two Mandrils to have one and the fame Plain, as is ne- 
:effary. And, having done all this, he perfuades himfelf, that it is 
sxceeding difficult, if not impoffible, for two contrary motions, 
vhere fo many pieces are, to reft for along timefteddy arid 
irm, as is requifite for the not fwarving from it a hair’s thick- 
aefs, fince lels than that can change all. 
He goes on 5 and, feeing that this Inventor fpeaks of Glaffes of a 
houfand^ten thoufand foot,which he fuppofed not impoffible 
o be made by this Engine , difconrfes of what is neceffary for 
he making Glaffes of fuch bignefs 5 which he believes this /»- 
,entor may perhaps not have thought of. Wherefore he affirms, 
hat if the Table, made by himfelf for the Apertures of Glaffes 
which is that, that is above delivered) be continued unto a 
houfand feet, by taking always the Subduplicate proportion of 
lengths, it will be found,that for pretty good ones, the Aperture 
nuftbeof 15. Inches; for good ones, more than IS. and for 
uch as are excellent, more than 21. Inches : whence it may be 
udged,what piece of Glafs, and of what thicknefs it muft be, to 
efift the working* But he proceeds to fpeak of the Inclination , 
vhichthc Mandril muft have upon \ho.Plain of theAtng.when the 
^j n r, ffiouldhave ro.or 12 Inches; and finds, that it would make 
nit 6 <or. 7. minutes of inclination, and that a Glafs would have 
efs Convexity, and confequently,lefs difference from a Glafs per- 
: edrly plain, than the 7. or 8 .part of a Line. And then he leaveth it 
o be judged, whether a Glafs of fuch a Length being found, we 
night to hope, that a Turn can be firm enough to keep fuch a 
>iece of Glafs in the fame Inclination, fo that a Mandril do not 
■ecede fome Minutes from it: and, though even the Glafs could 
se faftned perfe<ftly perpendicular to th eMandnl, that thefe 
wo Mandrils could beput'in one’and the fame place, & that that 
itde Inclination, which is requifite, could be given, and the 
MandrilhQ continued to be preffed in that jfame Inclination, ac- 
:ording as the Glafs is worn. All which particulars, he conceives 
;o be very hard in the pra&ice $ not to mention, that the weight 
sf the Glafs,that fhould be inclined to the Horizon, as ’tis repres- 
ented by Mr. Bool ^ would make it Aide upon the Cement, and fo 
chance 
