He goes on, and, seeing that this _Inventor_ speaks of Glasses of a thousand, & ten thousand foot, which he supposed not impossible to be made by this _Engine_, discourses of what is necessary for the making Glasses of such bignesses; which he believes this _Inventor_ may perhaps not have thought of. Wherefore he affirms, that if the _Table_, made by himself for the _Apertures_ of Glasses (which is that, that is above delivered) be continued unto a thousand feet, by taking always the _Subduplicate proportion_ of _Lengths_, it will be found, that for pretty good ones, the _Aperture_ must be of 15. Inches; for good ones, more than 18. and for such as are excellent, more than 21. Inches: whence it may be judged, what piece of Glass, and of what thickness it must be, to endure the working. But he proceeds to speak of the _Inclination_, which the _Mandril_ must have upon the _Plain_ of the _Ring_, when the _Ring_ should have 10. or 12 Inches; and finds, that it would make but 6 or 7. minutes of inclination, and that a Glass would have less _Convexity_, and consequently, less difference from a Glass perfectly plain, than the 7. or 8. part of a Line. And then he leaveth it to be judged, whether a Glass of such a Length being found, we ought to hope, that a _Turn_ can be firm enough to keep such a piece of Glass in the same Inclination, so that a _Mandril_ do not recede some Minutes from it: and, though even the Glass could be fastned perfectly perpendicular to the _Mandril_, that those two _Mandrils_ could be put in one and the same Plain, & that that little Inclination, which is requisite, could be given, and the _Mandril_ be continued to be pressed in that same _Inclination_, according as the Glass is worn. All which particulars, he conceives to be very hard in the practice; not to mention, that the weight of the Glass, that should be inclined to the _Horizon_, as 'tis represented by Mr. _Hook_, would make it slide upon the _Cement_, and so {60} change the _Center_; and that the Glass is not pressed at the same time by the _Ring_ but in one part on the side, _vid._ about a fourth; and that the parts of the Glass are not equally worn away, &c. What then, _saith he_, would becom of a Glass of 10000 feet, which, according to the said Table, would have more than four feet, or four feet and nine inches, or five feet, seven inches _Aperture_, and of which the _Ring_, though it were two feet nine inches, would have but one minut of _Inclination_, and the Glass of 5 feet _Aperture_ would have but 4 minuts, and the curvity of it would be less than the eight part of a Line.