Page:Philosophical Transactions - Volume 001.djvu/63

 Inclination, that would be necessary for great Glasses, and to make the two Mandrils to have one and the same Plain, as is necessary. And, having done all this, he persuades himself, that it is exceeding difficult, if not impossible, for two contrary motions, where so many pieces are, to rest for a long time steddy and firm, as is requisite for the not swarving from it a hair's thickness, since less than that can change all.

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 ; 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 he judged what piece of Glass, and of what thickness it must be, to 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 two Mandrils could be put in one and the same, & 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 Rh