good one, when it is an Inch or two out of it. And I have a good one by me at present, of 36. foot, that will bear an Aperture, if Saturn or the Moon in the twilight, be look'd on with it, of 31 Inches over, and yet the thickest part of the Glass is a great way out of the middle. And I must take the liberty to doubt, whether ever my Animadversor saw a long Glass, that was otherwise; as he might presently satisfie himself by a way I could shew him (if he did not know it) whereby the difference of the thickness of the sides might be found to the hundreth part of a Line.
As to the exceeding exactness of the Figure of Long Object-Glasses, 'tis not doubted, but that it is a matter difficult enough to be attained any way: but yet, I think, much easier by Engine than by Hand; and of all Engines, I conceive, none more plain and simple, than that of a Mandril. And for making spherical Glasses by an Engine, I am apt to think, there hardly can be any way more plain, and more exact, than that which I have described; wherein there is no other motion, than that of two such Mandrils, which may be made of sufficient strength, length, and exactness, to perform abundantly much more, than I can believe possible to be done otherwise than by chance, by a man's hands or strength unassisted by an Engine, the motion and strength being much more certain and regular. I know very well, that in making a 60. foot Glass by the strength of the hand, in the common way, not one of ten that are wrought, will happen to be good, as I have been assured by Mr. Reeves; who, I am apt to think, was the first that made any good of that length. For the Figure of the Tool in that way is presently vitiated by the working of the Glass, and without much gaging will not do any thing considerable. Besides, the strength of a man's hands, applied to it for the working and polishing of it, is very unequal, and the motions made, are very irregular; but in the way, I have ventured to propose, by Mandrils, the longer the Glass and Tool are wrought together, the more exact they seem to be, and if all things be ordered, as they should be, the very polishing of the Glass, does seem most of all to rectifie the Figure.
As to what he objects, that the Tool does only touch the Glass in a Mathematical Circle; that is true, perhaps, at first, but before the Glass is wrought down to its true Figure, the Edge of the Tool