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 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_ {67} will be worn or grownd away, so as that a Ring of an inch broad may be made to touch the _Spherical Surface_ of the Glass; nay, if it be necessary (without much trouble, especially in the grinding of longer Glasses) the whole _Concave Surface_ of the _Tool_ may be made to touch a Glass. Besides, that as to the keeping a quantity of the same sand and Powders of several finesses, according as the glass wears, the same is possible to be don, as with the same Sand wrought finer by working in the Ordinary way. The giving the _Inclination_ to the _Mandrils_, is not at all difficult; though perhaps to determine the length exactly which the Glass so made shall draw, is not so easie: But 'tis no matter, what length the Glass be off, so it be made good, whether 60 or 80 foot, or the like. Nor is it so very difficult, to lay them both in the same _Plain_. And to keep them _steddy_, when once fix'd, is most easie.