Having premised this, he discourses upon Mr. _Hook_ his _Turne_, intimating first of all, that he was impatient to know what kind of _Turne_ this was, imagining, that it had been tried, and had succeeded, as coming from a Society that professeth, they publish nothing but what hath been maturely examin'd. But that he was much surprised when he saw the _Micrography_ of Mr. _Hook_, and found there, that his _Engine_ was published upon a _meer Theory_, without having made any Experiment, though that might have been made with little charge and great speed; expence of Money and Time being the onely thing, that can excuse those who in matter of _Engines_ impart their inventions to the publick, without having tried them, to excite others to make trial thereof. Whereupon he proposes some difficulties, to give the _Inventor_ occasion to find a way to remove them. He affirms therefore, that though it be true in the _Theory_, that a _Circle_, whose _Plain_ is inclined to the _Axis_ of the _Sphere_ by an _Angle_, whereof half the _Diameter_ is the _Sine_, and which touches the _Sphere_ in its _Pole_, will touch in all its parts a _spherical Surface_, that shall turn upon that _Axe_. But that it is true also, that that must be but a _Mathematical Circle_, and without _Breadth_, and which precisely touches the Body in its middle: Whereas in the practice, a _Circle_ capable to keep Sand and Putty, must be of some _breadth_; and he knows not whether we can find such a dexterity of keeping so much of it, and for so long a time, as needs, upon the Brim of a _Ring_ that is half an Inch broad. He adds, that it is very difficult to contrive, that the middle of the Glass do always precisely answer to the Brim of this _Ring_, seeing that the position of the Glass does always change a little in respect of the _Ring_, in proportion as 'tis worn, and as it must be pressed because of its inclination. He believes it also very hard, to give to the _Axis_ or to the _Mandril_, which holds the Glass, that little {59} _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 exceedingly 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.