em, may spoil the best _Mould_,
before they be finished. Besides, that the strength of Man is so limited,
that he is unable to work Glasses beyond a certain bigness, so as to finish
and polish them all over so well, as _small_ Glasses; whereas yet, the
bigger they are, the more compleat they ought to be: And if any weight or
Engine be used to supply strength, there is then danger of an unequal
pressure, and of wearing away the Engine; In the mean time, the preciseness
and delicateness is {58} greater than can easily be imagined. Wherefore he
could never, having some experience of this preciseness, conceive, that a
_Turn-lathe_, wherein must be two different, and in some manner contrary
motions, can move with that exactness and steddiness, that is required,
especially, for any considerable length of time.

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