the. Aperture, the Glass must be ground of a very large sphere; for, by
that means, the longer the Glass be, the bigger aperture will it bear, if
the Glasses be of an equal goodness in their kind. Therefore a six will
indure a much larger Aperture then a three foot Glass, and a sixty foot
Glass will proportionably bear a greater Aperture then a thirty, and will
as much excel it also as a six foot does a three foot, as I have
experimentally observ'd in one of that length made by Mr. _Richard Reives_
here at _London_, which will bear an Aperture above three inches over, and
yet make the Object proportionably big and distinct; whereas there are very
few thirty foot Glasses that will indure an Aperture of more then two
inches over. So that for _Telescopes_, supposing we had a very ready way of
making their Object Glasses of exactly spherical Surfaces, we might, by
increasing the length of the Glass, magnifie the Object to any assignable
bigness. And for performing both these, I cannot imagine any way more
easie, and more exact, then by this following Engine, by means of which,
any Glasses, of what length soever, may be speedily made. It seems the most
easie, because with one and the same Tool may be with care ground an Object
Glass, of any length or breadth requisite, and that with very little or no
trouble in fitting the Engine, and without much skill in the Grinder. It
seems to be the most exact, for to the very last stroke the Glass does
regulate and rectifie the Tool to its exact Figure; and the longer or more
the Tool and Glass are wrought together, the more exact will both of them
be of the desir'd Figure. Further, the motions of the Glass and Tool do so
cross each other, that there is not one point of eithers Surface, but has
thousands of cross motions thwarting it, so that there can be no kind of
Rings or Gutters made either in the Tool or Glass.

The contrivance of the Engine is, only to make the ends of two large
_Mandrils_ so to move, that the Centers of them may be at any
convenient distance asunder, and that the _Axis_ of the _Mandrils_