through the Poppet head C, is screwed a Ring of a hollow
_Cylinder_ K, or some other conveniently shap'd Tool, of what wideness
shall be thought most proper for the cize of Glasses, about which it is
to be imploy'd: As, for Object glasses, between twelve foot and an
hundred foot long, the Ring may be about six inches over, or indeed
somewhat more for those longer Glasses. It would be convenient also and
not very chargeable, to have four or five several Tools; as one for all
Glasses between an inch and a foot, one for all Glasses between a foot
and ten foot long, another for all between ten and an hundred, a fourth
for all between a hundred and a thousand foot long; and if Curiosity
shall ever proceed so far, one for all lengths between a thousand and
ten thousand foot long; for indeed the principle is such, that
supposing the _Mandrils_ well made, and of a good length, and supposing
great care be used in working and polishing them, I see no reason, but
that a Glass of a thousand, nay of ten thousand foot long, may be as
well made as one of ten; for the reason is the same, supposing the
_Mandrils_ and Tools be made sufficiently strong, so that they cannot
bend; and supposing the Glass, out of which they are wrought, be
capable of so great a regularity in its parts as to refraction: this
hollow _Cylinder_ K is to contain the Sand, and by being drove round
very quick to and fro by means of a small Wheel, which may be mov'd
with ones foot, serves to grind the Glass: The other _Mandril_ is
shap'd like this, but it has an even neck instead of a taper one, and
runs in a Collar, that by the help of a Screw and a joynt made like M
in the Figure, it can be still adjustned to the wearing or wasting
neck: into the end of this _Mandril_ is screwed a Chock N on which with
Cement or Glew is fastned the piece of Glass Q that is to be form'd;
the middle of which Glass is to be plac'd just on the edge of the Ring
and the Lath OP is to be set and fixt (by means of certain pieces and
screws the manner whereof will be sufficiently evidenc'd by the Figure)
in such an Angle as is requisite to the forming of such a Sphere as the
Glass is design'd to be of; the geometrical ground of which being
sufficiently plain, though not heeded before, I shall, for brevities
sake, pass over. This last _Mandril_ to be made (by means