rst Part, together with Instructions teaching the
manner of using them.

_Thirdly_, That Astronomers will find in this Book many very remarkable
things, concerning the _Apparent Diameter of the Sun_ and the other Stars,
the Motion of the _Libration of the Moon_, the _Eclipses_, _Parallaxes_,
and _Refractions_: And that this Author shews, that there is a great
difference between _Optical_ and _Astronomical_ Refraction, which _Tycho_
and many others have confounded; undertaking to prove, that, whereas these
_Astronomers_ have believed, that the remoter any Star is, the less is its
Refraction, on the contrary the Refraction is the greater, the more a Star
is distant. And among many other things, he ingeniously explicates the two
contrary Motions of the Sun, from East to West, and _vice versa_, by one
onely Motion upon a _Spiral_, turning about a _Cone_.

_Fourthly_, That he represents, How uneasie it is to establish sure
Principles of this Science, by reason of the difficulties of making exact
Observations. So, for example, in the Observation of the _Equinox_, every
one is mistaken by so many _Hours_, as he is of _Minutes_, in the Elevation
of the _Pole_, or the Diameter of the Sun, or the Refraction, or in any
other circumstance. In the Observation of the _Solstice_, the error of one
only _Second_ causeth a mistake of an _Hour_ and an _half_: mean time 'tis
almost impossible to avoid the error of a _Second_; and even the sharpest
sight will not be able to perceive it, except it be assisted with an
Instrument of a prodigious bigness. For to mark _Seconds_, though Lines
were drawn as subtil as the single threds of a Silk-worms Clew, (which are
the smallest spaces to be discerned by the sharpest Eye) by the Calculation
made by this Author there would need an Instrument of 48. feet _Radius_,
since Experience shews, that there needs no more at most, than 3600. threds
of Silk to cover the space of an _inch_. But, suppose one could have a
_Quadrant_ of this bigness, who can assure himself, that dividing it into
{396} 324000. parts (for so many _Seconds_ there are in 90. _Degrees_)
either in placing it, or in observing, he shall not mistake the thickness
of a single thred of Silk? He adds, that Great Instruments have their
defects, as the small ones: For in those, that are _Movable_, if the thred,
on which the Lead hangs, is any thing big, it cannot exactly mark
_Seconds_; if it be very fine, it breaks, because of its great length