within the body of the _Atmosphere_; which conclusion he
grounds upon this, that a _medium_, whose parts are unequally _dense_, and
mov'd by various motions and transpositions as to one another, will produce
all these {31} visible effects upon the rays of Light, without any other
_coefficient_ cause: and then, that there is in the Air or _Atmosphere,
such_ a variety in the constituent parts of it, both as to their _density_
and _rarity_, and as to their divers mutations and positions one to
another.

He concludeth with two _Celestial Observations_; whereof the _one_ imports,
what multitudes of Stars are discoverable by the _Telescope_, and the
variety of their magnitudes; intimating with all, that the longer the
Glasses are, and the bigger apertures they will indure, the more fit they
are for these discoveries: the _other_ affords a description of a _Vale_ in
the _Moon_, compared with that of _Hevelius_ and _Ricciolo_; where the
Reader will find several curious and pleasant Annotations, about the Pits
of the _Moon_, and the Hills and Coverings of the same; as also about the
variations in the _Moon_, and its _gravitating_ principle, together with
the use, that may be made of this Instance of a gravity in the _Moon_.

As to the _Inventions of Art_, described in this Book, the curious Reader
will there find these following:

1. A _Baroscope_, or an Instrument to shew all the Minute Variations in the
_Pressure of the Air_; by which he affirms, that he finds, that before and
during the time of rainy weather, the Pressure of the Air is less; and in
dry weather, but especially when an _Easterly_ Wind (which having past over
vast Tracts of Land, is heavy with earthy particles) blows, it is much
more, though these changes be varied according to very odd Laws.

2. A _Hygroscope_, or an Instrument, whereby the _Watery steams_, volatile
in the Air, are discerned, which the Nose it self is not able to find.
Which is by him fully described in the Observation touching the _Beard of a
wild Oate_, by the means whereof this Instrument is contrived.

3. An Instrument for _graduating Thermometers_, to make them _Standards_ of
_Heat_ and _Cold_.

4. A _New Engine_ for _Grinding Optick Glasses_, by means of which he
hopes, that any Spherical Glasses, of what length {32} soever, may be
speedily made: which seems to him most easie, because, if it succeeds, with
one and the same Tool may be ground an _Object Glass_ of any length or
breadt