is Observation, because in Fact the
Signs here mentioned are no other than Nature's apparatus for a Storm
of Thunder and Lightning, which will be perfectly understood by
attending a little to the Causes of these Meteors. Lightning is a great
flame, very bright, extending every way to a great distance, suddenly
darting upwards, there ending, so that it is only momentaneous. The
Matter which produces the Fire, is the Oil of Plants, attenuated by the
heat of the Day, and raised on high. Then whatever has exhaled from the
Earth that is sulphureous or Oily, which is dispersed up and down in
the Atmosphere, and is not continuous, is set on Fire by Turns, and the
Flame dilates itself as far as the Tract of that Exhalation reaches.
Some other Substance pendant and floating in the Air meets with this
also, with which it excites an effervescence, takes Fire and flashes
along with it. Thunder is another bright Flame, rising on a sudden,
moving with great Velocity through the Air, according to any
Determination upwards from the Earth horizontally, obliquely, downwards
in a right Line, or in several right Lines as it were in serpentine
Tracts joined at various Angles, and commonly ending with a loud Noise
or Rattling.

IT is observed that it thunders most when the Wind blows from the
South, and least when it blows from the East. The great Principle of
Thunder is Sulphur, as is evident from the Smell it leaves behind it;
but in order to occasion such an Explosion, there must be other
Ingredients mixed therewith, especially Nitre, of which the Air is
always full, besides other Things, of which it is impossible to give
any Account. The Tracts of this Sort of Matter fly about in the Air,
and are as it were Lines of Gunpowder, and as in the firing of that
Powder, the Fire begins at one End, and pursuing its Aliment proceeds
to the other Extremity, and so the whole Mass of Powder is fired; we
may from thence account for the Phaenomenon of Thunder. For in like
Manner those inflamed Tracts which are suspended in the Air, flash from
a Flame that runs from one Extreme to the other, wherever the Vein of
Nourishment leads it. Hence those Rays of Thunder, which seem to be
brandished through the Air, and sometimes to be split in two or more
Tracts, and sometimes to return back, at other Times to be projected in
Lines that are joined by various Angles, and this only because the
Flame meets with Tracts lying in various Situations that cohere one
w