"The theory of the air seemeth at present," he says,
"to be perfectly well understood, and the differing
densities thereof at all altitudes; for supposing the
same air to occupy spaces reciprocally proportional to
the quantity of the superior or incumbent air, I have
elsewhere proved that at forty miles high the air is
rarer than at the surface of the earth at three thousand
times; and that the utmost height of the atmosphere,
which reflects light in the Crepusculum, is not fully
forty-five miles, notwithstanding which 'tis still
manifest that some sort of vapors, and those in no
small quantity, arise nearly to that height. An instance
of this may be given in the great light the
society had an account of (vide Transact. Sep., 1676)
from Dr. Wallis, which was seen in very distant counties
almost over all the south part of England. Of
which though the doctor could not get so particular a
relation as was requisite to determine the height thereof,
yet from the distant places it was seen in, it could
not but be very many miles high.
"So likewise that meteor which was seen in 1708, on
the 31st of July, between nine and ten o'clock at night,
was evidently between forty and fifty miles perpendicularly
high, and as near as I can gather, over Shereness
and the buoy on the Nore. For it was seen at London
moving horizontally from east by north to east by
south at least fifty degrees high, and at Redgrove, in
Suffolk, on the Yarmouth road, about twenty miles
from the east coast of England, and at least forty miles
to the eastward of London, it appeared a little to the
westward of the south, suppose south by west, and
was seen about thirty degrees high, sliding obliquely
downward. I was shown in both places the situation
thereof, which was as described, but could wish some
person skilled in astronomical matters bad seen it,
that we might pronounce concerning its height with
more certainty. Yet, as it is, we may securely conclude
that it was not many more miles westerly than Redgrove,
which, as I said before, is about forty miles more
easterly than London. Suppose it, therefore, where
perpendicular, to have been thirty-five miles east from
London, and by the altitude it appeared at in London--
viz., fifty degrees, its tangent will be forty-two miles,
for the height of the meteor above the surface of the
earth; which also is rather of the least, because the
altitude of the place shown me is rather more than
less than fifty degrees; and the like may be concluded
from the altitude it appeared in at Redgrove, near
seventy miles distant. Though at this very great
distance, it appeared to move with an incredible
velocity, darting, in a very few seconds of time, for
about twelve degrees of a great circle from north to
south, being very bright at its first appearance; and
it died away at the east of its course, leaving for some
time a pale whiteness in the place, with some remains
of it in the track where it had gone; but no hissing