_dry weather_, but especially when an _Eastern Wind_ (which having past
over vast tracts of Land is heavy with Earthy Particles) blows, it is much
more, though these changes are varied according to very odd Laws.

The Instrument is this. I prepare a pretty capaceous Bolt-head AB, with
a small stem about two foot and a half long DC; upon the end of this D
I put on a small bended Glass, or brazen _syphon_ DEF (open at D, E and
F, but to be closed with cement at F and E, as occasion serves) whose
stem F should be about six or eight inches long, but the bore of it not
above half an inch diameter, and very even; these I fix very strongly
together by the help of very hard Cement, and then fit the whole Glass
ABCDEF into a long Board, or Frame, in such manner, that almost half
the head AB may lye buried in a concave Hemisphere cut into the Board
RS; then I place it so on the Board RS, as is exprest in the first
figure of the first Scheme; and fix it very firm and steady in that
posture, so as that the weight of the _Mercury_ that is afterwards to
be put into it, may not in the least shake or stir it; then drawing a
line XY on the Frame RT, so that it may divide the ball into two equal
parts, or that it may pass, as 'twere, through the center of the ball.
I begin from that, and divide all the rest of the Board towards UT into
inches, and the inches between the 25 and the end E (which need not be
above two or three and thirty inches distant from the line XY) I
subdivide into Decimals; then stopping the end F with soft Cement, or
soft Wax, I invert the Frame, placing the head downwards, and the
Orifice E upwards; and by it, with a small Funnel, I fill the whole
Glass with Quicksilver; then by stopping the small Orifice E with my
finger, I oftentimes erect and invert the whole Glass and Frame, and
thereby free the Quicksilver and Glass from all the bubbles or parcels
of lurking Air; then inverting it as before, I fill it top full with
clear and well strain'd Quicksilver, and having made ready a small ball