283

Feb: 13. 1678/9.

mr. Henshaw the vice Prest. tooke the chair .

vpon the Reading the minutes of February the 6th. the Society
againe entered vpon the Debate concerning the causes and Reason
of the motion of the mercury of the Barometer which . . .
And It was Conceiued that the reason of it did proceed from the Gra
uitation of the air which did at some times Presse more and
at some times Lesse. This variety of Pressure was conceiued
to proceed from two Causes the one was that the air at such time
as the quicksiluer rose higher had a new accession of air at
the top thereof which caused it to haue a greater height and
consequently a greater pressure, And the other was that there
were new accessions to the air from the Earth which made
the same height of air haue differing grauitation and
consequently differing pressure, the former being explained
by a cylinder of the air of greater altitude whose parts con
tinued of the same grauitation, the other by a cylinder of
the Air of the same or possibly lesser altitude but the
parts thereof greater grauity and more Dense. This was
further explayned by shewing that the heat and cold
working vpon the same cylinder /quantity/ of the air, though
it would make the same cylinder of a greater &
Lesser altitude according as the heat did expand it
and the cold did contract it, and yet not at all
alter the pressure thereof, there being in both
the same quantity of grauitating parts -
The substance of this Discourse mr. Henshaw moued
might be drawn vp, and that the whole theory
as soon as could be, might be fully explained it
being become an Instrument of Generall Vse
and the causes & reasons thereof very generally Com
monly Debated amongst the Learned. -

Ref: CELL/RS/HF_285  © Centre for Editing Lives and Letters