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neer the top. then a glasse made of the forme of an Inuerted syphon -
like the annexed figure and conteining at the bottom a pretty quan
tity of mercury, this ^ ___ tube being gradually sunk down into the water it
was very obuious how the pressure of the water vpon the in that part
of the syphon, which was open ^ /to the water/ did depresse it and rayse the same in the
other part wch. was open to the air and Excluded the water. And It was
plainly shewed that the cylinder of mercury kept vp by the pressure of the water
was always about a 14th part in length of the length of the water cylinder
between the surface of the Quicksiluer and the top of the water. It was far
ther Explaind by him and shewd that that the air and water in this Represented
in all circumstances materiall in this tryall the aether & air in the comon
Barometer, that the mercury was common in both. for as twas supposd that
the Barometer at the top of the cane admitted the aether & excluded the air
soe in this, it admitted the air & excluded the water -


He further shewd how the alteration of the specifique grauity of the air didalter the pressure though the cylinder or altitude of the pressing atmosphere
were the same, which to make more plaine by An Experiment, the fresh wa=
ter wth. wch. the greater glasse was filld was powred out & it was filld wth. a very
strong solution of salt. care being first taken to obserue the exact
comparatiue height of the mercuriall cylinder to the fresh water cylinder
and it now appeard very plaine that the same altitude of salt water
kept vp a cylinder of mercury much higher than did the fresh water -
Here by the way Also mr. Hooke shewd how the pressure of the air Decreasd as
by assending a mountaine approach is made neerer to the top, /of the air/ and he men
tiond aso also that he had formerly brought into this Society Diuers Discourses &
experiments by wch. he had manifested that the pressure of the air did actually
soe decrease in grauity according to the expt. was made further from the
center of the earth. euen after the same manner as the Decrease of the
pressure of the water was very visible in this Instrument. And also added that he
was preparing an instrument for some tryalls to that purpose which he would shortly shew.
These were an ^ /2/ other Instruments of the like nature wch those other he hath for
merly produced since their meeting. wch. /are/ was not Designed to shew one ^ /or two/ single Expt
and noe more but to be & constant & standing Instruments whereby all the
phenomena of Grauitation and pressure may be explaind, by hundreds of
Expts. -

Further when a query was made why the Longer end of the syphon was not seald
vp, but remaind open, which was otherwise in the Barometer. mr. Hooke explaind
the same and shewd that it was by reason the spring of the air that was included
would vary its pressure vpon the rising or falling of the cylinder of mercury -
which being left open the weight of the air did not noe more than the aether did in the

Ref: CELL/RS/HF_147  © Centre for Editing Lives and Letters