( 5IS7 ) 
though as to its particular Effccas I leave them to the fpe- 
cuIatioQ of Phyfitians 5 and ftiall only add^that to keep this 
Conjefture of mine (for I propofe it as 00 other ) from 
feeding as groundlefs as extravagant 5 I will aoocx an Ex- 
peritneot that you will not perhaps diflike 5 juft as I find it 
regifter'd among fome of my loofe Papers, 
I caufed to be blown at the flame of a Lamp three fmall 
round Gla(s*bubbics about the bignefs of Hazel-nuts^ and 
furnifliM each of them with a ftiort and flendcrftem, by 
whoie means they were <b nicely poifed in water^that a ve- 
ry finall change of Weight would make them either emerge^ 
if they but lightly leaned on the bottom of the Veflel ,^ or 
fiokjif they floated on the top of theWaier. 
This being done at a time when the Atmofphere was of 
a convenient Weighty (and fucha feafon is not ordinarily 
difficult to be chofen within fome reafonable time to him 
that wants neither attention nor a good Barofcope ) I put 
them in a wide.mouth'd Clals furnifli'd with common Wa^ 
ter, and leaving them in a quiet place^where yet they were 
frequently in my eye, and were fufFcr'd to continue many 
weeks (or fome monthsj) I obfervedj as I expeded , that 
fometimes th ey would be at the top of the Water, and re- 
main there for divers days, or perhaps Weeks 5 and fome- 
times would fall to the Bottomjaod after having continued 
there for fome time (longer or (horter) they would again 
emerge* And though fometimes (efpecially if I removed 
the Veflel that contaiii'd them to a Southern Window^) 
they would rife to the Top or fall ta the Bottom of the 
Water 3 according as the Air was hot or cold 5 yet 'twas 
not difficult to diftinguifh thofe motions from thofe pro- 
duced by the varying Gravity of the Atmofphere. For 
when the Beams of the Sun , or heat of the Atnbient Air, 
by rarifying the Air included in the Bubbles, made that 
^Air drive out fome of the Water, and coofequeotly made 
the whole Bubble ( confifting of Glafs, Air and Water) ' 
fomewbat lighter than a bulk pi Water equal to itjthough 
the bubble did tnecejD&rily fwim as long as the iDcluded 
Tttt t ^ Aic 
