[ 34' ] 
Under with Mercury j and having ftopt the uppermoil Pipe at 
F with the little Iron ftopple K and clos»d it at the other 
joints, we let the Mercury run out gently at O into the bladder 
V, till it remain'd Tufpended at the ufual height ( as in the 
Baromctrc ) leaving the fpace above it between the glafles o$ 
and // void of Air. We then found the ObjcdV, which before 
appear'd in the Axis at x, rais'd confiderably above it| and 
we reduc'd it to appear at x by removing it from a to The 
Axis therefore, of the vifual Ray xa (which was alfo the 
Axis of the Cylinder J xa, falling Perpendicularly on the 
void fpace in the Cylinder paft through ic without any Re- 
fraction : But emerging obliquely into the Air , it was Re- 
framed towards the Perpendicular pc, and there receiv'd a 
new dirc(^tion to x. And therefore the fpace ax fiibftcnded the 
Angle of Rcfradion acx^ which we meafur'd and found a« 
follows. 
The height of the Objed above the Axis j inches dephs 
of vifual Ray ax the unrefrafted -j o, 42 y 
The Diftance of the Objed from the Refradingl 
Plain, (^<7. about 51 feet or 5 
, therefore the Angle of Refradion acx w^s o, a'. 2%^ 
The AngleofEmerfion /7C/i f by thcconftruftionZ 70 
of the Cylinder) was S ' ^ * 
Therefore th€ Angle of Incidence /7<r;v=::? 
And therefore univerfally (according 
to the known Laws of Refradion ) 
The fines of the Angles of Incidence being 100000 
The fines of the Angles of Emerfion are 100036 
And the Refradive Power of the Denfe -^ir 56 
j*''By the Refraftive Power of a Pellucid body I mean that 
Properly in it whereby the Oblique Rays of Light are diverted 
from their dired Courfej and v^'hich is meafur'd by the Pro- 
portional Differences alwavsObfervd between the fines of the 
Angles of Incidence and Emerfion. 
This Property is not always proportional to the Denfity ( at 
leaft not to the Gravity ) of the Refrading Medium, For the 
Refradtive power of Glafs to that of Water is as 5^ to 34, 
whereas its Gravity is as 87 to 345 that is, thefquares of their 
Refradive Powers are ( very near ) as their refpcdive Gra- 
vities. And thereare fome fluids which iho lighter than Water 
