410 ME M O I R S of the 
by the infertion of the glafs tube T 5 the joints were very care^ 
fully clofed up with cement, and then covered over with pieces 
of a bladder tied hard with ftrong thread 5 there was alfo a 
bladder tied below each joint at ra, and when it was filled with 
water, it was tied about at n, fo that no air could come to the ce- 
ment, or infinuate itlelf thro' its pores or fiffures, if any happened 
to be unftopped 5 with all this precaution, the experiment luc- 
ceeded at lall, after this manner 5 the object a, which was a 
black thread faftened in a little frame over a piece of white pa- 
per, was placed in the axis of the cylinder aca^ the pipes and 
cylinder were filled with mercury, and having flopped the upper 
end of the pipe at F with the Imall iron flopple K, and clofed it 
at the upper part of the tube and other joints, the mercury was 
let run gently at O into the bladder «, till it remained fufpended 
at the ufual height, as in the barometer, leaving the upper part of 
the tube and the cavity of the cylinder between the glafTes 00 and 
// void of air 5 then the objea, which before appeared in the 
axis at x, was raifed conliderably above it, and it appeared again 
at X by removing it from ^ to a 5 the axis therefore of the vifual 
ray, which was alfo the axis of the cylinder xca, falling perpen- 
dicularly upon the void fpace, pafled thro' it without any refrac- 
tion ^ but emerging obliquely into the air, it was refracted to- 
wards the perpendicular pCy and received a new direftion to a • 
and therefore the diflance a a fubtended the angle oT refradion 
a cay all which was mcafured, and was as follows 5 
The height of the objed above the axis, or the uno "*" ' ^'''^^^' 
refradted vifual ray <^ a — r o^o 425 
The diltance of the object from the refracting plane? 
a f about 5 1 feet or — £ ^^^ 000 
d m -T 
Therefore the angle of refraction ^<:a was ^ 00 oi 23 
The angle of emerfion pea (by the conllruaion of? 
the cylinder) was — > ^2 30 00 
Therefore the angle of incidence pc2i =: pca7 
+ ^<:awas — , ^ r ^^ ^7 57 
fraaroV^^'''^'''^ univerfally, according to the known laws of re- 
The fines of the angles of incidence being - loocoo 
The fines of the angles oi emerfion are -^ ^ iooo'>(^ 
And the rcfraftive power of the denle air -^ ^ l^ 
By 
