C 311 ] 
Cc moved together. The 'Torricellian vacuum then 
<£ occupied a fpace of about thirty inches. In make- 
£c ing the experiment, when the room was darken- 
u ed, a wire from the prime conductor of the com- 
<c mon electrical machine communicated with one 
£C of the bafons of mercury, and any non-eleCtric 
tc touching the other bafon, while the machine was 
cc in motion, the eleftricity pervaded the vacuum in 
i( a continued arch of lambent flame , and as far as 
<c the eye could follow it , without the leafl diver gen - 
“ cy.” 
I imagine Dr. Watfon , who has defcribed this ex- 
periment, did not, when he made the experiment, at- 
tend to a lingular appearance of light upon one of the 
furfaces [of the quicklilver, becaufe he has taken no 
notice of it. However, I had a mind to make the 
experiment myfelf, and try whether I could not ma- 
nage the quicklilver, fo as to have more than two vi- 
lible furfaces, in order that I might have that re- 
markable appearance repeated. 
To do this, I let a very fmall quantity of air into 
the tube, by which means four columns of quicklil- 
ver were obtained, reckoning great and fmall toge- 
ther ; and lix vifible furfaces, three of which I called 
upper, and three under furfaces. But the figure in 
the plate N°. IX. will give you a better idea of the 
inftrument, than I am able to do by writing. 
When the column of quickfilver on the left hand 
was electrified, and the other on the right commu- 
nicated with the earth, the Itream of light was vi- 
lible in a darkened chamber, and the general appear- 
ance, all the way through the vacuum , was a light of 
a feeming uniform denfity, excepting at the upper 
furfaces, for above one tenth of an inch, reckoning 
from 
