[ 3 6 5 3 
of which was fcrewed a hollow brafs cap, nearly of an 
hemifpherical figure. Into the top of one of thefe caps 
was adapted a brafs box of oiled leathers, through 
which was admitted a flender brafs rod of a length 
tufficient to reach within eight inches of the other 
extremity of the tube. Into the top of the other 
brafs cap was fattened a brafs rod, like the former, 
only of eight inches in length. Thus the extremity 
of one of thefe brafs rods might at pleafure, with- 
out letting in the air, be made to touch the other; 
and for the better obferving what difference in effedt 
would arife from an increafe of furface, a fmall brafs 
circular plate was made to fcrew into each of thefe 
extremities. As the fight of this inttrument will 
convey to you at once a more clear idea than the 
mod accurate defcription, I take the liberty of lay- 
ing it before you. 
The intent of being able to bring the extremities 
of thefe rods near together, and to feparate them 
again to what diftance you pleafed, was, that it 
might without difficulty be determined, whether, 
" and to what diftance, the eledlrical fluid would ma- 
nifeft itfelf in vacuo, further than in air of the fame 
denfity with the external. 
The tube then thus fitted, and made dry both 
within and without, was placed in a cylinder of brafs, 
of about two inches long, and of a diameter juft fuf- 
ficient to admit the brafs cap before-mentioned ; and 
round the rim of this brafs cylinder, to prevent the 
ingrefs of air, was adapted a narrow piece of wet 
leather. Thefe being placed upon the plate of the 
air-pump, which flood upon cakes of wax, a piece 
of wire patted from the prime condu&or to the long 
brafs 
