258 Mr. Cavallo’s Defer iption of a 
When the inftrument is to be ufed, it muft be placed upon 
a table, a window, or other convenient iupport, a bottle elec- 
trometer is placed near it, and is connected, by means of a 
wire, with one of the tin tubes AD, BC ; and by another 
conducting communication the tin plate muft be connected 
with the electrified fubftance, the electricity of which is re- 
quired to be collected on the plate ABCD : thus, for inftance, 
if it be required to colleCt the electricity of the rain, or of the 
air, the inftrument being placed near a window, a long wire 
muft be put with one extremity into the aperture A or B of 
one of the tin tubes, and with the other extremity projecting 
out of the window. Jf it be required to colleCt the eleCtricity 
produced by evaporation, a fmall tin pan, having a wire or 
foot of about fix inches in length, muft be put upon one of 
the tin tubes, fo that the wire going into the tube the pan may 
ftand about two or three inches above the inftrument. A 
lighted coal is then put into the pan, and a few drops of wa- 
ter poured upon it will produce the delired effeCt. Thus far 
may fuffice with refpeCt to the mechanical defeription of the 
inftrument : the power and ufe of it will be made apparent by 
the following experiments. 
Exp. 1. Communicate to the tin plate ABCD a quantity of 
eleCtricity, for inftance, as much as would very fenfibly affeCt 
a common cork-ball electrometer; then, if the lateral frames 
GHM, NOP, ftand upright, as in fig. 1. the electrometer W 
will fhew no divergency ; but if the frames are opened and 
let down, as in fig. 2. the balls of the electrometer W will 
immediately repel each other, and by the approach of an ex- 
cited piece of fealing-wax, the quality of the eleCtricity may 
be eafily ascertained after the ufual manner. — Put up the lateral 
frames again, and the eleCtricity will apparently vanifh ; — let 
them down, and the eleCtricity will re-appear, and fo on. 
' If 
