The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

64 STATIC ELECTRICITY.
which may be done by applying to the cylinder, as it revolves,
a rag wetted with spirits of wine. The production of electricity is greatly promoted by applying, with the hand to the
cylinder, a piece of soft leather, five or six inches square, covered with amalgam. This is, in fact, equivalent to giving a
temporary enlargement to the cushion.
The use of the oiled silk flap is to prevent the dissipation of
the electricity evolved on the glass by contact with the air; it
is thus retained on the cylinder till it encounters the points of
the prime conductor, by which it is rapidly drawn off. It is
usual to cover with a varnish of gum lac, those parts of the
glass beyond the ends of the rubber, with a view of preventing
the escape of the electricity through the metallic caps at the
extremities of the cylinder, and the inside of the flap is also
sometimes coated with a resinous cement, consisting of four
parts of Venice turpentine, one part of resin, and one of bees'
wax, boiled together for about two hours in an earthen pipkin
over a slow fire.
Fig. 4.
When the cylindrical machine is arranged for the development of either positive or negative electricity, the conductor is
placed with its length parallel to the cylinder, and the points