Of late years the principle of induction, which is the secret of the Leyden jar and electrophorus, has been applied in constructing "influence" machines for generating electricity. Perhaps the most effective of these is the Wimshurst, which we illustrate in figure 10, where PP are two circular glass plates which rotate in opposite directions on turning the handle. On the outer rim of each is cemented a row of radial slips of metal at equal intervals. The slips at opposite ends of a diameter are connected together twice during each revolution of the plates by wire brushes S, and collecting combs TT serve to charge the positive and negative conductors CC, which yield very powerful sparks at the knobs K above. The given theory of this machine may be open to question, but there can be no doubt of its wonderful performance. A small one produces a violent spark 8 or 10 inches long after a few turns of the handle. The electricity of friction is so unmanageable that it has not been applied in practice to any great extent. In 1753 Mr. Charles Morrison, of Greenock, published the first plan of an electric telegraph in the Scots Magazine, and proposed to charge an insulated wire at the near end so as to make it attract printed letters of the alphabet at the far end. Sir Francis Ronalds also invented a telegraph actuated by this kind of electricity, but neither of these came into use. Morrison, an obscure genius, was before his age, and Ronalds was politely informed by the Government of his day that "telegraphs of any kind were wholly unnecessary." Little instruments for lighting gas by means of the spark are, however, made, and the noxious fumes of chemical and lead works are condensed and laid by the discharge from the Wimshurst machine. The electricity shed in the air causes the dust and smoke to adhere by induction and settle in flakes upon the sides of the flues. Perhaps the old remark that "smuts" or "blacks" falling to the ground on a sultry day are a sign of thunder is traceable to a similar action.