At pp. 71 and 88, 1746 edition, and at p. 88, Prop. XI. of the 1752 edition of this same “Essay,” Wilson says that during the year 1746 he discovered a method of giving the shock of the Leyden jar to any particular part of the body without affecting any other portion; that he increased the shock from the jar by plunging it into water, thereby giving it a coating of water on the outside as high as it was filled on the inside; and that the accumulation of electricity in the Leyden jar is always in proportion to the thinness of the glass, the surface of the glass and that of the non-electrics in contact with the inside and outside thereof. It was in this same year, 1746, that Wilson first observed the _lateral shock_ or _return stroke_, which was not, however, explained until Lord Mahon, third Earl of Stanhope, published his “Principles of Electricity,” in 1779. On the 13th of November, 1760, a paper of Mr. Wilson’s was read before the Royal Society, in which he detailed several of his ingenious experiments on the _plus_ and _minus_ of electricity, and showed that these can be produced at pleasure by carefully attending to the form of bodies, their sudden or gradual removal and the degrees of electrifying. He had previously noticed that when two electrics are rubbed together, the body whose substance is hardest and electric power strongest is always electrified positively and the other negatively. Rubbing the tourmaline and amber together he produced a _plus_ electricity on both sides of the stone and a _minus_ on the amber; but, rubbing the diamond and the tourmaline, both sides of the tourmaline were electrified _minus_ and the diamond _plus_. When insulated silver and glass were rubbed, the silver became _minus_ and the glass _plus_. He further observed that when directing a stream of air against a tourmaline, a pane of glass or a piece of amber, these were electrified _plus_ on both sides. Prof. Faraday subsequently showed that no electrical effect is produced in these cases unless the air is either damp or holds dry powders in suspension, the electricity being produced by the friction of particles of water in the one case and by the particles of powder in the other. Sir David Brewster, who thus mentions the latter fact, likewise singles out two more of Mr. Wilson’s observations, viz. that when a stick of sealing-wax is broken across or when a dry, warm piece of wood is rent asunder, one of the separated surfaces becomes vitreously and the other resinously electrified.