HISTORY AND PROGRESS. 5 



of the jar with, for example, positive electricity, the charge 

 acts upon the natural electricity of the outer coating, which, 

 during the operation, should be connected by a conductor 

 with the earth, decomposes it, and repels the positive element, 

 attracting and retaining the negative element on the outer 

 coating. If the communication between the knob and the 

 source of electricity be broken, the charge will remain accumu- 

 lated on the inner coating of the jar ; and if a connection 

 be then made between the knob and the outer coating by 

 means of a wire or of a discharger, as shown in the figure, 

 the opposite electricity accumulated on the coatings of the 

 jar will rush towards each other through the conductor, 

 producing, on its approach to complete communication, a 

 spark of brilliant light. 



Suppose that, instead of the short discharger, a wire of 

 several yards' length were employed, the effect would be the 

 same. And it was virtually to ascertain the maximum 

 length of this wire that formed the purpose of those re- 

 searches of Grey, Desaguilliers, Watson, and others, to which 

 we are indebted for the first suggestion of a telegraph. 



10. "Watson, in 1747, stretched a wire across the Thames, 

 over old Westminster Bridge. One end was fixed to the 

 exterior coating of a Ley den jar, the interior coating being 

 connected to earth through the body of the experimenter, 

 and the other end held by a person who grasped an iron rod. 

 The moment the latter dipped the rod into the river, both 

 felt a shock. 



Subsequently, in the same year, Watson transmitted an 

 electric discharge through 2,800 feet of wire and the same 

 distance of earth at Stoke Newington ; and on the 14th of 

 August, in the same year, repeated his experiments on a con- 

 siderably larger scale, transmitting the electric impulse 

 through 10,600 feet of wire suspended between wooden poles 

 erected on Shooter's Hill. 



Franklin made similar experiments in 1748 across the 

 Schuylkill, at Philadelphia, and Du Luc, about the same 

 date, across the Lake of Geneva. 



But up to this time the experiments had been conducted 



