Mr. James Alexander, of New York, having proposed to Franklin that the velocity of the electric discharge might be measured by discharging a jar through a long circuit of river-water, Franklin, in his reply, explained that such an experiment, if successful, would not determine the actual velocity of electricity in the conductor. He compared the electricity in conductors to an incompressible fluid, so that when a little additional fluid is injected at one end of a conductor, an equal amount must be extruded at the other end--his view apparently being identical with that of Maxwell, who held that all electric displacements must take place _in closed circuits_. "Suppose a tube of any length open at both ends.... If the tube be filled with water, and I inject an additional inch of water at one end, I force out an equal quantity at the other in the very same instant. "And the water forced out at one end of the tube is not the very same water that was forced in at the other end at the same time; it was only one motion at the same time. "The long wire, made use of in the experiment to discover the velocity of the electric fluid, is itself filled with what we call its natural quantity of that fluid, before the hook of the Leyden bottle is applied at one end of it. "The outside of the bottle being at the time of such application in contact with the other end of the wire, the whole quantity of electric fluid contained in the wire is, probably, put in motion at once. "For at the instant the hook, connected with the inside of the bottle, _gives out_, the coating or outside of the bottle _draws in_, a portion of that fluid.... "So that this experiment only shows the extreme facility with which the electric fluid moves in metal; it can never determine the velocity. "And, therefore, the proposed experiment (though well imagined and very ingenious) of sending the spark round through a vast length of space, by the waters of Susquehannah, or Potowmack, and Ohio, would not afford the satisfaction desired, though we could be sure that the motion of the electric fluid would be in that tract, and not underground in the wet earth by the shortest way." In his investigations of the source of electricity in thunder-clouds, Franklin tried an experiment which has been frequently repeated with various modifications. Having insulated a large brass plate which had been previously heated, he sprinkled water upon it, in order, if possible, to obtain electricity by the evaporation of the water, but no trace of electrification could be detected. During his visit to England, Franklin wrote many letters to Mr. Kinnersley and others on philosophical questions, but they consisted mainly of accounts of the work done by other experimenters in England, his public business occupying too much of his attention to allow him to conduct investigations for himself. In one of his letters, speaking of Lord Charles Cavendish, he says:--