It will be observed that he speaks of crowds coming to see the experiments, and this confirms what I have already shown of the strong interest in science which prevailed at that time in Pennsylvania, and which had evidently first aroused Franklin. In fact, a renewed interest in science had been recently stirred up all over the world, and people who had never before thought much of such things became investigators. Voltaire, who resembled Franklin in many ways, had turned aside from literature, and at forty-one, the same age at which Franklin began the study of electricity, had become a man of science, and for four years devoted himself to experiments. Franklin was by no means alone in his studies. Besides the crowds who were interested from mere curiosity, there were three men--Ebenezer Kinnersley, Thomas Hopkinson, and Philip Syng--who experimented with him, and it was no mere amateurish work in which these men were engaged. Franklin was their spokesman and reported the results of his and their labor by means of letters to Mr. Peter Collinson. Within six months Hopkinson had observed the power of points to throw off electricity, or electrical fire, as he called it, and Franklin had discovered and described what is now known as positive and negative electricity. Within the same time Syng had invented an electrical machine, consisting of a sphere revolved on an axis with a handle, which was better adapted for producing the electrical spark than the tube-rubbing practised in Europe. The experiments and the letters to Collinson describing them continued, and about this time we find Franklin writing a long and apparently the first intelligent explanation of the action of the Leyden jar. Then followed attempts to explain thunder and lightning as phenomena of electricity, and on July 29, 1750, Franklin sent to Collinson a paper announcing the invention of the lightning-rod, together with an explanation of its action. In these papers he also suggested an experiment which would prove positively that lightning was a form of electricity. The two phenomena were alike as regarded light, color, crooked direction, noise, swift motion, being conducted by metals, subsisting in water or ice, rending bodies, killing animals, melting metals, and setting fire to various substances. It remained to demonstrate with absolute certainty that lightning resembled electricity in being attracted by points; and for this purpose Franklin proposed that a man stand in a sort of sentry-box on the top of some high tower or steeple and with a pointed rod draw electricity from passing thunder-clouds.