Page:Popular Science Monthly Volume 25.djvu/694

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tial and physiological checks to increase of population; the final result, perhaps, being one in which the birth-rate and death-rate shall become closely allied, and a virtually stationary condition of population ensue. We have here indications of a rich promise for the future of the human race. If the numbers of mankind become thus checked, while wealth continues to grow, and culture, with its advanced needs, becomes a general possession, the standard of desire must rise, until absolute want may no longer mean, as now, physical misery and starvation, but may mean the deprivation of what would now be considered luxuries beyond the reach of the poor. In such a case the population of the earth could never sink, as now, to press upon the sharp edge of absolute destitution. It would be too far above this limit to sway so far downward, and misery from want of food might become an obsolete tradition of the past.



THE first lightning-conductor was erected by Benjamin Franklin upon his own house in Philadelphia in 1752. The invention is, therefore, now a little more than one hundred and thirty years old. Franklin was led to the investigations which resulted in its construction by the fortuitous circumstance that, about six years previously, he had been present at a lecture on electricity delivered in Boston by Dr. Spence.[1] In the same year—that is, in 1746—he received a present from Peter Collinson, a member of the Royal Society in London, who was also the agent of the Library Company in Philadelphia, of one of the London electric tubes, and an account of some experiments that had recently been made by Dr. Watson, Martin Folkes, Lord Charles Cavendish, Dr. Bevis, and others of their contemporaries. The idea had already suggested itself to these investigators that the luminous gleam which was elicited from glass tubes when they were rubbed in dark cellars, in performance of the frequently repeated and fashionable experiment of the day, might possibly be of a kindred nature to the lightning of the thunder-storm. In a book describing some "physico-mechanical experiments" that he had made, published in London in 1709, Francis Hawksbee remarked that the luminous flash and crackling sounds produced by rubbing amber were similar to lightning and thunder. In 1720 Stephen Gray, the pensioner of the Charterhouse, so celebrated for his electrical investigations, boldly and uncompromisingly affirmed that, "if great things might be com-

  1. It is, perhaps, worthy of remark that, in this lecture, the experiments were made by the primitive instrumentality of a glass rod and silk pocket-handkerchief.