678 THE POPULAR SCIENCE MONTHLY. 



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, be- 

 comes 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 con- 

 sidered 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. 



-♦♦♦- 



PEOTECTIOK AGAINST LIGHTOTISTG. 



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 construc- 

 tion by the fortuitous circumstance that, about six years previously, 

 he had been present at a lecture on electricity delivered in Boston by 

 Dr. Spence.* In the same year — that is, in 1746 — ^he received a pres- 

 ent 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- 



* 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. 



