Page:Popular Science Monthly Volume 9.djvu/55

Rh  friction produces heat then and there upon the surfaces rubbed, while the force expended in overcoming the electric attraction may be converted into a spark which shall appear a thousand miles away from the place where it was generated.

Theoretic conceptions are incessantly checked and corrected by the advance of knowledge, and this theory of electric fluids is doubted by many eminent scientific men. It will, at all events, have to be translated into a form which shall connect it with heat and light, before it can be accepted as complete. Nevertheless, keeping ourselves unpledged to the theory, we shall find it of exceeding service both in unraveling and in connecting together electrical phenomena.



HE year 1875 completed the third quarter of the nineteenth century, a period distinguished by the activity which has prevailed in every branch of scientific inquiry, but particularly distinguished as a remarkable period of geographical exploration and discovery.

The history of geographical knowledge is a history of its rapid acquisition in periods very limited in point of time, but of great activity, and of long intervals of repose, in which comparatively little was done, or a great deal lost that had been previously acquired. For the last twenty-five years we have been living in one of those periods of exceptional activity, for at no time has an interest so wide-spread been manifested for geographical exploration since that great age of maritime discovery, that began in the early part of the fifteenth century with the exploration of the western coast of Africa by the Portuguese, and culminated in the circumnavigation of the globe by Magellan. The comparatively small limits of about a century is all that is embraced from the time (1418), when Prince Henry of Portugal, surnamed the Navigator, took up his abode upon the promontory of Sagres to devote the residue of his life to the fitting out of expeditions for the exploration of the coast of Africa beyond Cape Bojador, a region then wholly unknown, and the year 1519, when Magellan entered the Pacific by the discovery of the straits that bear his name. Within that period the captains of Prince Henry had sailed around the continent of Africa; Columbus had discovered America; his companion, Nuñez de Balboa, the Pacific; Sebastian Cabot had followed