It may be objected that Guericke made no invention, because he did not conceive the idea of making a machine or instrument and did not, in fact, produce one: that he merely made a discovery. The author admits that such an objection would have great reasonableness, and that Guericke's feat is a little hard to class. It is classed by many as an invention, however, and the present author is inclined to class it so; because there seems no reason to doubt that Guericke first conceived the idea of doing what he did do, and that he did produce a device whereby an actual motion of a rubbed ball at one place caused actual motion at another place, through the medium of a current of electricity that traversed a conductor joining the two places. The device is sometimes spoken of as the first telegraph instrument. Guericke (like Gilbert) was more distinctly an experimenter than an inventor,--and (like Gilbert) his work was not only in electricity, but in most of the other branches of science. Of the two, Guericke seems to have covered a wider field, and to have been more distinctly an inventor. His celebrated experiment of holding two hollow hemispheres together, then exhausting the air from the hollow sphere thus formed, and then demonstrating the force of the atmosphere by showing that sixteen horses could not pull the hemispheres apart, indicates just the kind of clear apprehension of the laws of Nature that characterizes the inventor. By some, Guericke is esteemed the inventor of the first electric light, because by rubbing a sulphur ball in a dark room he produced a feeble electric illumination. Of Guericke's discoveries and inventions, the only one that has survived as a concrete apparatus is the air pump; but it is doubtful if the direct influence on history of the air pump, great as it has been, has actually been any greater than the indirect influence of his less widely known discoveries and experiments. [Illustration: Hero's Engines]