Frederick's first invention was his successful invasion of Silesia. This may be called by some "an invention of the devil," and perhaps it was inspired by him. But even if Frederick's conception came straight from the devil, it was a brilliant conception, nevertheless, as the conceptions of the devil himself are popularly supposed to be. So original in conception and so perfect in development was Frederick's invented plan, that he had seized the capital of Silesia before Austria had taken any real defensive measures of any kind. During the first half of Frederick's reign, or twenty-three years (from 1740 to 1763), he was engaged continually in war or preparation for war; and in both activities he had to plan to fight against odds that often seemed overwhelming. They would have overwhelmed any man, except a man like Frederick. It is true that Frederick had two advantages, the best trained army, and the fact that all his forces, military and political, were united under one head--his own. But it is the verdict of history that even these advantages were far from sufficient to explain his victories; that his victories cannot be explained except on the ground that Frederick showed a generalship superior to that of his foes. In what did its superiority consist? A careful study of his campaigns, even if it be not in detail, shows that Frederick was able to invent better plans than his adversaries, to invent them more quickly, and to carry them into effect more promptly. If he had been born under other stars, he might have exercised his inventiveness in such ways as men like Guericke, for instance, did; as is shown by his gathering around him, in the peaceful period of the latter half of his reign, a company selected from the most eminent philosophers and scientists of the age; and as is shown with equal clearness by his admirably conceived and executed measures for the better government of his country. The middle of the eighteenth century is especially distinguished by the success of some extraordinary and brilliant experiments with electrical apparatus. One of the most important in results occurred about 1746, in the town of Leyden, where Muschenbroek invented a device that made possible the accumulating and preserving of charges of electricity. This appliance consisted of merely a glass jar, coated on the outside and the inside with tin foil. It was a most important invention, and it is still in general use, and called the Leyden jar. The Leyden jar was soon put to practical work in electrical investigations, notably by the Royal Society in London; and many valuable demonstrations were made with it. Among these were the firing of gunpowder by the electric spark that passed when both surfaces of tin foil were connected by an external conductor; and the transfer of the spark over a distance of two miles, by using one discharging conductor or wire two miles long, the earth acting as the return conductor.