It is a little surprising that the phenomenon of electrical repulsion should have escaped the attention of one so skilled in experimentation as Gilbert. Yet such was the case; and Gilbert even went so far as to deny its very existence, saying, "Electrics attract objects of every kind; they never repel." This error reminds one of Gilbert's own saying that "Men of acute intelligence, without actual knowledge of facts, and in the absence of experiment, easily slip and err." Just twenty-nine years after Gilbert had penned this aphorism, there appeared in Ferrara an extensive work on electric and magnetic philosophy, by the Jesuit Cabeo, in which this electrical repulsion was recognized and described. Having rubbed one of his electrics, Cabeo noticed that it attracted grains of dust at first and afterward repelled them suddenly and violently. In the case of threads, hairs or filaments of any kind, he observed that they quivered a little before being flung away like sawdust. This self-repelling property of electricity, described in the year 1629, opened up a new field of inquiry, which was actively explored by a number of brilliant electricians in England and on the Continent. This was especially the case after the building of the first frictional machine by Otto von Guericke in 1672. The burgomaster of Magdeburg had already acquired European fame by the original and sensational experiments on atmospheric pressure which he made in presence of the Emperor and his nobles in solemn diet assembled (1651). Von Guericke seems to have been of a mind with Gilbert concerning writers on natural science who treat their subjects "esoterically, miracle-mongeringly, abstrusely, reconditely, mystically"; for he affirms that "oratory, elegance of diction or skill in disputation avails nothing in the field of natural science." Von Guericke's machine consisted of a ball of sulphur, with the hand of the operator or assistant as rubber. Some years later, the sulphur ball was replaced by Newton (some say Hauksbee) by a glass globe, which, in turn, was exchanged for a glass cylinder by Gordon, a Scotch Benedictine, who was Professor of natural philosophy in the University of Erfurt. In 1755, Martin de Planta, of Sus, in Switzerland, constructed a plate-machine which was subsequently improved by Ramsden of London. The frictional machine, as it was rightly called, has been superseded by the influence machine, a type of static generator which is at once efficient, reliable and easy of operation. The best known form for laboratory use is that of Wimshurst (1832-1903), of London.