GUERICKE, HEINRICH ERNST FERDINAND (1803-1878), German theologian, was born at Wettin in Saxony on the 25th of February 1803 and studied theology at Halle, where he was appointed professor in 1829. He greatly disliked the union between the Lutheran and the Reformed churches, which had been accomplished by the Prussian government in 1817, and in 1833 he definitely threw in his lot with the Old Lutherans. In 1835 he lost his professorship, but he regained it in 1840. Among his works were a Life of _August Hermann Francke_ (1827, Eng. trans. 1837), _Church History_ (1833, Eng. trans. by W. T. Shedd, New York, 1857-1863), _Allgemeine christliche Symbolik_ (1839). In 1840 he helped to found the _Zeitschrift fur die gesammte lutherische Theologie und Kirche_, and he died at Halle on the 4th of February 1878. GUERICKE, OTTO VON (1602-1686), German experimental philosopher, was born at Magdeburg, in Prussian Saxony, on the 20th of November 1602. Having studied law at Leipzig, Helmstadt and Jena, and mathematics, especially geometry and mechanics, at Leiden, he visited France and England, and in 1636 became engineer-in-chief at Erfurt. In 1627 he was elected alderman of Magdeburg, and in 1646 mayor of that city and a magistrate of Brandenburg. His leisure was devoted to scientific pursuits, especially in pneumatics. Incited by the discoveries of Galileo, Pascal and Torricelli, he attempted the creation of a vacuum. He began by experimenting with a pump on water placed in a barrel, but found that when the water was drawn off the air permeated the wood. He then took a globe of copper fitted with pump and stopcock, and discovered that he could pump out air as well as water. Thus he became the inventor of the air-pump (1650). He illustrated his discovery before the emperor Ferdinand III. at the imperial diet which assembled at Regensburg in 1654, by the experiment of the "Magdeburg hemispheres." Taking two hollow hemispheres of copper, the edges of which fitted nicely together, he exhausted the air from between them by means of his pump, and it is recorded that thirty horses, fifteen back to back, were unable to pull them asunder until the air was readmitted. Besides investigating other phenomena connected with a vacuum, he constructed an electrical machine which depended on the excitation of a rotating ball of sulphur; and he made successful researches in astronomy, predicting the periodicity of the return of comets. In 1681 he gave up office, and retired to Hamburg, where he died on the 11th of May 1686. His principal observations are given in his work, _Experimenta nova, ut vocant, Magdeburgica de vacuo spatio_ (Amsterdam, 1672). He is also the author of a _Geschichte der Belagerung und Eroberung von Magdeburg_. See F. W. Hoffmann, _Otto von Guericke_ (Magdeburg, 1874).