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 604 OHM OILS AND FATS badly obstruct the navigation. At the lowest stage, generally in August and September, the river may be forded at several places above Cin- cinnati. In the winter it is often frozen over, and for several weeks floating ice prevents its navigation. The rate of its current varies with the stage of the water from 1 to 3 m. an hour. The only rapids are at Louisville, and these are not insurmountable to all the steamboats. In 2 m. the fall is about 27 ft. A canal was long since constructed past these rapids at Louisville, through which steamers of 3,000 tons may pass. The country bordering the Ohio is for the most part a thriving agricul- tural region, and many prosperous towns and cities have grown up within the present cen- tury on its banks. Manufactures are en- couraged by the mines of coal and iron ore that abound in the country traversed by this river and its tributaries, and the products of these add largely to the immense transporta- tion carried on by the boats. The character and extent of these operations are particularly noticed in the descriptions of the several states and large towns on the borders of the river. The tributaries of the Ohio from both sides are numerous, and many of them are impor- tant rivers, as the Muskingum and Miami of Ohio, the Wabash of Indiana and Illinois, and the Sandy, Licking, Kentucky, Cumberland, Green, and Tennessee of Kentucky. OHM. I. Georg Simon, a German physicist, born in Erlangen, March 16, 1787, died in Mu- nich, July 7, 1854. He was the son of a lock- smith, and in his boyhood worked in his father's shop. He studied at Erlangen, taught mathematics in various places, and in 1817 was appointed professor in the Jesuit college in Cologne. In 1818 he published a work on the elements of geometry. He devoted himself particularly to the investigation of the laws governing galvanic currents, and by a com- bination of mathematical and experimental investigation, carried on for many years, he at length discovered and established the law which forms the basis of the mathematical the- ory of electricity. (See GALVANISM.) His dis- coveries were first announced in 1825-'6 in scientific journals, and more completely in his Die galvanische Kette, mathematiscJi ~bearbeitet (Berlin, 1827 ; translated into English in Tay- lor's " Scientific Memoirs," vol. xi., London, 1841). The fundamental theorem of his doc- trine is known as " Ohm's law." In 1826 he resigned his professorship, and was director of the polytechnic school in Nuremberg from 1833 to 1849, when he was appointed profes- sor of physics at Munich. In 1841 the royal society of London conferred upon him the Copley medal. He also published Elemente der analytischen Geometric (Nuremberg, 1849), Grundzuge der PhysiTc (1854), and other works. II. Martin, a German mathematician, brother of the preceding, born in Erlangen, May 6, 1792, died in Berlin, April 1, 1872. He studied at the university of Berlin, and in 1817 was ap- pointed professor of mathematics and physics , in the gymnasium at Thorn. In 1821 he re- moved to Berlin, and in 1839 became a full professor in the university. He delivered courses of lectures at the academy of architec- ture from 1824 to 1831, and at the schools of artillery and engineering from 1833 to 1852 ; and he also taught in the military school from 1826 to 1849. He published Versuch eines vollkommen consequenten Systems der Mathe- matik (9 vols., Nuremberg, 1822-'52); LeTir- luch der Mechanik (3 vols., Berlin, 1836-'8) ; Geist der maihematiscJien Analysis (2 parts, 1842-'5 ; the first part translated into English by A. J. Ellis, London, 1843) ; and Die Drei- einiglceit der Kraft (Nuremberg, 1856). OHMACHT, Landolin, a German sculptor, born in Wtirtemberg about 1761, died in Strasburg, March 31, 1834. He was at first a joiner, stud- ied under Canova in Eome, and settled in Strasburg in 1801. His principal works are: " The Judgment of Paris," in the royal garden at Munich ; the statue of Neptune at Mtinster, and that of Desaix between Kehl and Stras- burg ; the mausoleum of the emperor Rudolph in the cathedral of Spa ; the statue of Luther at Weissenburg, and that of " Venus leaving her Bath," which is regarded as his master- piece. Among his best known busts are those of Lavater, Klopstock, Raphael, and Holbein. OILS AND FATS, an important natural group of organic compounds found in the various parts of plants, particularly the seeds, and in animals, principally in the adipose tissues. (See ADIPOSE SUBSTANCES.) In vegetables there are two kinds of oils, totally distinct and having a different chemical formation, viz.,: the fixed, which are analogous to the animal oils and fats, and the volatile or essential oils; and there is also a class of oils and fats which are the result of destructive distillation. These last and the volatile oils will be found treated under the heads COAL PKODTTCTS, PARAFFINE, PETEOLEUM, and ESSENTIAL OILS. The natural oils and fats, which alone are the subjects of this article, are now regarded as the compound ethers of glycerine, a triatomic alcohol (see GLTOEEINE), and may be artificially formed by the action of this alcohol upon certain mono- basic acids. The principal elements in their composition are carbon and hydrogen, oxygen entering as a constituent in smaller propor- tions ; the solidity of the fatty body being gen- erally in proportion to the amount of carbon, and its fluidity in proportion to that of oxygen. When separated from the organism the fatty bodies which are solid at ordinary tempera- tures are called fats, while those which are liquid are called oils. The fatty bodies taken from warm-blooded animals are generally solid at ordinary temperatures, but those obtained from fish and other cold-blooded animals are principally liquid. One of the distinguishing characteristics of the oils and fats is that they are lighter than water, the specific gravity va- rying from 0'91 to 0'94. They are chiefly com-