366 ACROPHYSICS. 



often kept in contact with the poles of magnets, to strengthen them 

 by inductive action. 



5. Electro- Magnetism, and Thermo-Electridty, are kindred 

 branches of electric science, treating of the relations between gal- 

 vanism, and magnetism and calorics. It was first discovered by 

 Professor Oersted, in 1820, that a galvanic current has a peculiar 

 action on a magnetic needle placed near it. Supposing the current to 

 pass through the centre of a watch, from the face to the back, it 

 tends to carry the north pole of any magnet around it, in the direction 

 in which the hands of the watch move; and the south pole in the 

 opposite direction. If the magnet, or needle, turn on a pivot, 

 exterior to the galvanic current, it will soon come to rest, in a trans- 

 verse position, as the above forces require. These facts were applied 

 by Schweigger, to the invention of the galvanometer ; in which, if 

 a very feeble galvanic current circulate around a coil of insulated 

 wire, it will be rendered manifest, by acting on a magnetic needle 

 suspended within the coil. 



In 1820, Arago discovered that a bar of steel can be magnetized 

 by a galvanic current made to circulate around it ; and hence, that a 

 spiral coil of insulated wire, so long as a galvanic current is passing 

 around it, acts as a magnet; manifesting polarity; attracting other 

 magnets ; and speedily magnetizing a steel bar placed within it. 

 This furnishes a new mode of forming artificial magnets ; and it led 

 to Ampere's theory, that the peculiar properties of magnets are owing 

 to galvanic currents circulating around their elementary particles. 

 On this ground, it is now generally admitted that the earth's magnet- 

 ism is produced by such currents, circulating around it, and caused 

 by the heat of the sun. This is farther confirmed by Dr. Seebeck's 

 thermo-electric discovery, that if a ring be formed of two metals, one- 

 half of it, for example, of bismuth, and the other of copper, soldered 

 together, then, on heating one of the junctures, a galvanic current will 

 pass through the same, from the bismuth to the copper. 



6. Meteorology, so named from the Greek ^fffwpa, meteors, 

 relates to atmospherical phenomena ; their causes, and the means of 

 observing and foretelling them. This science, if it has now reached 

 that dignity, engaged the attention of Aristotle, and Theophrastus ; 

 and, in more recent times, of Leslie, Dalton, and other philosophers : 

 and it is at present the subject of profound study and careful observa- 

 tion. Of our own countrymen, Dr. Franklin first observed that all 

 extensive storms, in the United States, travelled towards the north- 

 east : and Mr. Redfield first traced their course from the southern 

 to the northeastern states ; while he ascribed them, generally, to a 

 gyratory or whirling motion of the air. Mr. Espy, who has made 

 extensive and important researches on this subject, ascribes their 

 formation chiefly to the action of heat : and Dr. Hare considers elec- 

 tricity as an equally prominent agent in their production. 



If the air be heated, at any one place, it becomes rarified, and rises, 

 as balloons do; spreading itself out above; while currents of cooler 

 air converge to that place below, to supply the partial vacuum. This 

 principle explains the trade winds, blowing towards the equator; 

 and the monsoons, of the Indian ocean, (p. 163). As the heated air 



