1832.1 Progress of European Science. 371 



important result demonstrated that the earth was not a magnet, according to the 

 common property of which the poles are distinct and distant from one another. 



The intricate formulae of M. Biot when simplified, were found to agree with an 

 empyric relation discovered by M. Kraft of Petersburgh, in examining some ob- 

 servations of the magnetic dip in different parts of the world ; namely, " that the 

 tangent of the dip of the needle in any place is equal to double the tangent of 

 the magnetic latitude of that place." It followed also from Biot's formula?, 

 that the intensity of the dipping needle ought to vary inversely as the square 

 root of four, minus three times the square of the sine of the dip; and that of 

 the horizontal needle, inversely as the square root of three, plus the square of 

 the secant of the dip : conditions which have in a great measure been verified, but 

 which call for further confirmation in every quarter of the globe. 



These laws are entirely inconsistent with those of a permanent magnetic 

 body ; while Mr. Barlow shows, they are the fundamental laws of a body which 

 receives its transient magnetism by induction j and resemble precisely the condi- 

 tions of the iron sphere upon which his experiments were made in 1819*, where 

 the remarkable fact was pointed out, that all the magnetic power of an iron 

 sphere resides on its surface. 



Still an insuperable obstacle seemed to oppose itself to any rational hypothesis, 

 relative to the cause of the earth's magnetic power. At that time only one means of 

 inducing magnetism was known, which was by the approximation of a permanent 

 magnet to a ball, or mass of simple iron, and of one or two other metals ; the cause 

 therefore, remained inexplicable, until the important discovery of M. Oersted 

 that a wire conducting an electric current was during the interval of transmission 

 in a state of magnetic induction. A number of interesting facts were immediately 

 brought to light, founded upon this grand discovery, by the researches of M. Am- 

 pere, Biot, and Faraday. Mr. Barlow sought in it directly for the explanation 

 of the phenomena of the iron sphere ; and after many experiments to prove that the 

 force of each particle of the galvanic wire on each particle of the needle varies in- 

 versely as the square of the distance, and that the nature of the force is tangential 

 that is, such as would place a needle always at right angles to the direction of the 

 wire, (facts more fully elicited by M. Ampere's investigations,) he applied it 

 to the construction of a globe which should exhibit the whole phenomena of terres- 

 trial magnetism as due to the superficial action of galvanic currents. This idea 

 was put to the test of experiment with the most satisfactory results. A wooden 

 globe of 16 inches diameter was encircled by a series of coils of wire in circles 

 representing the latitude lines of the actual globe or parallels to the earth's mag- 

 netic equator. When the two ends of the coils of wire were connected with the 

 two poles of a galvanic battery, a state of magnetism was induced precisely re- 

 sembling the state of things on the earth : and a saiall needle placed in any situa- 

 tion upon its surface assumed the dip and variation corresponding to the locality. 



Thus Mr. Barlow was the first to prove the existence of a force competent to 

 produce all the phenomena of terrestrial magnetism, without the aid of any body 

 usually called magnetic ; and indeed it had been shewn by M. Biot's laws, that no 

 position of a single magnet, nor the arrangement of any number of such bodies 

 within the globe, could cause an exhibition of the same phenomena, particularly as 

 relates to the intensity of the needle's magnetism. 



As far as the discovery of M. Oersted went, however, it did not appear how a 

 system of electrical currents could have existence on the earth, without a particur 

 * Essay on Magnetic Attraction, 1820. 



