THE earth's magnetism — BAUER. 201 



constant. The monsoon, wliicli favors or opposes the progress of the steamer 

 along the Red Sea, is the same which wafted to and fro the ships of Solomon. 

 Eternal snows occupy the same regions and whiten the same mountains, and 

 springs well forth at the same elevated temperature, from the same sources, 

 now as in the earliest recorded history. But the magnetic state of our globe 

 is one of swift and ceaseless change. A few years suffice to alter materially 

 and the lapse of half a century or a century to obliterate and completely re- 

 model the form and situation of those lines on its surface which geometers have 

 supposed to be drawn in order to give a general and graphical view of the 

 direction and intensity of the magnetic forces at any given spoch. 



REGARDING LONGITUDE DETERMINATIONS AT SEA. 



One important result of Halley's voyage and of the publication 

 of his chart was the awakening of renewed interest in the improve- 

 ment of methods for determining the longitude at sea. Kecalling 

 Halley's instructions, we note that one of the objects of his expedi- 

 tion was " to improve the knowledge of the Longitude." 



When the discovery was made that the magnetic declination varied 

 from place to place, the idea immediately occurred to Columbus, as 

 also to Cabot, that the longitude might be determined at sea by means 

 of this fact. Antonio Pigafetta, who accompanied Magellan on 

 his first voyage around the world in 1522, definitely proposed, in 

 his book on navigation, this method of longitude determination. 

 The line of no magnetic declination, which at that time passed 

 through the Azores, was regarded as the natural meridian from 

 which to count longitude. Wlien later it was found, as was first re- 

 marked by J. de Acosta in his Historia Natural : Sevilla, 1590, that 

 there were four such lines, it was again thought that these quadrantal 

 divisions could be utilized for reckoning longitudes. In 1674 Charles 

 II appointed a commission to examine into the pretensions of a 

 scheme devised by Henry Bond for ascertaining the longitude by 

 the " variation of the compass." 



Halley's chart, however, definitely showed that it would be, in 

 general, futile to attempt to determine the longitude by means of 

 an element so variable and so irregular in its distribution as is the 

 magnetic declination. Nevertheless, the hope that some magnetic 

 phenomenon might yet serve to aid in the solution of this problem 

 did not die immediately. 



In 1721 we find AVilliam Whiston, Newton's successor at Cam- 

 bridge, installing dip circles on a number of vessels, with instructions 

 to observe diligently the magnetic dip in order to determine whether 

 by means of this element the longitude could be better found at 

 sea than by the magnetic declination; he likewise hoped thus to 

 determine the latitude at sea. 



It is also interesting here to note that when Dr. Johnson was 

 at Oxford, he gave in 1756 to the Bodleian Library a thin quarto 

 of 21 pages, entitled "An Account of an Attempt to ascertain the 



