202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



Longitude at Sea by an exact Theory of the Variation of the Mag- 

 neticai Needle, etc.," by Zachariah AVilliams, published at London 

 in 1755; Johnson entered it with his own hand in the library cata- 

 logue. Boswell relates that Johnson himself wrote the English 

 version for Williams, and, in order to make it more extensive!}'' 

 known, also had an Italian translation prepared by his friend, 

 Signor Baretti. 



For fully three centuries the idea that the longitude could be de- 

 termined at sea with the aid of some magnetic element, though proved 

 to be fallacious, served a most useful purpose by furnishing the nec- 

 essary incentive to observe the magnetic elements. This is a striking 

 illustration of the soundness of the position taken by Maxwell when 

 he said : " I never try to dissuade a man from trying an experiment; 

 if he does not find what he wants, he may find out something else." 

 It was indeed true of these magnetic longitude seekers that they 

 failed in their purpose, but the}^ contributed data of inestimable 

 value to the advancement of our knowledge of the earth's magnetism. 



Before leaving this subject it might be said that Halley himself 

 proposed an astronomical method for solving the longitude problem, 

 and, with Newton, he was responsible for the act of 1714 offering a 

 reward to any person who should devise a satisfactory method for 

 the determination of the longitude at sea. He also improved some 

 of the instruments used in navigation. 



Another result of Halley's various voyages deserves mention here, 

 though not immediately concerned with the subject of our lecture, 

 namely, his theory of the cause of the trade winds.^ On certain 

 editions of his Variation Chart there was given, in addition to the 

 lines of equal magnetic variation, a " View of the Generall and Coast- 

 ing Trade Winds and Monsoons or Shifting Trade Winds." 



COMPLEXITY OF THE EARTH'S MAGNETISM. 



Reference has already been made to Halley's attempts, before his 

 magnetic expedition, to establish a theory respecting the phenomena 

 of the compass needle. Thus in 1683 he published in the Philo- 

 sophical Transactions of the Royal Society "A Theory of the Varia- 

 tion of the Magnetical Compass," and in 1692, in the same Transac- 

 tions, "An Account of the Cause of the Change of the Variation of 

 the Magnetic Needle." 



In these papers Halley rejected the hypothesis which had been 

 accepted up to that time, and on the basis of which elaborate tables 

 of the magnetic declination had been constructed by previous investi- 

 gators, namely, that the directions assumed by a compass needle in 

 various parts of the earth could be accounted for by a simple magnet- 



1 Soe Miscellanea Curiosa, vol. I, pp. 61-80, and pi. 2, 



