95 



But to devise the means by which the phenomenon might be produced, 

 and, unassisted, to bring the experiment to a successful conclusion, — of 

 all this, it is not too much to say, that it required in the observer the 

 possession of experimental skill and genius of the highest order. 'Nov 

 was Dr.* Lloyd content with the mere exhibition of the phenomenon of 

 conical refraction ; he also examined carefally the elementary rays of 

 which the emergent cone is composed, and succeeded in establishing 

 experimentally the simple and elegant law by which the position of 

 the planes of polarization of these rays is regulated. Passing now 

 from optics to magnetism, we find that Dr. Lloyd's labours have been 

 perseveringly and successfully directed to the improvement of the 

 methods by which the intensity of the earth's magnetic force is mea- 

 sured. In a communication read before the Academy as far back as 

 1843, and printed in the twenty-first volume of our Transactions, he 

 has pointed out a mode of reducing the error attending the determina- 

 tion of this quantity, by the ordinary method, to less than one-fifth of 

 its amount. Adopting Biot's law of magnetic distribution, he has deter- 

 mined a relation between the lengths of the magnets employed, which 

 not only simplifies the calculation, but also efi'ects the above-mentioned 

 important reduction in the error resulting from that observation. He 

 has also, by a series of direct experiments, verified the accuracy of the 

 method adopted, and thus incidentally given an important confirmation 

 of the truth of the law of magnetic distribution which had been assumed. 

 The same subject is resumed in a paper read before the Academy in the 

 year 1858, in which Dr. Lloyd points out a fatal imperfection attend- 

 ing the ordinary mode of calculating the intensity of the earth's mag- 

 netic force, rendering that method quite inapplicable in high magnetic 

 latitudes. The method proposed by Dr. Lloyd is wholly free from this 

 imperfection; and, besides, requires for its application only the use of the 

 dip circle — a vast advantage to the travelling observer, inasmuch as it 

 reduces to the smallest possible number the instruments which he is 

 compelled to carry with him. 



Doctor Lloyd, — The medal which I have now the honour of pre- 

 senting to you is a very inadequate token of the respect with which 

 the Council of this Academy regards your labours in the various de- 

 partments of physical science. Combining an exact knowledge of 

 theoretical principles with a refined tact and ingenuity in experimental 

 processes, you have devised methods of observation, the use of which 

 has greatly facilitated the accumulation of the means of future discovery. 

 You have employed these methods with diligence and success, in the 

 accurate determination of quantities which it was most important to 

 measure. You have also pointed out sources of error in received me- 

 thods of observation. Your colleagues here look forward with a lively 

 interest to the prosecution of those researches in terrestrial magnetism, 

 of which you have recently communicated accounts to the Academy. 

 Though these discoveries belong to a period later than that within 



