672 EEPOET— 1893. 



your Committee, co-operating with tlie leaders of physical science in other lands, 

 have secured international agreement on these fundamental points. 



Among the physical papers of the year I would mention a few as specially call- 

 ing for notice. Mr. E. H. Griffiths's re-determination of the value of the mechanical 

 equivalent of heat has j ust been published,^ and is a monumental work. With untiring 

 energy and great ability he struggled for five years against the difficulties of his 

 task, and has produced results which, with the exception of one group of experi- 

 ments, do not diifer by more than 1 part in 10,000 ; while the results of that one 

 excepted group differ from the mean only by 1 part in 4,000. 



The number of ergs of work required to raise 1 gramme of water 1° C. at 

 15° C. is 4'198 X 10'. Expressed in foot-pounds and Fahrenheit degrees, the 

 value of J is 77977. The value obtained by Joule from his experiments on the 

 friction of water, when corrected in 1880 by Rowland so as to reduce his readings 

 to the air thermometer, is 7785 at 12°-7 C. The result at this temperature of 

 Rowland's own valuable research is 780'1. Another satisfactory outcome of Mr. 

 Griffiths's work is the very exact accordance between the scale of temperature as 

 determined by the comparison of his platinum thermometer with the air thermometer, 

 which was made by Callendar and himself in 1890, and that of the nitrogen 

 thermometer of the Bureau International at Sevres. 



Another great work now happily complete is Rowland's ' Table of Standard 

 Wave-lengths.' " Nearly a thousand lines have been measured with the skill and 

 accuracy for which Rowland has made himself famous ; and in this table we see 

 the results achieved by the genius which designed the concave grating and the 

 mechanical ingenuity which contrived the almost perfect screw. 



Those of us who have seen Mr. Higgs's wonderful photographs of the solar 

 spectrum taken with a Rowland grating will rejoice to know that his map also is 

 now finished. 



Lord Rayleigh's paper on ' The Intensity of Light reflected from Water and 

 3Iercury at nearly perpendicular incidence,' ^ combined with the experiments on re- 

 flexion from liquid sujfaces in the neighbourhood of the polarising angle,* establishes 

 results of the utmost importance to optical theory. ' There is thus,' Lord Rayleigh 

 concludes, ' no experimental evidence against the rigorous application of Fresuel's 

 formulae ' — for the reflexion of polarised light — ' to the ideal case of an abrupt 

 transition between two uniform transparent media.' 



Professor Dewar has, during the year, continued his experiments on the lique- 

 faction of oxygen and nitrogen on a large scale. To a physicist perhaps the most 

 important results of the research are the discovery of the magnetic properties of 

 liquid oxygen, and the proof of the fact that the resistance of certain pure 

 metals vanishes at absolute zero.^ The last discovery is borne out by Griffiths 

 and Callendar's experiments with their platinum thermometers.® 



Mr. Williams's article on ' The Relation of the Dimensions of Physical Quanti- 

 ties to Directions in Space ' '' has led to an interesting discussion. Some of his 

 deductions will be noticed later. 



The title-page of the first edition of Maxwell's 'Electricity and Magnetism' bears 

 the date 1873. This year, 1893, we welcome a third edition, edited by Maxwell's 

 distinguished successor, and enriched by a supplementary volume, in which 

 Professor J. J. Thomson describes some of the advances made by electrical science 

 in the last twenty years. The subject matter of this volume might well serve as 

 a text for a Presidential Address. 



The choice of a subject on which to speak to-day has been no easy task. The 

 field of physics and mathematics is a wide one. There is one matter, however, to 

 which for a few minutes I should like to call your attention, inadequately though 

 it be. Optical theories have, since the year 1876, when I first read Sir George 

 Stokes's ' Report on Double Refraction,'^ had a special interest for me, and I think 



' Phil. Trans., vol. clxxxiv. ^ Phil. Mag., October 1892. 



■ Phil. Mag., July 1893. « Ibid., December 1892. 



=" Ihid., October 1892. ' ThUl., September 1S92. 



"" Ibid., January 1892. ^ British Ansociation Rrjwrt, 1SC2. 



