Royal Society. 225 



above referred to, but will admit, that during the several years of 

 his purely mathematical lectures, nothing could exceed the depth 

 or surpass the exquisite taste and elegance of all his original concep- 

 tions, both in analysis and geometry. Nor will it be denied by any 

 who were so happy as to possess the opportunity of judging, that 

 during the last three years and a half in which he filled the Chair of 

 Natural Philosophy, his earnest endeavour was ever to instil sound 

 and accurate physical conceptions into the minds of his hearers, and 

 to array them, when stated in mathematical language, in all the 

 charms which arises from true taste and appropriate refinement. 



In his first course of Lectures — on the rotation of a solid body 

 round a fixed point — he completely solved the case of a body aban- 

 doned to its own motions on receiving a primitive impulse in any 

 direction, and under the action of no external accelerating forces. 

 This problem he had finished several years before, and was preparing 

 it for publication, when he found that he had been just anticipated, 

 in many though not in all respects, by Poinsot, who published about 

 that time a very elegant little tract on the subject. During tlie same 

 course of Lectures he gave some interesting and original theorems 

 respecting the rotation of surfaces of revolution moving freely in 

 sjiace, and acted on by any external accelerating foixes, directed to 

 any number of fixed centres. 



In his course of Lectures on attractions, he gave some very beauti- 

 ful theorems respecting the attraction of a body of any nature and 

 form on a point distant a long way in comparison of its own dimen- 

 sions. And he gave some most simple and elegant geometrical 

 methods for finding the attraction of an homogeneous ellipsoid on 

 anj' internal point. The subject of attractions seems indeed to have 

 been a favourite one with him ; and he on sevei'al previous occasions 

 gave new and beautiful theorems in it, and in many important re- 

 spects improved the existing theories, keeping always in advance of 

 the knowledge of the time. He delivered also courses of Lectures 

 on part of Sir Isaac Newton's Principia, and on Heat, Electricity 

 and Magnetism. 



I now come to Professor MacCullagh's great course of Lectures 

 on "The Dynamical Theory of Light," which was on his part 

 (whatever other researches on tliat subject may have been elsewhere 

 made) the unaided creation of his own genius ; and was founded 

 on one single and simple hypothesis, on which as a basis (to bor- 

 row the language of Dr. Lloyd when speaking of Fresnel's beau- 

 tiful theory of double refraction), he " has reared the noblest fabric 

 which has ever adorned the domain of physical science, Newton's 

 system of the Universe alone excepted." If we now venture to say 

 that Professor MacCulIagh ranks as a ])hilosopher higher than Frcs- 

 nel in the region of Light (and if that be admitted, he will certainly 

 rank inferior to none on that sul)ject), it is not thereby designed to 

 institute any comparison between labours so different in their na- 

 ture as those of these two great men. Professor MacCulIagh may 

 be regarded as standing to Fresnel in the same relation as Newton 

 to Kepler. I'Vesnel undoubtedly discovered all the elegant laws of 



I' hit. Ma-r. S. .'J. Vol. 32. No. 211. March 1818. ^ Q 



