276 Obituary Notices of Fellows 



Poinsot's representation of the motion of a rigid body about a fixed 

 point by means of a material ellipsoid whose centre is fixed and which 

 rolls on a rough plane. This paper was read to the Royal Society in 

 1869, and printed in the " Transactions." He investigates expressions 

 for the pressure and friction, and arrives at a treatment of the problem 

 different from that of Sylvester, in the course of which some other 

 theorems presented themselves which were not without interest. 



His contributions to the " Quarterly Journal " are too numerous to 

 T>e discussed at any length. A complete list of his papers may be 

 found in the Eoyal Society's " Catalogue of Scientific Papers." We 

 may, however, mention the headings of a few, to show the varied 

 mature of his writings. In 1861 and 1862, he has a series of notes on 

 rtrilinear and quadriplanar co-ordinates, the latter being probably 

 preparatory to a treatise on " Quadriplanar Co-ordinates," which he 

 once informed the present writer he intended to publish. Then in 

 1867, he investigates the envelope of the Simson or pedal line of a 

 triangle, and shows that it is a three-cusped hypocycloid. In 1873, he 

 ihas an extension of Lagrange's equations. In 1875, he has two 

 good papers on hydro-dynamics. In the first, he supposes that a 

 cylindrical vessel is constrained to move in a given manner with fluid 

 inside and outside. He compares the problem to find the motion of 

 the fluid with that to determine the potential of an attracting film, 

 .and finally uses the known results of the second problem to solve the 

 .first. In the second paper, he solves the same problem when the 

 cylinder is replaced by an ellipsoidal vessel. The manner in which he 

 treats this problem is different from and simpler than that of his 

 predecessors Green and Clebsch in the same work. 



These hydro-dynamical researches were allied to the theory of 

 .attractions, and accordingly we find him writing on the latter subject 

 in 1877. The components of the attraction of a solid ellipsoid, 

 whose strata of equal density were similar to the boundary, had been 

 investigated by Poisson. Ferrers gave a method of deducing from 

 these the potential of a solid ellipsoid whose density varies as xAfz h , 

 which is easily applied when the integers /, g, h, are not large. He 

 .also explains a new device by which the potential of an ellipsoidal 

 shell may be deduced from that of the contained solid. 



Lastly, in 1882, he applied himself to study Kelvin's investigation 

 of the law of distribution of electricity in equilibrium on an uninfluenced 

 spherical bowl. In this he made the important addition of finding the 

 potential at any point of space in zonal harmonics. 



The writer is indebted for several of the dates mentioned in the 

 first part of this obituary notice to a brief fragment of an auto- 

 biography kindly lent to him by Mrs. Ferrers. 



E. J. E. 



