70 BEPOET— 1882. 



case of very large viscosity or very small. For very large viscosity the 

 result agrees with that of Darwin's and the displacement subsides without 

 oscillation. For very small, the time of oscillation agrees with that for a 

 perfect fluid, found by Thomson,' whilst the 'modulus of decay,' i.e. the 

 time in which the amplitude decreases to 1/e of its original value, is 

 (w — l)(2n + VjiJtja^, for an harmonic displacement of order n. This 

 for a sphere of water of the size of the earth is 1'84 x 10^' years. For 

 a globule of water oscillating under the action of its surface tension, the 

 time of oscillation is 2ir/j3, where /3^ = n(n — l)(n + 2)T/pa^ where T is 

 the surface tension. For the slowest oscillation (n = 2) the modulus of 

 decay of a globule of water is 14-3 a^ seconds, the radius a being ex- 

 pressed in centimetres. 



The effect of viscosity on the motion of waves has been discussed by 

 Stokes,^ Boussinesq,' Lamb,* and others. The rate of decay of the motion 

 obtained by Stokes is double the true value — an error caused, as both 

 Boussinesq and Lamb have pointed out, by an oversight of the former in 

 neglecting the potential energy of the motion. 



Postscript. 



I find I have omitted to notice a paper by Lodge ' On some Problems 

 connected with tlie Flow of Electricity in a Plane,' Phil. Mag. (V.), 

 vol. 1. (1870), in which the motions in plane triangles, rectangles, and 

 circles are treated. 



Report of the Committee, consisting of Professor Gr. Carey P^oster, 

 the late Mr. C. Hockin, Sir William Thomson, Professor Ayrton, 

 Mr. J. Perry, Professor W. G. Adams, Professor Lord Eayleigh, 

 Professor F. Jenkev, Dr. 0. J. Lodge, Dr. Johx Hopkixsox, Dr. A. 

 MuiRHEAD (Secretary), Mr. W. H. Preece, INIr. Herbert Taylor, 

 Professor Everett, and Professor Schuster, appointed for the 

 piorpose of constructing and issuing practical Standards for 

 use in Electrical Measurements. 



The Committee have to report that Mr. Taylor has continued the experi- 

 ments upon the temperature-coefficient of the resistance of metals and 

 alloys, the first results of which were communicated at the York meeting. 

 In consequence of Mr. Taylor's absence from the country, the details of 

 the farther experiments cannot be communicated at present ; but it may 

 be stated that they have shown the possible influence of the process of 



' See above p. 24. 



^ ' On the effect of the internal friction,' &:c., cited above. 



" 'Additions et eclaircissements au memoire intitule Essai sur la thfiorie,' &c. 

 Mem. de Sav. Etran. xxiv. (1877). 



* ' On the oscillations,' k.c., cited above. 



