34 REPORT — 1S76. 



cessioual phenomena of the liquid gyrostat were not noticeably different from those 

 of ordinary solid gyrostats, which were shown in action for the sake of comparison. 

 It is probable that careful observation without measurement might show very sen- 

 sible differences between the performances of the liquid and the solid gyrostat in 

 the way of nutational tremors produced by striliing the case of the Instrument 

 with the fist. 



No attempt at measurement either of speeds or forces was included in the com- 

 munication, and the author merely showed the liquid gyrostat as a rough general 

 illustration, which he hoped might be regarded as an interesting illustration of that 

 very interesting result of mathematical hydroldnetics, the quasi-rigidity produced 

 in a frictionless liquid by rotation. 



P.S. — Since the communication of this paper to the Association, and the de- 

 livery of my opening address v/hich preceded it on the same day, I have received 

 from Prof. Henry No. 240 of the ' Smithsonian Contributions to Knowledge,' of 



le 



portion of my previously pubfished statements which I had taken the occasion of 

 my address to correct, expressed in the following terms : — 



"I do not concur with Sir William Thomson in the opinions quoted in note, 

 p. 38, from Thomson and Tait, and expressed in his letter to Mr. G. Poulett 

 Scrope (' Nature,' Feb. 1, 1872) ; so far as regards fluidity or imperfect rigidity, 

 within an infinitely rigid envelope, I do not think the rate of precession would be 

 affected." 



Elsewhere in the same paper Gen. Barnard speaks of " the practical rigidity con- 

 ferred by rotation." Thus he has anticipated my correction of the statements con- 

 tained in my paper on the rigidity of the earth, so far as regards the effect of inte- 

 rior fluidity on the precessional motion of a perfectly rigid ellipsoidal shell filled 

 with fluid. 



I regret to see that the other error of that paper v.'hich I corrected in my opening 

 address had not been corrected by Gen. Barnard, and that the plausible i-easoning 

 which had led me to it had also seemed to him convincing. For myself I can only 

 say that I took the very earliest opportunity to correct the errors after I found them 

 to be errors, and that I deeply regret any mischief they mav have done in the mean 

 time. 



Addoichcm. 



Solid and Liquid Gijrodats. — The solid gvrostat has been regularly shown for 

 many years in the natural philosophy class of the University of Glasgow as a me- 

 chanical illustration of the dynamics of rotating solids, and it has also been exhibi- 

 ted in London and Edinburgh at conversaziones of the Royal Societies and of the 

 Society of Telegraph Engineers, but no account of it has yet been published. The 

 following is a brief description of it. 



The solid gjTOstat consists essentially of a massive flv-wheel, possessing great 

 moment of inertia, pivoted on the two ends of its axis in bearings attached to an 

 outer case which completely encloses it. Fig. 1 represents a section by a plane 

 through the axis of the fly-wheel, and fig. 2 a section by a plane at right angles to 

 the axis and cutting through the case just above the fiy-wheel. The containino-- 

 case is fitted with a thin projecting edge in the plane of the flj'-wheel, which "is 

 called the bearing-edge. Its boundary forms a regular curvilinear polygon of six- 

 teen sides with its centre at the centre of the fly-wheel. Each side of 'the polygon 

 is a small arc of a circle of railius greater than the distance of the corners from the 

 centre. The friction of the fly-wheel would, if the bearing-edge were circular, 

 cause the case to roll along it like a hoop ; and it is to prevent this effect that the 

 curved polygonal form described above and represented in the drawing is given to 

 the bearing-edge. 



To spin the solid gyrostat a piece of stout cord about forty feet long and a place 

 where a clear run of about GO feet can be obtained are convenient. The gyrostat 

 having been placed with the axis of its fly-wheel vertical, the cord is passed in 

 through an aperture in the case two and a half times round the bobbm-shaped 

 part of the shaft and out again at an aperture on the opposite side. Having taken 



