58 Proceedings of the Royal In'sli Academy. 



termined thickness. At the same time, it follows that this fluid must 

 have properties entirely diiierent from the ideal fluid assumed in 

 Mr. Hopkins' inquiries. I was led to affirm, that the fluid matter of 

 the inteiior of the earth possessed such an amount of viscidity as to 

 cause it to rotate together with its solid envelope, as if they consti- 

 tuted one continuous mass. 



Several years afterwards, by a veiy simple process of reasoning 

 fi'om physical and mechanical principles, M. Delaunay was led to 

 announce precisely the same conclusions with reference to the motions 

 of the solid and fluid parts of the earth. So remarkable a confijmation 

 of my views created much discussion, and some of those who had 

 adopted the conclusions of Mr. Hopkins seemed to call in question the 

 physical properties of fluids alluded to by M. Delaunay and myself. 

 The properties in question are the outward resistances of fluids to 

 solids in contact with them, and the internal resistances among the 

 fluid particles, when both the fliiids and solids are rotating. It is 

 scarcely necessary to say, that the phenomena of fluids in rotation 

 are connected with other physical questions, and they have a most 

 important connexion with questions of the practical application of 

 hydro-mechanics. Hence I may be pennitted to hope that the inquiry 

 I have commenced may be attended with some useful result. 



The first experiments I tried were similar to those quoted by 

 M. Delaunay, and made, under his direction, by M. Champagneur. 



I obtained a small glass globe, which could be more or less filled 

 with liquid by a small opening. A strong silk cord was looped round 

 the region of its equator, and to the opposite sides of this cord a pair 

 of silk threads were attached, which were fastened to a support close 

 to the ceiling of a lofty room. A steady and rapid rotation was com- 

 municated to the globe by the torsion of the threads. I half filled the 

 globe with common water, and placed a number of small pieces of 

 paper of the same size on its surface at different distances. Whenever 

 the globe was set in rapid rotation, the pieces of paper were at first 

 left behind, and therefore the water ; but when the rotation was long- 

 continued, the papers appeared to move at the same rate. If the 

 rotation was slow from the commencement, the papers seemed to 

 moA-e with precisely the same angular velocity as the glass globe. 

 With moderate velocities, if a piece of paper was placed in the centre 

 of the flat surface-hemisphere of water, it required some time to 

 partake of the motion of the vessel, but if placed near the glass it 

 immediately moved. 



However instructive these experiments were, they could only be 

 regarded as preliminary inquiries, suggestive of ideas to be tested by 

 more precise methods. 



I accordingly, with the assistance of the well-known mechanician 

 and instrument-maker, Mr. Spencer, dc'\"isecl an apparatus which might 

 be employed in the study of phenomena accompanying very slow rota- 

 tion of fluids or rapid rates of motion. I was particularly desirous of 

 being able to observe the effects of the internal viscidity of liquids by 



