154 Mb. E. Hunt on certain Phenomena connected with 



A on the descending side descend farther than it would otherwise have 

 done, and the centre of the line joining the two points will necessarily 

 move to a lower position. Here, again, Mr. Elliot's method, if followed 

 out, actually shows the reverse of what he would have it to do. 



Mr. EUiot says that a top spinning in an incHned position will rise 

 to a vertical position under certain circumstances, and he appears to 

 consider the explanation which I have just discussed as superior to 

 others, because it professes to explain this rising as well as the not 

 falHng. Now, I feel sure that a top can be made which will not rise 

 in a vacuum when spinning under circumstances where friction of 

 the point cannot make it rise. A top's rising under such circumstances 

 (when not in a vacuum) is due to its action on the surrounding air. 

 The case is very similar to that treated by Professor Magnus of Berlin, 

 in a paper to be found in Taylor's Scientific Memoirs for 1853. In this 

 paper the author, in discussing the cause of the deviation of projectiles, 

 shows that if a body rotates, and has at the same time a motion of 

 translation through the air, in a direction at right angles to its axis of 

 rotation, it will experience a greater pressure of air on one side than on 

 the other, in consequence of the opposition on that side of the rotatory 

 and rectilinear or translatory currents, these currents being both in the 

 same direction on the other side. Now, when a top spins in an inclined 

 position, its axis moves on the surface of a cone round the vertical line 

 passing through its point of sujsport, and this conical motion corre- 

 sponds to the motion of translation treated of by Professor Magnus. 

 The side of the top farthest from the vertical line moves in the direc- 

 tion of the conical motion, so that on that side the rotatory current 

 of air caused by the spinning motion is opposed to, and consequently 

 increases the pressure resisting the conical motion, whilst, on the side 

 nearest the vertical line, the rotatory cmTcnt coincides in direction with, 

 and reduces the pressure resisting the conical motion. A resultant 

 pressure thus arises, which tends to lift the top into the vertical 

 position. 



Since the top is spinning, this uplifting pressure tends, by the com- 

 position of rotatory motions, to produce a backward conical motion, 

 which cannot however take place in consequence of the forward conical 

 motion which the top has already, and the effect of the air pressure is 

 partly to reduce the forward conical motion, and partly to lift up the 

 top. For the air pressure can only bear a small proportion to the 

 pressure tending to make the top fall by gravity, and it can only reduce 

 the forward conical motion in that small proportion ; whilst, supposing 

 the conical motion to be reduced to the full extent of that proportion, 

 the causes producing the air pressure will still be in operation to an 



