1858.] on Hotatory Stability, 485 



perfect workmanship, to turn with the least possible friction, yet 

 without looseness or displacement. An immense rotatory velocity 

 must be communicated to the disk, by machinery, of wljjch its 

 suspension must be quite independent, so that the moving power 

 can be instantaneously withdrawn. All these conditions are ful- 

 filled in the form of the machine which, after repeated trials, has 

 been adopted by Prof Smyth, exhibited by him at the Paris 

 Exhibition 1855, and successfully tried on board Mr. Stephenson's 

 yacht Titania, on his voyage to Teneriffe, in 1856.* 



To explain the principle of its action, we must consider the 

 general conditions of the case. 



In tha first instance, it may be necessary to bear in mind that a 

 telescope pointed to a star may undergo a motion of translation to 

 any extent and in any direction, provided it retain strict parallelism 

 to its original position, that is, remain pointed to exactly the same 

 altitude and azimuth, and it will retain the object in its field just as 

 if it were absolutely fixed. Thus, if a ship be going in a perfectly 

 straigjit course on a perfectly level sea, or even if its motion on the 

 waves were confined to one straight direction up and down, this 

 parallelism might be retained. But this is not the actual case ; 

 even if the course be perfectly straight the pitching and rolling 

 take place by angular motions^ in planes respectively parallel and 

 perpendicular to the length of the ship. The slightest angular 

 motion is destructive to the parallelism of the telescope. It is 

 against this, therefore, tnat it is our object to guard. 



The grand principle of fixity of the plane of free rotation^ is 

 that which enables the revolving disk to retain parallelism to its 

 original plane, however the external frame or pivots supporting the 

 whole be moved. From this principle the revolving disk resists 

 all angular change of position in directions perpendicular to its 

 plane: but it offers no resistance to any motion in that plane. 

 Thus, if set spinning horizontally, it will resist all angular motion 

 impressed in a vertical plane, either by the pitching of the ship in 

 the direction of its length, or its rolling in the direction of its 

 breadth : but it will not resist lateral motion in a horizontal plane, 

 such as any shifting of the ship's head towards the points of the 

 compass. 



Thus a free revolving disk in gymbals externally turning on 

 pivots horizontally resting on supports fixed to the deck, will suffice 

 to preserve the telescope from all deviation due to pitching and 

 rolling. The addition of another disk, freely revolving in a vertical 

 ^ plane, whose external pivots turn vertically in a frame attached to 

 * the top of the former internal frame, the upper pivot projecting 

 through it, and carrying a small platform for the telescope, and the 



♦ See a paper "On the angular disturbances of Ships, &c.," by Prof. C. P. 

 Smyth, F.R.S. Trans, of Royal Scottish SocietjT of Arts, Vol. iv., Part 4. 1857. 

 Also Astron. Society Notices, Vol. xvii., p. 36. 



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