486 Hev. B, Powell, [Feb. 26, 



whole, of course, balanced below, will preserve the telescope from 

 any lateral deviations of the ship. And the combination of the 

 two wil] give a plane retaining its parallelism against all three 

 causes of disturbances. But under favourable circumstances this 

 last cause of disturbance is but small : so that this addition may 

 be often of little importance. 



The invariable platform of this revolving apparatus may then 

 equally be applied to support either a telescope, or the artificial 

 horizon before mentioned, whether simply or in conjunction with 

 the sextant, or reflecting circle. By a mere enlargement of the 

 scale of the machine, the same stand which carries the telescope 

 might be made to carry the observer also, which would be a 

 material convenience for any nice observations. 



But the essential condition is the preservation of perfect equi' 

 librium about the centre of motion. Now if this were secured by 

 proper compensation for the observer in one position, the slightest 

 change of position on his part would vitiate that arrangement. 



The observer, instead of being an extraneous source of dis- 

 turbance incapable of producing any impression on the balanced 

 and revolving system, now becomes a part of it, and thus impresses 

 upon it a fresh motion arising from every slight change of posture, 

 which alters the exact position of the centre of gravity of the whole. 

 This effect, however, would not manifest itself directly, but being 

 compounded with the rotation, would show itself in a precessional 

 motion, fatal to the stability of the direction of the telescope. 



The case is not one of great importance practically, though 

 interesting theoretically. A modified arrangement to meet it has 

 been devised by Prof. Smyth, which will be understood by the 

 following considerations : — 



Two free revolvers turning horizontally are placed with the 

 lines of the pivots of their interior rings horizontally in directions 

 (p and q) at right angles to each other, (which we will call the 

 disks p and q respectively,) in a common exterior frame, itself 

 turning on pivots in the direction (q) in a third exterior frame, 

 whose pivots are on fixed supports in the direction (p). 



Now supposing the whole to remain perfectly balanced, if any 

 angular motion in a vertical plane be communicated to the fixed 

 supports, tending to turn the whole about (p), then the revolving 

 disk {q), in order to retain its own parallelism, will cause both the 

 outer and inner general frames together to retain their parallelism, 

 by turning about (p) through an angle equal and opposite to that 

 through which the supports are moved ; while the disk {p) simply 

 retains its position along with the frames. 



An angular motion given to the supports about (q) will cause 

 the outer frame to move through that angular space. But the 

 revolving disk (p), in order to retain its parallelism, will cause the 

 inner general frame to move through an angle equal and opposite 

 to that of the supports about (q), and the disk {q) will simply 

 retain its position. 



