484 Hev. B, Powell [Feb. 26, 



the use of the telescope, especially those requiring one of consider- 

 able power, the same requisite of invariable stability of direction 

 is yet more indispensable, but hitherto unattained. 



One of the most important desiderata of nautical astronomy 

 has always been the means of observing at sea the eclipses of 

 Jupiter's satellites, — so frequently recurring and affording so simple 

 and direct a means of obtaining the longitude. The same want 

 exists also with respect to a variety of other astronomical observa- 

 tions which it is often desirable, if not necessary, to make at sea. 

 Accordingly, this object has engaged the attention of many inven- 

 tors of schemes for supporting the telescope, and the observer with 

 it, so as to be free from the motion of the ship. 



In general, to procure stability on ship-board, it seemed an 

 obvious recource simply 1o suspend any object which it was desired 

 to keep steady by cords from a fixed point in the vessel. But 

 a body thus suspended is like a plumb line, when the point of 

 support is itself set in motion : it acquires a part of that motion 

 and becomes a pendulum ; and it oscillates more irregularly and 

 violently from the accumulation of motions impressed upon it con- 

 tinually by every fresh motion of the ship. The case is th^ same 

 as that just considered in Troughton's top. 



There is, indeed (as Prof. Smyth remarks) a semblance of 

 steadiness in that (for example) articles keep their places on a 

 table so suspended ; a glass of water placed on it does not spill : 

 but this is only a case of the same kind as when an object so sus- 

 pended and whirled round, is retained in its place from centri- 

 fugal force ; its surface keeps perpendicular to the string, not 

 parallel to the horizon. Nairi^e's or Irwin's " Marine Chair," for 

 carrying the observer an(J his telescope, was simply an application 

 of this principle. It was tried on board ship, especially in a voyage 

 to the West Indies, by the late Dr. Maskelyne : and though some- 

 what prematurely rewarded by the Government, was found not to 

 answer : though no one seemed fully aware of the cause of its 

 failure, till Sir J. Herschel (in the Admiralty Manual) pointed 

 out the principle just stated, and showed that this free suspension 

 must tend to perpetuate disturbances rather than destroy them. 



In these cases the centre of gravity is below ; this is what con- 

 stitutes the table or chair, a pendulum. If it were suspended in 

 gymbals, so that the centre of gravity should be at the point of 

 suspension, the tendency to oscillate from this cause would be over- 

 come. But still any slight cause might disturb the level : there 

 would be no principle of permanent stability. 



Thus, to produce this desired stability for a plane or stand on 

 which the telescope is to be rested, we must have recourse to the 

 free revolving disk accurately balanced within gymbals, on its 

 centre of gravity. The balancing must be perfected by means of 

 the adjustable i)lugs before-mentioned both in the disk, and 

 in the gymbul frames ; the pivots of the gymbals must be of 



