NATURAL PHILOSOPHY. 173 



standing the motion of the ship. The method was, however, found practically 

 to fail; and the failure has been since traced to another mechanical principle. 

 The pivot partakes in the irregular motion of the ship. When the disk is not 

 revolving, this motion is in turn communicated to the disk, and the centre of 

 gravity being below, the very circumstance which gives it stability on land, 

 causes it to acquire an oscillatory movement. When in rotation this does 

 not show itself directly, but is compounded with the rotation, and causes a 

 processional motion, which is fatal to its use as a horizontal reflector. Hence, if 

 the centre of gravity coincided -with the point of support, as would be most 

 readily done by suspending the revolving disks in gymbals in the manner of 

 Bonenbergcr's machine, this cause of irregularity would be avoided. By 

 this means it would preserve its original level; but this would not nece^sa- 

 rily, or usually, be the true horizontal level. To obtain the true Jtorizontal point 

 another contrivance has been made by the same inventor, which has been 

 fully described in the " Notices of the Royal Astronomical Society," A*ol. 

 xviii., p. do, January, 18-58. But for other classes of observation on board 

 ship which involve the use of the telescope, especially those requiring one of 

 considerable 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. In general, to pro- 

 cure stability on ship-board, it seemed an obvious recourse simply to subtend 

 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 continually by every fresh 

 motion of the ship. The case is the same as that just considered in Trough- 

 ton's top. Nairne's or Irwin's " Marine Chair," for carrying the observer 

 and 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 somewhat prematurely rewarded by the govern- 

 ment, was found not to answer; thoutrh 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 to destroy them. Thus, to 

 produce the 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 bal- 

 anced irifltin gymbals, on its centre of gravity. The balancing must be per- 

 fected by means of adjustable plugs both in the disk and in the gymbal 

 frames; the pivots of the gymbals must be of 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 which its suspension must be quite independent, so that the moving 

 power can be instantaneously withdrawn. All these conditions are fulfilled 

 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 success- 

 fully tried on board Mr. Stephenson's yacht Titania, on his voyage to Tcn- 

 criffe, in 185G. The grand principle of ^fixity in the plane of free rotation, is 

 that which enables the revolving disk to retain parallelism to its original 

 plane, however the external plane or pivots supporting the whole be moved. 



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