172 ANNUAL OP SCIENTIFIC DISCOVERT. 



exhibited, they created unbounded surprise. In a former lecture before the 

 Loyal Institution (18o4), he had discussed the principle of "composition of 

 rotations/' and those applications of it which had been found in certain 

 rotatory phenomena of projectiles, illustrated the same by the gyroscope in 

 its several earlier forms as successively modified by Bonenberger, Atkinson, 

 Fessel, and Wheatstone, showing the identity of these results on a small 

 scale with the grand cosmical phenomenon of the procession of the equinoxes. 

 Since the date of that lecture, the striking results produced by merely carry 

 ing out the same principles, and applying the gyroscope to demonstrate 

 directly the fact of the earth's rotation, as well as under other conditions to 

 point to the poles, by M. Foucault, have become familiarly known. 



In recurring to the subject on the present occasion, the object is to explain 

 another application of the same principles, like the former, very obvious u-ln-n 

 once disclosed; but which, nevertheless, remained unknown and unthought of 

 until it was pointed out and actually effected by the inventions of Professor 

 C. P. Smyth; the use of rotatory apparatus for giving an invariable plane or 

 platform for astronomical instruments used at sea. To render the subject intel- 

 ligible it is necessary to recur to two simple first principles in dynamics, 

 which, when distinctly apprehended, give the clue to the whole of the appli- 

 cations. The first of these is the tendency of a body in rotation to retain 

 that rotation in the same plane, when perfectly balanced, irrespective of the 

 motion of external objects, which is termed "the fixity of the plane of 

 rotation." The second is "the composition of rotatory motion; " or that 

 when a force is impressed on aboil}- in rotation, it does not show itself 

 directly, but is compounded with the first motion, so that the rotation takes 

 an intermediate direction, or the axis shifts its position in space; this being 

 the cause of the motion of the earth's axis, giving rise to the precession of 

 equinoxes, it is called generally a " processional motion." This first princi- 

 ple is that chiefly referred to in the inventions about to be described, where 

 the effect depends essentially on the great amount of resistance thus offered 

 to any angular motion impressed by an extraneous cause on a perfectly- 

 balanced revolving heavy disk. The most important observations requisite to 

 be made at sea are those of the altitudes of the heavenly bodies, on which de- 

 pend both the determination of the latitude, and the correction of time essential 

 to finding the longitude ; and for this purpose there is a necessity for a well- 

 defined horizon, which it is often impossible to obtain from the atmosphere in 

 its lower parts, when the sun or star can be distinctly seen above, and this more 

 especially at night; yet the safety of the ship may essentially depend on such 

 an observation. Hence various plans have been resorted to for obtaining an 

 artificial horizon. Simple reflection from the surface of a liquid can hardly 

 ever be practicable, on account of the motion of the ship, though it is the usual 

 substitute on land. The most celebrated attempt to substitute some other 

 principle was an application of rotatory motion, devised by the late Mr. Trough- 

 ton, in 1820. It consists in causing a disk, truly balanced on a fixed pivot, to 

 spin round with great velocity, so as to keep up its motion during the time 

 required for an observation, known by the name of " Troughton's top." The 

 disk carried a plane reflector on its upper surface; and being a cylinder hol- 

 lowed out at its lower end, and the point of support \vithin, the centre of grav- 

 it}* is thrown below, so that it is in stable equilibrium when at rest. The 

 velocity is communicated by a separate train of wheels, from which it can bo 

 instantly detached. Thus, from the principle ofjixity of theplane of rotation, 

 it was expected that the reflecting surface would preserve its level, notwith- 



