THE SMITHSONIAN INSTITUTION. 1Y7 



itself, its north pole all the time pointing nearly in the direction of 

 the so-called north star. Were it not so, our seasons would not be 

 preserved. Let this horizontal wooden ring represent the plane of 

 the eclii>tic, the lamp in the centre the sun, and the six-inch globe 

 revolving on tlie axis which I hold, the earth. As I carry the globe 

 around the ring, with the e'i|uator oblique to it, and keep the axis di- 

 rected to the same point in the sky, you perceive that the upper pole 

 is now in the light of the lamp ; now, after a quarter revolution, the 

 light just reaches to both poles ; and now, when carried half round, 

 the upper pole is turned av.^ay from the light, and the lower one to- 

 wards it; and, once more, after three-quarters of a revolution, both 

 poles are again in the edge of the enlightened hemisphere. The axis 

 being held ])arallel to itself, and the globe all the time spinning upon 

 it, you perceive that the upper hemisphere in the first position has 

 the long days and short nights of summer ; in the second, the equal 

 days and nights of autumn ; in the third, the short days and long 

 nights of winter ; and in the last, the equal days and nights of spring. 

 In the lower hemisphere, all these facts are reversed. So, also, the 

 moon's axis is not exactly perpendicular to the plane of its orbit ; and, 

 as its equator continues parallel to itself, we alternately see the north 

 and south poles of the moon presented to us — a phenomenon called 

 the moon's libration in latitude. In the foregoing illustration, we 

 have only to suppose the wooden ring to be the moon's orbit, and the 

 small globe the moon, while the lamp in the centre occupies the place 

 of the earth. 



In like manner, the orbits and equators of all the planets and satel- 

 lites in the system show plainly a tendency to maintain a parallelism 

 at all times. That these planes are not really and precisely parallel, 

 is the result of disturbing influences, to be noticed presently. 



In order to show this tendency experimentally, it is necessary that 

 the revolving body should be free to place its axis in all directions. 

 This is done by sv/inging it in gimbals, somewhat like the mariner's 

 compass. The instrument before you was called by the inventor, the 

 late Professor Walter R. Johnson, the Rotascope.* It very much re- 

 sembles Bohnenberger's apparatus for illustrating the precession of 

 the equinoxes, but is many times larger, and has several appendages 

 for various experiments on rotatory motion. The outer brass ring is 

 free to revolve on a vertical axis in the wooden frame ; the inner ring 

 can revolve freely on a horizontal axis in the outer one ; and the spheroid 

 in the inner ring has its axis perpendicular to tliat of 

 the ring itself. (Fig. 1.) Thus, you perceive, the 

 spheroid, by means of the rectangular axis, is free to 

 revolve in any plane whatever. I now set it spinning, 

 (by looping a cord upon the small pin in the axis, 

 winding it up, and then drawing the ends apart till 

 it is unwound and detached,) and elevate somewhat 

 that end of the axis which is nearest to you, that you 

 may see its position better. I now take up the frame 

 in my hands, and carry it about the platform, and 

 turn it to every point of the compass, and tip it over to any angle^, even 



""" See Professor W. R. Johnson's "Description of the Rotascope," in the American 

 Journal of Science aud Aits, for January, 1832, p. 265, et seq. The instrument usod in 

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