184 TENTH ANNUAL REPORT OF 



If the attraction of the sun and moon was greater than it is, we 

 reason that the precession would he more rapid; and if less, it would 

 be slower. The experiment is easily modified, to shoAv the correct- 

 ness of these conclusions. I take off the weight, and put on a heavier, 

 and the horizontal movement is hastened ; if I put on a smaller v.'eight 

 you sec it slackened ; and finally, if I remove the weights altogether, 

 the phenomenon ceases, as it should do. 



Once more, we know that if the earth were to revolve more rapidly 

 on its axis than it now does, the present attraction of the sun and 

 moon would produce less effect to change the axis ; in other words, 

 the precession would he slower, and vice versa. In illustration of 

 this, observe that, as the spheroid loses some of its velocity, (with a 

 given weight on the ring,) the horizontal circulation is gradually gain- 

 ing speed ; and so it will continue to do as long as 1 let the experi- 

 ment continue. 



We may here notice why the precession is so excessively slow : 



1st. The ring of matter on which the sun and moon act is an ex- 

 ceedingly small fraction of tlie whole earth. 



2d. It is not the ivhole attraction of those bodies upon the ring which 

 causes this disturbance, but only that part by which it exceeds or falls 

 short of the attraction on tlie internal portions. 



3d. It is not even the lohole of this difference, but only that com- 

 ponent which is perpendicular to the plane of the ecliptic. 



4th. The ring cannot move alone, in obedience to this influence, but 

 must carry the entire earth with it. 



No wonder, then, that the effect is almost too small to be observed. 

 We may well say, when explaining the seasons, that the earth's axis 

 is, in every part of its orbit, parallel to itself. 



It is interesting to see so delicate a phenomenon as the precession 

 of the equinoxes completely accounted for. A cause is found, which 

 is not only right in the direction of its action, but exactly right, too, 

 in quaniiiy, to c^use this almost insensible disturbance. It is just as 

 small as it shoidcl be, considering that the earth is as Jarge as it is, 

 and as heavy as it is, and revolves as often as it does; that the ring is 

 as small as it is_, inclined as it is to the ecliptic, and confined as it is to 

 the earth ; that the su7i and moon are just as massive and as distant 

 as they are, and varying as they do their relations to the line of the 

 equinoxes. All these, and still other conditions, being j\(st as they 

 are, if the precession Avas any faster or any slower than about fifty 

 seconds in the year — that is, about the width of the sun in forty 

 years — then this motion would not be accounted for. But, be- 

 sides the agreement of calculated results with the observed facts, 

 which so iew are able to appreciate, the same phenomenon can be 

 shown by experiments ; a body being made to revolve like the earth, 

 and a force being brought to act on it as the sun does on the earth, 

 the phenomenon is artificially produced before our eyes. 



It is to be observed that if the equator, having an inclination of 

 23^° to the ecliptic, directs that inclination every way in the course 



at the screws L and E, revolve horizontally in the direction of the double-shaft arrow. 

 As the component forces are always at right angles with each other, their resultant is 

 perpetually reproduced. 



