178 AN EMPIRICAL STUDY OF GYRATING BODIES. 



Evidently, we have here no force that accounts for the 

 tilting of the protuberance from the ecliptic. Yet such 

 a movement certainly occurs. 



Reflecting upon this in the light of the laws of gyra- 

 ting bodies, I recollected that any force tending to send 

 the gyroscope forward in its horizontal movement more 

 rapidly than it would go under the influence of gravity 

 alone, made it rise against gravity, or, in other words, 

 made it go in a direction opposite to the force that was 

 pulling it down. 



Could this law — it is our "first law 1 ' — apply to the 

 earth \ Does it ever gyrate more rapidly than is due to 

 the pull, at that very instant, of the sun and moon ? As 

 these act in the same sense, we may, for convenience, 

 speak only of one, say the moon ; remembering that what 

 is true of that is true, although in a less degree, of the 

 sun. 



When the moon is nearest to us, and at the same time 

 the angle between it and the equator is greatest, then, 

 evidently, its tilting power is at its maximum. But 

 soon the moon's distance from the earth begins to in- 

 crease, and the angle between it and the earth's equator 

 to grow less, and hence its tilting power also diminishes ; 

 but as this does not affect the precessional velocity of 

 the previous instant it follows that the earth, in this sec- 

 ond instant, gyrates more rapidly than it would if it 

 were acted upon only by the pull of the moon ; and 

 therefore the equator ought to rise. In other words, it 

 ought to recede from the plane of the ecliptic, and it 

 should continue to recede as long as the tilting force con- 

 tinues to grow less. When the tilting force ceases to 

 grow smaller, the receding should stop. If the tilting 

 force becomes zero, both precession and nutation should 

 cease. 



When the tilting force begins to grow larger, preces- 

 sion recommences, and the equator commences to tilt to- 



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