18S5.] NEW YORK ACADEMY OF SCIENCES. 39 



end of the cross moves away from the plumb to the right, while 

 the lower moves away from it also, but to the left. If, while 

 the left hand remained stationary, the cross had been allowed 

 to drop freely, the top and bottom would evidently acquire a 

 certain horizontal motion, one to the right, the other to the left. 

 If now the T-square l)e quickly turned over so that the top 

 and bottom chai)ge places, this will not interfere vrith motion 

 previously acquired ; the Ijottoni (which h'ls now become the 

 top) will continue to move to the left, while that which a 

 moment before was the top will move to the right, and as the 

 motion continues (as in case of a pendulum), the ends of the 

 cross are puslied back to where they were, and the instrument 

 rises to its first position. So long as this continues, the instru- 

 ment will maintain its position, or at least it would if the 

 change was instantaneous. As it is not, it falls slowly. 



This answers the first question. 



Secondly, why does the instrument rotate around the central 

 point, in a direction always tlie opposite of that of the top of 

 the disk i 



Take the T-square again. Let it fall again a few inches. 

 As in the first experiment, the top, when the T goes down, 

 gets a motion towards the right — (if held as liefore) — but, 

 before the instrument can be reversed, it must go half way, 

 aud point, instead of up and down, horizontally. Evidently 

 the motion which sends the upper end to the right, Avill push 

 the instrument (if the top was revolved towards the south), 

 towards the north. Hence we have the horizontal motion. 



Another question naturally suggests itself. Why, it may be 

 asked, is the horizontal motion slow in proportion as that of the 

 disk is rapid ;' 



All depends upon the movement of the arms of the T. If 

 this tm'ns slowly, it has more time to give motion to the ends 

 of its arms, consequently they push it around faster. If the 

 T turns very quickly, it falls a very short distance (has so 

 little time), hence the ends of the arms get very little motion, 

 and, of course, can impart but little. Hence a quick motion of 

 the disk makes a slow horizontal movement, and a slow motion 

 of the disk makes a quick horizontal movement. 



A careful consideration of the above will render it easy to 

 see why the Gyroscope ceases to maintain itself if the lateral 

 (or horizontal) motion is stopped, for in order to maintain 



