ON AN INSTRUMENT TO ILLUSTRATE POINSOT's THEORY OF ROTATION. 247 



That by working the vertical bolts, we can make the axis of the instrument 

 the centre of this closed curve. It will then be one of the principal axes of 

 inertia. 3rdly. That, by working the nut on the axis, we can make the order 

 of colours either red, yellow, green, blue, or the reverse. When the order of 

 colours is in the same direction as the rotation, it indicates that the axis of the 

 instrument is that of greatest moment of inertia. 4thly. That if we screw the 

 two pairs of opposite horizontal bolts to different distances from the axis, the 

 path of the instantaneous pole will no longer be equidistant from the axis, but 

 will describe an ellipse, whose longer axis is in the direction of the mean axis 

 of the instrument. 5thly. That if we now make one of the two horizontal axes 

 less and the other greater than the vertical axis, the instantaneous pole will 

 separate from the axis of the instrument, and the axis will incline more and more 

 till the spinning can no longer go on, on account of the 'obliquity. It is easy 

 to see that, by attending to the laws of motion, we may produce any of the 

 above effects at pleasure, and illustrate many different propositions by means of 

 the same instrument. 



