57 



rigid external crust, the force must accumulate until it 

 exceeds the resistance, and thus frequent adjustments, 

 per saltern, may ensue. It is probable, therefore, that the 

 earth's form is undergoing a slow progressive change ; 

 and if so, it must be taken into account by those who 

 venture to speculate on the causes of earthquakes, vol- 

 canic eruptions, and the upheaval of mountains. 



IV. DESCRIPTION OF THE GYUOSCOPE. 



1. When the disc is in rapid motion it resists any force 

 which tends to alter its plane of rotation, because the 

 point at which the force is applied is conveyed to the 

 point diametrically opposite, before it has time to move in 

 any perceptible degree in the direction of the impressed 

 force. 



2. When a force is applied to turn the disc about a 

 vertical axis, whilst it is turning on a horizontal axis, the 

 plane of rotation is changed. Assume a point in the 

 upper edge of the disc. It tends to move in the direction 

 of a tangent, and when the disc is moved about a vertical 

 axis, the point assumed still tends, by its inertia, to con- 

 tinue its motion in the same direction as before, that is, 

 now obliquely to the disc, and therefore tending to alter 

 its plane of rotation. 



3. When a weight is suspended to the extremity ot 

 the horizontal axis on which the disc turns, the plane of 

 the disc is not sensibly drawn' out of the perpendicular, 

 but a motion is produced about the vertical axis. Assume 

 a point in the upper edge of the disc. Place a thread on 

 it by means of a piece of sealing wax, leaving the two 

 ends free from the point of attachment. Pull one thread 

 in the direction of the tangent, thus producing a rotation 

 of the disc : pull the other at right angles to the plane of 

 the disc, thus producing the same effect as tbc wciglit. 

 This force cannot perceptibly alter the plane of rotation, 



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