THE MOTION OF THE EARTH. 13 



will then be seen that the pendulum will still vibrate in the 

 direction of the line we have continued onward to the wall, 

 but that the line on the table is gradually withdrawn 

 from it. If we had no upright, we might turn the table 

 entirely round, without in the slightest degree altering 

 the line along which the pendulum performs its oscilla- 

 tions. Now, if from some elevated spot, say, from the 

 centre of the dome of St. PauPs, a long and heavy pen- 

 dulum is suspended, and if on the floor we mark the line 

 along which we set the pendulum free to vibrate, it will 

 be seen, as in the experiment with the table, that the 

 marked line moves away from under the pendulum. It 

 continues to vibrate in the plane it first described, 

 although the line on the earth's surface continues to 

 move forward by the diurnal rotation around the axis. 

 Similar to this is the law of the conservation of the axis 

 of rotation. If a common humming-top, the spindle of 

 which is its axis of rotation, is set spinning obliquely, it 

 will be seen that the axis will continue to point along 

 the line it took at the commencement of motion. 

 By placing a heavy sphere in a lathe, resting its project- 

 ing axial points on some moveable bearings, and then 

 getting the sphere into extremely rapid motion, one of the 

 bearings may be removed without the mass falling to 

 the ground. The rapidity of motion changes so con- 

 stantly and quickly the position of the particles which 

 have a tendency to fall, that we have motion balanced 

 against the force of gravitation in a striking manner ; 

 and we learn, from this experiment, the explanation 

 of the planetary and stellar masses revolving on their 

 axis at a speed sufficient to maintain them without sup- 

 port in space. A mass of matter, a sphere or a disc, 

 carefully balanced, is fixed in gymbals such as we employ 

 for fixing our compass needles, and it is set by some 

 mechanical contrivance in rapid rotation. The position 

 of the axis of rotation remains unaltered, although the 

 earth is moving; and thus, by this instrument, called the 



