2a 



the outer edge of its circumference attach a small disk by a 

 thread through its centre, to serve as its axis. Draw a straight 

 line between the two axes to represent the radius-vector, and then 

 make the large disk revolve on its axis, and it will be seen at 

 once that, in whatever direction it may be turned, the small disk 

 will remain as unaffected by the movement as if it were an in- 

 tegral part of the mass, and that the relations of the several points 

 of its circumference to the axis and the radius are affected by its 

 own movements only^ round its own axis, quite independently of 

 the movements of the large disk to which it is attached. The lid 

 of a small pill-box will answer nicely for the small disk, if a 

 thread be attached to one point of its circumference, having its 

 other end fixed to the centre of the larger disk, and the large 

 disk be made to revolve with the parallelism of the small disk 

 preserved, the thread will be drawn all round \\\q small disk, 

 showing incontestably that disk's rotation on its axis. 



Upon the face of a wall, or other flat perpendicular surface, 

 draw a circle one foot in diameter, and divide it into quadrants 

 by two diametric lines crossing at right angles in the centre. 

 Then take a stick, staff, or rod, one foot long, with four square 

 sides, and attach it by a hinge or other ligature to a fixed pivot 

 in the centre of the orbit. At the middle of the rod, which of 

 course will be the distance of the orbit from its centre, fix 

 another pivot, (on the rod itself, not on the wall,) and on that 

 pivot hang a string with an open loop, having at its other end a 

 plummet which shall keep it stretched as far as the centre of the 

 circle, when the rod is placed perpendicularly with its whole 

 length above that centre. In other words, the string, including 

 both loop and plummet, must be of precisely the length of half 

 the rod, or six inches. Mark the quadrants as N, W, S, E ; 

 N and S being at the opposite ends, N at the upper, and S 

 at the lower end of the circle's perpendicular diameter, while E. 

 will be at the right hand end, and W at the left hand end of 

 the cross diameter. Then attach the rod to the centre of the 

 circle on the line N S with its own middle or pivot c on the 

 orbit at N ; and on the latter pivot hang the looped string and 

 plummet. The lower half of the rod, while so attached to the 

 centre of the circle, will, of course, be identical, or virtually the 

 same, with the mathematical radius-vector of the pivot c to which 

 the plummet is hung. When thus prepared, set the machine 

 in motion, making the rod revolve round the central pivot of the 

 circle, from west to east, that is to say, from right to left at 

 starting, so continuing always forward in the same direction. 

 As the rod revolves, the plummet will be seen diverging from 

 it to the left, in the same proportion, degree for degree, moving 

 away from the centre of the circle, just as its own pivot c moves 



