46 FERGUSON'S LECTURES. 



LECT. wire C stretched from end to end of the trough, and made 

 ,^-v^/ fast by nut-screws on the outside of the ends. Let these 

 balls be so placed upon the wire C, that their common cen- 

 ter of gravity g may be directly over the center of the whirl- 

 ing-board. Then, turn the machine by the winch, ever so 

 swiftly, and the trough and balls will go round their cen- 

 ter of gravity, so as neither of the balls will fly off; be- 

 cause, on account of the equilibrium, each ball detains the 

 other with an equal force acting against it. But if the 

 ball E be drawn a little more towards the end of the 

 trough at A, it will remove the center of gravity towards 

 that end from the center of motion ; and then, upon 

 turning the machine, the little ball E will fly off, and 

 strike with a considerable force against the end A, and 

 draw the great ball B into the middle of the trough. 

 Or, if the great ball D be drawn towards the end B of 

 the trough, so that the center of gravity may be a little 

 towards that end from the center of motion, and the 

 machine be turned by the winch, the great ball D will 

 fly off, and strike violently against the end B of the 

 trough, and will bring the little ball E into the middle 

 of it. If the trough be not made very strong, the ball 

 D will break through it. 



Of the 12. The reason why the tides rise at the same abso- 

 lute time on opposite sides of the earth, and consequent- 

 ly in opposite directions, is made abundantly plain by a 

 new experiment on the whirling-table. The cause of 

 their rising on the side next the moon every one under- 

 stands to be owing to the moon's attraction : but why 

 they should rise on the opposite side at the same time, 

 where there is no moon to attract them, is perhaps not 

 so generally understood. For it would seem, that the 

 moon should rather draw the waters (as it were) closer 

 to that side, than raise them upon it, directly con- 

 trary to her attractive force. Let the circle abed 



