MOTION, MECFIANICS, ETC. l7l 



this frame are two balls, D and E, of unequal weights, con- 

 nected by a tube,/, and made to slide easily upon the wire, 

 C, stretched from end to end of the frame, and made fast 

 by nut-screws on the outside of the ends. Let these balls 

 be so placed upon the wire, C, that their common centre of 

 gravity, g, may be directly over the centre of the whirling- 

 board. Then turn the machine by the winch, ever so swiftly, 

 and the trough and balls will go round their centre of gravity, 

 so as neither of the balls will fly off; because, 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 frame at A, it will remove the 

 centre of gravity towards that end from the centre of mo- 

 tion ; and then, upon turning the machine, the little ball, E, 

 will fly off, and strike with considerable force against the 

 end, A, and draw the great ball, B, into the middle of the 

 frame, or if the great ball, D, be drawn towards the end, 13, 

 of the frame, so that the centre of gravity may be a little 

 towards that end from the centre of motion, and the 

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

 oft* and strike violently against the end, B, of the frame, and 

 will bring the little ball, E, into the middle of it. If the 

 frame be not made very strong, the ball, D, will break 

 through it. 



The earth's motion demonstrated. From the principles 

 thus established, it is evident, that the earth moves round 

 the sun, and not the sun round the earth ; for the centri- 

 fugal law will never allow a great body to move round a 

 small one in any orb whatever ; especially when we find, 

 that if a small body moves round a great one, the great one 

 must also move round the common centre of gravity be- 

 tween them. And it is well known, that the quantity of 

 matter in the sun is 227,000 times as great as the quantity 

 of matter in the earth. 



Now, as the sun's distance from the earth is at least 

 81,000,000 of miles, if we divide that distance by 227,000, 

 we shall only have 357 for the number of miles that the 

 centre of gravity betAveen the sun and earth is distant from 

 the sun's centre. And as the sun's semidiameter is one- 

 fourth of a degree, which, at so great a distance as that of 

 the sun, must be no less than 381,500 miles, if this be 

 divided by 357, the quotient will be 1268|, which shows, 

 that the common centre of gravity between the sun and 



