WHIRLING-TABLE. 



(liftance of U from tu ; it will be found, upon turning the 

 machine by the winch, that the balls U and V will raife 

 their refpeftive weights at very nearly the fame inftant of 

 time. This experiment confirms the famous propofition of 

 Kepler, viz. that the fquares of the periodical times of the 

 planets round the fun are in proportion as the cubes of 

 their diftances from him; and that the fun's attratlion is in- 

 verfely as the fquare of the dillancc from liis centre. 



8. Take off the firing E from the wheels D and^H, and 

 let the firing F remain upon the wheels D and G ; take 

 away alfo the bearer M X from the whirling-board </, and 

 inflead of it put on the machine A B ( Piute XXI. Jig. 14. 

 ^Jronomy\ fixing it to the centre of the board by the 

 pins c and d, fo that the end ef may rife above the board 

 to an angle of 30 or 40 degrees. On the upper part of 

 this machine, there are two glafs tubes a and 3, clofe flopped 

 at both ends ; each tube being about three-quarters full of 

 water. In the tube a is a little quickfilver, which naturaUy 

 falls down to the end a in the water ; and in the tube ^ is a 

 fraall cork, floating on the top of the water, and fmall 

 enough to rife or fall in the tube. While the board b with 

 this machine upon it continues at refl, the quickfilver lies at 

 the bottom of the tube a, and the cork floats on the water 

 near the top of the tube b. But, upon turning the winch 

 and moving the machine, the contents of each tube will fly 

 off towards the uppermoft ends, which are farthefl from the 

 centre of motion : the heaviefl with the greatefl force. 

 Confequently, the quickfilver in the tube a will fly off quite 

 to the end/, occupying its bulk of fpace and excluding the 

 water, which is lighter than itfelf : but the water in the 

 tube b, flying off to its higher end e, will exclude the cork 

 from that place, and caufe it to defcend toward the lowefl 

 end of the tube ; for the heavier body, having the greater 

 centrifugal force, will pofTefs the upper part of the tube, 

 and the lighter body will keep between the heavier and the 

 lower part. 



This experiment demonfirates the abfurdity of the Car- 

 tefian doftrine of vortices : for, if the planet be more denfe 

 or heavy than its bulk of the vortex, it will fly off in it far- 

 ther and farther from the fun ; if lefs denfe, it will come 

 down to the loweft part of the vortex, at the fun : and the 

 whole vortex itfelf, unlefs prevented by fome obllacle, 

 would fly quite off, together with the planets. 



9. If a body be fo placed upon the whirling-board of the 

 machine {Jig. 13.) that the centre of gravity of the body 

 be direftly over the centre of the board, and the board be 

 moved ever fo rapidly by the winch B, the body will turn 

 round with the board, without moving from its middle ; for, 

 as all parts of the body are in equilibrio round its centre of 

 gravity, and the centre of gravity is at refl in the centre of 

 motion, the centrifugal force of all parts of the body will be 

 equal at equal diftances from its centre of motion, and there- 

 fore the body will remain in its place. But if the centre of 

 gravity be placed ever fo little out of the centre of motion, 

 and the machine be turned Iwiftly round, the body will fly 

 off towards that fide of the board on which its centre of 

 gravity lies. Then, if the wire C {Jig. 15.) with its little 

 ball B be taken away from the femi-globe A, and the flat 

 fide efof the femi-globe be laid upon the whirhng-board, 

 fo that their centres may coincide ; if then the board be 

 turned ever fo quickly by the winch, the femi-globe will 

 remain where it was placed : but if the wire C be fcrewed 

 into the femi-globe at d, the whole becomes one body, whofe 

 centre of gravity is at or near d. Fix the pin c in the centre 

 of the whirling-board, and let the deep groove i cut in the 

 flat fide of the femi-globe be put upon the pin, fo that the 

 pin may be in the centre of A (fee _/^. 16. where the 



groove is reprefented at b), and let the board be turned by 

 the winch, which will carry the little ball B [Jg. 15.) with 

 its wire C, and the femi-globe A, round the centre pin c i ; 

 and then, the centrifugal force of the little ball B, weighing 

 one ounce, will be fo great as to draw off the femi-globe 

 A, weighing two pounds, until the end of the groove at e 

 ftrikes againfl the pin c, and fo prevents A from going any 

 farther : otherwife, the centrifugal force of B would have 

 been great enough to have carried A quite off the whirling- 

 board. Hence we fee, that if the fun were placed in the 

 centre of the orbits of the planets, it could not pofllbly re- 

 main there ; for the centrifugal forces of the planets would 

 carry them quite off, and the fun with them ; efpecially 

 when feveral of them happened to be in one quarter of the 

 heavens. For the fun and planets are as much connefted 

 by the mutual attraftion fiibfifting between them, as the 

 bodies A and B are by the wire C fixed into them both. 

 And even if there were but one planet in the whole heavens 

 to go round ever fo large a fun in the centre of its orbit, its 

 centrifugal force would foon carry off both itfelf and the 

 fun : for the greateft body placed in any part of free fpace 

 could be eafily moved ; becaufe, if there were no other 

 body to attraft it, it would have no weight or gravity of 

 itfelf, and confequently, though it could have no tendency 

 of itfelf to remove from that part of fpace, yet it might be j 

 very eafily moved by any other fubflance. I 



10. As the centrifugal force of the light body B will not 

 allow the heavy body A to remain in the centre of motion, 

 even though it be twenty-four times as heavy as B ; let the 

 ball A {Jig. 17.) weighing fix ounces be connefttd by the 

 wire C with the ball B, weighing one ounce ; and let the 

 fork E be fixed into the centre of the whirling-board; then, 

 hang the balls upon the fork by the wire C in fuch a man- 

 ner, that they may exaftly balance each other, which will 

 be when the centre of gravity between them, in the wire at 

 d, is fupported by the fork. And this centre of gravity is 

 as much nearer to the centre of the ball A than to the 

 centre of B, as A is heavier than B ; allowing for the 

 weight of the wire on each fide of the fork. Then, let the 

 machine be moved, and the balls A and B will go round 

 their common centre of gravity d, keeping their balance, 

 becaufe either will not allow the other to fly off with it. 

 For, fuppofing the ball B to be only one ounce in weight, 

 and the ball A to be fix ounces ; then, if the wire C were 

 equally heavy on each fide of the fork, the centre of gravity 

 d would be fix times as far from the centre of B, as from 

 the centre of A, and confequently B will revolve with a 

 velocity fix times as great as A does ; which will give B 

 fix times as much centrifugal force as any fingle ounce of 

 A has : but then, as B is only one ounce, and A fix ounces, 

 the whole centrifugal force of A will exaftly balance that 

 of B ; and therefore each body will detain the other, fb as 

 to make it keep in its circle. 



Hence it appears, that the fun and planets muft all move 

 round the common centre of gravity of the whole fyfteai, 

 in order to preferve that jufl balance which takes place 

 among them. 



1 1 . Take away the forks and balls from the whirling- 

 board, and place the trough A B {Jig. 18.) thereon, fix- 

 ing its centre to that of the board by the pin H. In this 

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

 nefted by a wire f, and made to Aide eafily upon the wire 

 C ftretched from end to end of the trough, and made fall 

 by nut-fcrews on the outfide of the ends. Place thefe balls 

 on the wire C, fo that their common centre of gravity g may 

 be direftly over the centre of the whirhng-board. Then, 

 turn the machine by the winch ever fo fwiftly, and the 



trough 



