W H I 



irough and balls will go round their centre of gravity, fo as 

 neither of them will fly off; becaufe, on account of the equili- 

 brium, each ball detains the other with an equal force ailing 

 againft it. But if the ball E be drawn a little more towards 

 the end of the trough at A, it will remove the centre of 

 gravity towards that end from the centre of motion ; and 

 then, upon turning the machine, the little ball E will fly off", 

 and ftrike with a confiderable force againft the end A, and 

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

 if the great bail D be drawn towards the end B of the 

 trough, fo 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 off, and 

 ftrike violently againft 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 ftrong, the ball D will break 

 through it. 



12. Mr. Fergufon has explained the reafon why the tides 

 rife at the fame time on oppofite fides of the earth, and con- 

 fequently in oppofite direftions, by the following new expe- 

 riment on the whirling-table. For this purpofe, let a i c d 

 i.fis- "9' ) rcprefent the earth, with its fide c turned toward 

 the moon, which will then attraft the water fo as to raife 

 them from c to g : and in order to fhew that they will rife 

 as high at the fame time on the oppofite fide from a to e ; 

 let a plate A B [Jig. 20. ) be fixed upon one end of the 

 flat bar DC, with fuch a circle drawn upon it as a 3 c </ 

 (Jig- 19.) to reprefent the round figure of the earth and 

 fea ; and an ellipfe as efg h to reprefent the fwelling of 

 the tide at e and g, occafioned by the influence of the moon. 

 Over this plate AB fufpend the three ivory balls f,y, ^, 

 by the filk lines h, i, i, faftened to the tops of the crooked 

 ■wires H, I, K, fo that the ball at e may hang freely over 

 the fide of the circle c, which is fartheft from the moon M 

 at the other end of the bar ; the ball at_/"over the centre, and 

 the ball at g over the fide of the circle g, which is neareft 

 the moon. The bally may reprefqnt the centre of the earth, 

 the ball g water on the fide next the moon, and the ball e 

 water on the oppofite fide. On the back of the moon M 

 is fixed a fliort bar N parallel to the horizon, and there 

 are three holes in it above the little weights p, q, r. A 

 filk thread is tied to the line i, clofe above the ball g, 

 and pafTing by one fide of the moon M goes through a 

 hole in the bar N, and has the weight p hung to it. Such 

 another thread n is tied to the line i, clofe above the bally, 

 and pafiing through the centre of the moon M and middle 

 of the bar N, has the weight q hung to it, which is fighter 

 than the weight p. A third thread m is tied to the line h, 

 clofe above the ball e, and pafTiiig by the other fide of the 

 moon M, through the bar N, has the weight r hung to 

 it, which is lighter than the weight q. The ufe of thefe 

 three unequal weights is to reprefent the moon's unequal 

 attraftion at different diftanccs from her ; fo that if they 

 are left at liberty, they will draw all the three balls to- 

 wards the moon with different degrees of force, and 

 caufe them to appear as mjig.2\, m which cafe they 

 are evidently farther from each other than if they hung 

 freely by the perpendicular lines h, i, k. Hence it appears, 

 that as the moon attrafts the fide of the earth which is 

 neareft her with a greater degree of force than fhe does 

 the centre of the earth, flie will draw the water on that 

 fide more than the centre, and caufe it to rife on that 

 )ide ; and as flie draws the centre more than the oppofite 

 fide, the centre will recede farther from the furface of the 

 water on that oppofite fide, and leave it as high there as 

 the raifed it on the fide next her. For, as the centre wiU 

 be in the middle between the tops of the oppofite elevations, 



VV H I 



tliey muft of courfe be equally high on both fides at the 

 fame time. 



However, upon the fuppofition, the earth and moon 

 would foon come together ; and this would be the cafe, if 

 they had not a motion round their common centre of gra- 

 vity, to produce a degree of centrifugal force, fufBcient to 

 balance their mutual attraftion. Such motion they have ■ 

 for as the moon revolves in her orbit every month at the 

 diflanceof 240,000 miles from the earth's centre, and of 

 234,000 miles from the centre of gravity of the earth and 

 moon, the earth alfo goes round the fame centre of gravity 

 every month at the diftance of 6000 miles from it, ;. e. from 

 it to the centre of the earth. But the diameter of the 

 earth being, in round numbers, 8000 miles, its fide next the 

 moon is only 2000 miles from the common centre of gra- 

 vity of the earth and moon, its centre 6000 miles from it ; 

 and its fartheft fide from the moon 10,000 miles. Confe- 

 quently the centrifugal forces of thefe parts are as 2000, 

 6000, and 10,000 ; i. e. the centrifugal force of any fide of 

 the earth, when it is turned from the moon, is five times 

 as great as when it is turned toward the moon. And as 

 the moon's attraftion, expreffed by the number 6000, at 

 the earth's centre, keeps the earth from flying out of this 

 monthly circle, it muft be greater than the centrifugal 

 force of the waters on the fide next her ;' and confe- 

 quently, her greater degree of attraftion on that fide is 

 fufficient to raife them ; but as her attraftion on the op- 

 pofite fide is lefs than the centrifugal force of tke water 

 there, the excefs of this force is fufBcient to raife the water 

 juft as high on the oppofite fide. 



To prove this experimentally, let the bar D C with its 

 furniture be fixed on the whirling-board of the machine, 

 (Jig- 14-) ^y pulhing the pin P into the centre of the 

 board ; which pin is the centre of gravity of the whole bar 

 with its three balls e, f, g, and moon M. Now, if the 

 whirling-board and bar be turned flowly round by the winch, 

 till the ball f hangs over the centre of the circle, as in 

 Jig. 22, the ball ^ will be kept towards the moon by the 

 heavieft weight p (Jig. 20.), and the ball c, on account of 

 its greater centrifugal force, and the lefs weight r, will fly 

 off as far to the other fide, as in^^. 22. And thus, whilft 

 the machine is kept turning, the balls i- and^ will hangover 

 the ends of the ellipfe Ifi. So that the centrifugal force 

 of the ball e wiU exceed the moon's attraftion juft as much 

 as her attraftion exceeds the centrifugal force of the ball g, 

 whilft her attraftion juft balances the centrifugal force of 

 the bally, and makes it keep in its circle. Hence it is evi- 

 dent, that the tides muft rife to equal heights at the fame 

 time on oppofite fides of the earth. See Fergufon's Lec- 

 tures on Mechanics, left. 2, and Defag. Ex. Phil. vol. i. 

 left. 5. 



WHIRL-WIND, a wind that rifes fuddenly, and is ex- 

 ceedingly rapid and impetuous when rifen, but is foon fpent. 

 In this cafe, the gufts of wind proceed from different quar- 

 ters at the fame time, and meet in a certain place, where the 

 air acquires a circular, or rotatory, or fcrew-hke motion, 

 either afcending or defcending, as it were, round an axis, 

 which axis is fometimes ftationary, and at other times moves 

 on in a particular direftion. This phenomenon, called a 

 whirl-wind, gives a whirhng motion to duft, fand, water, 

 part of a cloud, and fometimes even to bodies of great 

 weight and bulk ; carrying them either upwards or down- 

 wards, and lailly, fcattering them about in different direc- 

 tions. 



There are divers forts of whirl-winds, diftinguifhed by 

 their peculiar names : as, the prejler, typho, turbo, exhydrla, 

 and ecnephias. 



The 



