ASTRONOMY.

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(142.) The examples of the great differences that may existbetween apparent and real motion, which we have already pro-duced (Art. 1,) i” vindication of the Earth ’s motion, arc thusconsidered and demonstrated by Dr. Keil: “ If,” says he,“ while a ship is moving very fast forward, a ball of load, or anyother heavy metal, were let fall from the topmast, the p lengersin the ship will observe the ball to fall perpendicularly downwards,and it will fall upon the deck just by the foot of the mast, alterthe same manner as it would fall were the ship at rest. But not-withstanding this, the true motion of the ball is not in the perpen-dicular, but in at) oblique line, in which it descends; and a spec-tator in another ship which is at anchor, will easily observe thisobliquity and curvity of its way, while it falls through the air.The reason of this appearance is easily shewn : for, according tothe first and principal law of Natural Philosophy, a body onceput into motion, endeavours to retain that motion, and to continuemoving in the same direction. Now, the ball while it was heldat the topmast went forward with the ship, and had its motioncommunicated to it; and therefore after it is left to fall, it will re-tain the same force to go forward as it did before ; and at thesame time, its weight carrying it downwards, it will both go for-ward and descend : for the two forces, one communicated by the'ship, and the other from gravity, will not hinder or diminish oneanother, they not being contrary. It will-therefore be moved asfast forward, and as much downward, as it would be, did the twoforces act upon it separately at different times. By these twoforces acting together, the rectitude of its way is only hindeted,which it would have, did the perpendicular and the horizontalforces act separately; and the real way of the ball through the airis a curve line, exactly like that which a body fakes when it is -thrown according to an horizontal direction : and in such a line itwill be observed to move by a spectator placed near it in anothership which is at rest. Besides, since the ball and mast are bothmoved forward with the same velocity, they will always remain atthe same distance fmn each other, and therefore the ball willtouch the deck just hv the foot of the mast. Moreover, the mo-tion of the ball forward is common to the ship, and all its parts,as likewise to the passengers that are relatively at rest in the.ship :but we have before shewed, that the common motion could notbe observed by the passengers in the ship, and therefore it cannotbe perceived neither while the ball is falling. Wherefore the onlymotion that can be seen, wU) be that which is imprest upon it byits gravity, which is peculiar to the ball, and by which it de'-scends. T. iierefore the passengers will see the ball descending onlyin a perpendicular line. Experiments have often been made,which demonstrate that all we have said is exactly true. If anyperson sitting at the ship’s head should throw a ball towards thestern with the same velocity that the ship goes forward, that ballwould neither go forward nor backward ; and if there were nogravity it would remain immoveable: but, because gravity actsupon it, it will really descend in a perpendicular line, and a "spec-tator in a ship at anchor would observe it descending in a rightline. For the force imprest upon it, when it is thrown, will onlydestroy the first force communicated to it from the ship, to whichthe projectile force is contrary and equal. But for all this thepassengers will not perceive this perpendicular and direct motion,but they will see the ball go towards the stern with the same force,ns it really would have doin', had the ship been at rest, and theball been thrown with the same force to the stern. But if the ve-locity with which the ball is thrown toward the stern should beless than that of the ship, the real motion of the globe will be for-ward, in the same direction in which the ship goes, but slowerthan it; for the whole motion communicated by the ship will notbe destroyed, and there will still remain a part of its former mo-tion, by which it will be carried forward, though not so fast as be-fore. But tbe passengers will perceive no such motion, but theywill observe the ball to be moved in a line directly contrary to itsreal motion, with that very velocity that it would have, were itthrown when the ship is at rest. Ilence it is plain that bodiesmay appear to have a motion directly contrary to their real andabsolute motion. But some mav object, that the ball thus thrownwill really hit the stern of the ship, and impress on it a consider-able blow, which it could not do, had it not a motion towards thestern. But this difficulty is easily removed ; for though they thatare within the ship see the ball’ go and hit the stern, a spectatorwithout, who is not in motion, will observe that the bull does notvo1 .1 —no 19.

come upon the stern and give it a stroke, but (hat the stern rushesupon tbe ball, and acts upon it with all il> force. And the torceof the stroke which each body receives is the same as if the shipbad been at vest, and tbe bail had fallen upon it v.-.th the samevelocity that the stern docs really come aga.nst the ball; tor it isknown from the laws of motion, that if there he any two bodies,A and B, equal or unequal, (he force of (he stroke will be thesame, whether B with a certain.degree of velocity conus upon Awhich is at rest; or il B shall be at rest, and A with the same ve-locity rushes upon B ; or if both bodies move the same way, butA moving faster by its greater velocity gives B an impulse, theforce of the stroke will be the same as if B were at rest, and Acame upon it with the difference of their velocities, that is, by theexcess wherewith the velocity of A is greater than that of B. Orlastly, if A and B have contrary motions, and hit one against theother, the greatness of the stroke will be the same as if one of themstood still, and the other came against it with the sum of their ve-locities. In one word, whatever the real velocities of the bodiesmay be, so long as their relative velocities, or the velocities bywhich they approach each oilier, remain the same, the force of thestroke will likewise remain the same. Hence it is, that in a ship,however swiftly it may sail, all our apparent motions, and themotions of every thing in the ship, do all appear to be the samethat they would, were the ship at rest; and it is observable thatflies and other insects keep the same motion in regard to one ano-ther, whether the ship is at rest, or sails uniformly forward withany degree of velocity, let it be never so great. And it is uni-versally true, that all bodies that are shut up in any one place,preserve the same motions in regard of one another, and all ap-pearances will be the same, whether the place remains immove-able, or has a direct uniform motion forward.” Keil’s Astron.Lect. II.

(143.) Nutation, or deviation of the Earth ’s axis is a small ap-parent motion in the fixed stars, first discovered by Bradley, theperiod of which is IS years. It is caused by the attraction of theMoon upon the spheroid of the Earth . AVe have before shewnthat the precession of the equinoxes is 50^- seconds every year.Of these 50”-j-, which are caused by the action of the Sun andMoon, 36" are produced solely by the action of the latter. Buton account of the variation of the lunar nodes, and the Moon ’sconsequent difference of declination in the same points of the zo-diac, there will necessarily result an inequality in the annual pre-cession of the equinoxes in different years. This inequality is what istermed the nutation, by the effect of which the distances of the starsfrom the equator will be altered. Dr. Bradley, from observationsmade between the years 1727 and 1747, determined the quantityof this very delicate equation of the precession to be 9 seconds.

Of the Atmosphere and Ivefraction of Lit.ht.

(144.) The Earth is covered to a considerable altitude with abody of air decreasing in density, and containing vapours and othermatter which compose the atmosphere. See Atmosphere. AVhena rav of light coming from any of flic heavenly bodies, enters thismedium it is turned from its rectilineal course into that of a curve,see Optics , and thus meets the eye in a different direction.Let Z Y, Plate XXV, fig, 7, he a quadrant of a vertical circle, A Ba quadrant on the Earth in the. same plane, and Gila quadranton the surface of the atmosphere; (hen (lie light from the star S,falling on the denser medium at E, will be refracted (by the lawsof optics) towards the perpendicular ET, and still more as it pro-ceeds, on account of the increasing density: hence it passes throughthe atmosphere in a curve, and meets the eye at A in a tangent tothis curve, and therefore it will appear higher than its true place,as at Q, so that to bring the apparent altitudes to the true ones, thequantity of refraction must be subtracted. The higher tliev risethe less are the rays refracted, and when the heavenly bodies arein the zenith, they sutler no retraction, as will appear from theprinciples of optics. By means of this refraction we have the Sunin view every day about seven minutes longer than we otherwiseshould have. Tables of refractions have been calculated by va-rious astronomers as Sir I. Newton , Mr. Thomas Simpson, Dr.Bradley, Mr. Mayer, &c. The following specimen is taken fromDr: Bradley’s table, which is esteemed the most correct, andchiefly used by astronomers. For the method of calculating thesetables, see Mr. Simpson’s Dissert, p. 46, 4to. Gregory’s Astron.Vol. 1- Pa. 66, and Vince’s Astrou. Vol, I. 4lo. ch. 7.

4 U (145.) Dr.