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is called the curve of perpetual produced in a great measure by congelation. The equation to which the same causes. is found by making

ATOM, à particle of matter inн

divisible, on account of solidity, 32 = M- + E. cos. 2 L,

hardness, and impenetrability, and this line, at the equator,

is which preclude all division, and elevated 15577 feet above the leave no vacancy for the admislevel of the sea.

sion of any foreign force to sepaН.

rate or disunite its parts. Whence on = E.cos. 2 Lt M-32

ATOMICAL Philosophy, is the 15577

doctrine of Atoms; a system which

= 294, accounts for the origin and forma

27 + 58 - 32 tion of things from the hypothesis, and conseq. H = 7642 + 7933.cos. :hat atoms are endowed with 2 L. Which formula

to weight and motion. agree very nearly with actual ob ATTRACTION, in Physics, a çe. servation.

neral term used to denote the See Play fair's “ Outlines of Na cause, power, cr principle, real or tural Phil.” p. 285 ; see also a dif-imaginary, by which all bodies ferent formula for expressing the mutually tend towards each other, line of perpetual congelation, Les and cohere, till separated by some lie, “ Elements of Geometry,” 2d other power. The Jaws, phenoedition, p. 495.

inena, &c. of attraction, form the ATMOSPHERE of the Planets.- chief subject of Newtonian philoSince the planets and their satel. sophy, these being found to obtain liies are allowed to be bodies of a in almost all the wonderful operanature similar to the earth we in-tions of nature. habit, there are few who attempt The principle of attraction, in co deny that the planets are sur. the Newtonian sense of it, was rounded with atmospheres analo- first hinted at by Copernicus. gous, in most respects, lo that Kepler calls gravity a corporeal whose properties have been ex- and mutual affection between siplained in the preceding articles. milar bodies, in order to their These atmospheres are flattened union. And he pronounced more towards the poles, and protuberant positively, that po bodies what. at the equator. But this oblate- ever were absolutely light, but ness has its limits; and in the case only relatively so; and, consewhere it is greatest, the ratio of quently, ihat all matter was sub. the polar and equatorial diameierjected to the pɔwer and law of is as 2 to 3. The atmosphere can- gravitation. not extend itself at the equator 10 The first who, in this country, a greater distance than to the adopted the notion of attraction, place where the centrifugal force was Dr. Gilbert, in his book De is exactly equal to the force of Magnete; and the next was the gravity. With regard to the sun, celebrated Lord Bacon. In France this point is remote from its centre it was received by Fermat and to a distance measuring the radius Roberval; and in Italy, by Galileo of the orbit of a planet which and Borelli. But till Newton apwould make its revolution in the peared, this principle was very same period as that luminary em- imperfectly defined and applied. ploys in its rotation. The solar Before Newton, no one had enatmosphere cannot, therefore, ex tertained such correct and clear tend to the orbit of Mercury ; and notions of the doctrine of univer. consequently it cannot produce the sal attraction as Dr. Hooke, who zodiacal light, which appears to observes, that the hypothesis upon extend even to the orbit of the which he explains the systein of earth.

the world is founded upon the ATMOSPHERIC Tides, are cer- three following principles: 1. That tain periodical changes in the all the celestial bodies have not atmosphere, similar, in some re-j only an attraction or gravitation spects, to those of the ocean, and towards their proper centres, but

that they mutually attract each san, and communicate to it a ten other wiihin their sphere of actidency towards each of them. vily. 2. That all bodies which The satellites of Uranus tend tohave a simple and direct motion, wards Uranus, and Uranus towards continue to move in a right line, his satellites : the satellites of Sa. if some force, which operates turn tend towards Saturn, and Sa. without ceasing, does not constrain lurn towards them. The case is the them to describe a circle, an el same with regard to Jupiter and lipse, or some other more compli- his satellites. The earth and moon cated curve. 3. That attraction is tend likewise reciprocally the one so much the more powerful, as the towards the other. The proporattracting bodies are nearer to Lionality of the areas described by each other. But Hooke was not the satellites to the times of de. able to solve the general problem scription, concur with the equality relative to the law of attraction, of action and re-action, to render which would occasion a body to these assertions unequivocal. describe an ellipse round another All the satellites have a tenden. quiescent body placed in one of cy towards the sun; for they are its foci; this discovery being re- all animated by a regular motion served for Newton,

about their respective planets, as Attraction may be considered as if they had been immoveable; it regards celestial bodies, terres. whence it results that the salellites trial bodies, and the minuter par- are impelled with a motion com. licles of bodies. The first case is mon also to their planets; that is usually denoted by the word at. to say, that the same force by traction, or universal gravitation, which the planets tend incessantly the second by gravitation, and towards the sun, acts also upon the the third by the words affinity, satellites, and that they are car. chemical attraction, or molecular ried towards the sun with the same attraction. Many philosophers are velocity as the planets. And since now of opinion, that it is the same the satellites tend towards the sun, force contemplated under different it follows that the sun tends toaspects, yet constantly subject towards them, because of the equathe same law.

lity of action and re-action. At a finite distance, all the bo Observations have convinced us dies in nature are said to attract that Saturn deviates a little from one another in the direct ratio of his path when he is near Jupiter; the masses, and the inverse ratio whence it follows, that Saturn and of the square of the distance. Jupiter tend reciprocally the one

According to a law of Kepler, towards the other. deduced from observation, the It therefore appears, that all the radij vectores of planets and co-heavenly bodies tend reciprocally mets describe about the sun areas towards one another: but this tenproportional to the times; but this dency, or rather the attractive law can only have place so long force which occasions it, apperas the force which incessantly tains not solely to their aggregate de ects each hese bodies rom mass ; al heir inoleculæ partake the right line is constantiy direct- of it, or contribute to it. If the sun ed towards a fixed point, which is acted exclusively upon the centre the origin of the radii vectores. of the earth, without attracting The tendency, therefore, of the each of its particles, the undulaplanets and comets towards the tions of the ocean would be incom. sun, follows necessarily, from the parably greater, and very differproportionality of the areas de- ent from those which are daily scribed by the radii vectores to presented to our view. The ten. the times of description: this tendency of the earth towards the dency is reciprocal. It is, in fact, sun is, therefore, the result of the a general law of nature, that action sam of the attractions exerted up. and re-action are equal and con. on all the moleculæ, which conse. trary: whence it results, that the quently attract the sun in the raa planets and comets re-act upon thel tio of their respective masses ;

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besides, every body opon the eartlı the diminution that is occasioned
is attracted towards iis centre pro- by the augmentation of the square
portionally to its mass. It re-acts of the moon's distance. Now, it
iherefore upon it, the attraction may be shown that the revolution
following the same ratio, If it of the moon about the earth is a
were otherwise ; if all the parts of phenomenon of the same kind
the earth did not exert upon one and to be accounted for in the
another a reciprocal attraction, the same manner (that is, by consider.
centre of gravily of the earth ing the joint operation of the pro-
would move by a constantly-acce jectile and gravitating forces, as
lerating motion, till at length it the curvilinear motion of a stone,
would be lost beyond the limits of bullet, or any other projectile
the system,

the surface of the earth. If
The attraction is therefore uni. we had engines of a suflicient
versal, reciprocal, and propor- force to project a body in a right
tional to the niass. It remains to line parallel to the horizon, with
demonstrate, that this force is in the velocity of 24326 Paris feet
versely as the square of the dis- |(nearly five English miles) in a

second of time, that body, setting
Observations have shown, that aside the resistance of the air,
the squares of the periodic limes would revolve about the earth
of the celestial bodies are propor. like a moon. For, 24326 is a mean
tional to the cubes of the mean proportional between 39231600, the
distances. Farther, it is rigorously diameter of the earth, and 15t, the
demonstrahle, that when bodies space described in the first second
circulate in such manner that the of time by a heavy body falling
squares of the periodic times are from quiescence towards the earth.
proportional to the cubes of the And the periodical time of such a
distances, the central force which projectile would be nearly equal
actuates them is in the inverse to i hour, 24 minutes, 27 seconds.
ratio of the square of the distance: If this body could be carried to
therefore, supposing the planets the distance of the moon, and pro.
to move in circular orbits (froinjected in the same direction as
which they, in fact, differ but that in which the moon moves,
little), they are solicited towards with such a velocily as would
the sun by a force which varies carry it through 188489 Paris feet
inversely as the square of the dis, in a minute, it would revolve about
tance. This supposition is not ri. the earth in the same orbit as is
gorous. But the constant relation described by the moon. We know
of the squares of the periodic from experience, that the motion
times, to the cubes of the dis- with which a body near the sur.
tances, being independent of the face of the earth tends to its cen.
eccentricity, would doubtless sub- tre, is such as in a second of time
sist in the case where the eccen- makes it descend through 15.4 Paris
tricity vanishes, that is, if the

planets moved in circular orbits. decrease inversely as the square

Supposing this motion to
Indeed, the truth of the position of the distance ; at the distance of
may be readily established with
regard to elliptical orbits : but we semidiameters of the earth, it

the moon, which is equal to 60
omit the demonstration, rather

would be 60 X 60 times less than at
than protract this article to loo the surface of the earth; and
great a length.

therefore at that distance would
If the planets revolve about the be sufficient to make a body, de-
sun in virtue of a central force, scend through 257, Paris feet in a
which is reciprocally as the square
of the distance, it is natural to in- minute of time. This is, in fact,
fer that the moon is retained in the space through which the moon
her orbit by a central force direct at the distance of 60 semidiameters
ed towards the earth, and which of the earth, descends from the
only differs from the gravity of tangent of ils orbit, towards the
terrestrial bodies, in the ratio of centre of the earth iu a minute of


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time. For that space is a third of the latitudes of the two stations proportional to the diameter of the resulting from the observations, moon's orbit, and the arc described would be greater than it really is. in the same time. And 235389600 If, then, the true difference of (the diameter of the moon's orbit their latitudes be determined by in Paris feet) is to 188489 (the arc measuring the distance between described in a minute,) as 188489 is the two stations on the ground, the to 1571. Thus the motion agrees in excess of the ditference, found by quanuity as well as in direction, the observations of the star above with the legitimate interences that found by this nieasurement, from the motions of projectiles must have been produced by the near the earth. And these pheno attraction of the mountain, and mena are so perfectly coincident the half of it will be the effect of and similar, that they inust be such attraction on the plumb-line referred to the same principles, at each observation, provided that namely, a projectile force and á

the mountain altracts equally on gravitating force varying inversely both sides. as the squares of the distances.

The first idea for determining the ATTRACTION of Mountains.-Ac- quantity of this attraction was suga cording to the Newtonian theory gested by Newton, in his treatise of attraction, this principle per of the System of the World; but vades the minutesi particles of no farther notice was taken of it matter, and the combined action till M. Bougner and M. de la Conof all the parts of the earth forms damine, in the year 1738, while the attractions of the whole. For they were employed in measuring the same reason, therefore, that a three degrees of the meridian, heavy body tends downwards in a

near Quito in Peill, thougint they perpendicular to the earth's sur- perceived a detection of their face, it must be attracted towards plumb-line from the effect of the the centre ota neighbouring moun attraction of Chini boraco, a mounlain by a force greater or less, ac-lain in that neighbourhood, which cording to the quantity of matter by a rough computation they supcontained in it; and the effect of posed to be equal to about the this attraction, or the accelerative 2000th part of the attraction of the force produced by it, must depend whole earth. on the distance of the mountain

Nothing was afterwards done, till from the gravitating body, because Dr. Maskelyn made a proposal to this force increases as the squares the Royal Society for this purpose, of the distances decrease. "Upon in the year 1772; and, in 1774, he these principles, it is obvious that was deputed to make the trial, ac. the plumb of a quadrant, or of companied with proper assistants, any other astronomical instrument, and furnished with the most accu. must be deflected from its proper rate instruments. For the scene of situation, by a small quantly, to- his observations, he made choice wards the mountain; and the ap

of the mountain Schehallien, in parent altitudes and zenith disa Scotland, the direction of which is tances of the stars, taken with the nearly from east to west: its mean instrument, be altered according height above the surrounding val. ly; viz. if the zenith distance of a ley about 2000 feet, and its highest star were observed at two stations, part above the level of the sea under the same meridian, one on 3550 feet. Two stations for obser. the south side of a mountain, the vations were selected, one on the other on the north; and the plumb north and the other on the south line of the instrument were at. side of the mountain. Every cir. tracted out of its vertical position cumstance that could contribute to by the mountain, the star must ap- the accuracy of the experiment pear too much to the north, by the was regarded; and from the obserobservation at the southern sta- vation of ten stars near the zenith, tion, and too much to the south, it was found that the apparent dit. by that at the northern station; ference of the altitudes of the two and, consequently, the difference stations was 54' 8"; and from a

• oz.

. lb.


measurement by triangles, formed ing all the larger and coarser sorts from two bases on different sides of goods; and all metals, except of the mountain.

gold and silver. Avoirdupoise These data seemed to offer the weight is thus divided : means of determining the mean 16 dr. or drams, 1 ounce density of the earth; but the calcu- 16 oz.

. 1 pound lation necessarily required great 28 lb. . • .l quarter. • qr. accuracy, as well as immense 4 qrs. ••• 1 hun. weght cwt. labour. The task, however, was 21) cwt. . 1 ton. undertaken by Dr. Hutton, who The avoirdupoise is less than the gave an account of it, and the retroy ounce, in the proportion of sult of his investigation, in the 700 to 768 ; but the avoirdupoise Phil. Trans. as also in his Tracts pound is greater than the troy lately published, from which it pound, in the proportiou of 700 to appears, that the mean density of 570 ; viz. the earth is to that of common 70002. troy = 76807. avoirdupoise. water in about the ratio of 5 to 1. 100lbs. troy = 576lbs. avoirdupoise.

Sir Richard Phillips (Essays, p. 8. AURORA, the inorning twilight, et seq.) treats the doctrine of at- or that faini light which appears in traction as a mere assumption; and the morning when the sun is within he sums up the objections which eighteen degrees of the horizon. may be urged against it in these It is produced by the refractive words:

power of the atmosphere. 1. “ That matter is not itself an AU KORA Borealis, or NORTHERN agent, or source of power.

Light, sometimes also called 2. “ That it has no consciousness streamers, is extraordinary of any distant matter to be moved meteor, or laminous appearance, or allracted.

showing itself in the night-time in 3. “ That it has no means of the northern parts of the heavens, knowing the relative quantity, and and most commonly in frosty wea. of moving accordingly.

ther. 4. " That the direction ot motion It is usually of a reddish colour in bodies said to attract one ano- inclining 10 yellow, and sends out ther, is from their further, remote, frequent corruscations of pale lighi, or opposite sides; consequently, which seem to arise from the horieach is assumed to receive a mu. zon in pyramidal undulating forms, tual impulse from parts in which and shoots with great velocity up neither are situated.

to the zenith, 5. “ That, through a vacuum, no AUSTRAL, the same as southern. obligation of equal mechanical mo AUTOMATON, a seemingly self. inenta, in a system of bodies exists, moving machine; or one so con. such as we discover to be the fact structed, by means of weights, in nature.

levers, pulleys, springs, &c. as to 6. “ That the sun, plancts, and move for a considerable time as if satellites, do not fall together, as it were endued with animal life. the system requires.

And according to this description, 7. “ That, for the first four rea-clocks, watches, and all machines sons, bodies do not attract or repel of that kind, are automata. one another in any other branch of AUTUMN, the third season of philosophy, more than in physics." the year: this begins at the descend.

Centre of ATTRACTION. See ingequinox, which, in the northern CENTRE.

hemisphere, is when the sun enters ATTRITION, the striking or rub- the sign Libra, about the twentybing of bodies against each other, second of August, and ends about

ATWOOD's Machine, a very in the same day in December. genious apparatus invented by the AUTUMNAL, something relating Jate Mr. Atwood, of Trinity College, to autumn, Cambridge, to illustrate the doc. AUTUMNAL Equinor, the time trine of accelerated motion.

when the sun enters the descend. AVOIRDUPOISE. Wright, a ing point of the ecliptic where it weight used in England for weighi. I crosses the equinoctial.

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