GRAVITY. 



equator is equal 0.00694 multiplied by the fquare of the 

 cofine of the latitude, the force of gravity at the equator 

 being taken as unity. 



The moll exaft inftniment we pofTefs, for meafurmg the 

 intenfity of the force, is the pendulum, whofe ofcillations 

 are immediately accelerated or retarded by the flightell al- 

 teration in the force of gravity, and it is only by means of 

 this inllrument that we are enabled to infer with prccilion, 

 the. cxafl fpace that a heavy body falls through in one fe- 

 cond of time. 



Gravity, The Nature of. — Of ihe nature of gravitation, 

 or the force of nravity, nothing more is known, than that it 

 is apparently an effential property of matter, or, at leaft, of 

 all matter that hitherto has become the objeft of human in- 

 vetlio-ation, for Newton was not difpofed to believe but that 

 matter might exilt which was not endowed with this pro- 

 perty, and fuggeits the fuppohtion that it is caufed by the 

 agency of an elallic medium pervading all ipace. This 

 medium, he fuppofes, to be much rarer within the denfe 

 bodies of the fun, the liars, the planets, and the comets, than 

 in the empty celellial fpaces between tliem, and to grow more 

 and more denfe at greater diitancts from tliem, fo thai all thefe 

 bodies are naturally forced towards each other by the excefs 

 of prefTure. Upon this fuppolilion, Dr. Young remarks, in 

 his Lectures, that the eflefts of gravitation miglit be pro- 

 duced by a medium thus conllituted, if its particles were 

 repelled by all material fubllances with a force decrealing 

 like other repulfive forces, limply as the diltances increafj ; its 

 deniity would tlien be every where fucli as to produce the ap- 

 pearance of an attraftion, varying like that of gravitation. 

 Such an ftiiereal medium would, therefore, have the advantage 

 of hmplicity, in the original law of its action, fmce tlie re- 

 pulfive force, which is known to belong to all matter, would 

 be fufficient, when thus modilied, to account tor tlie prin- 

 cipal phenomena of attraction. 



It may be quellioned whether a medium, capable of pro- 

 ducing the eflefts of gravitation in this manner, would alfo 

 be equally fufceptible of thofe modiiications which have 

 been fuppofed neceffary to the tranfniifiion of light. In 

 f ither cafe, it mull be fuppofed to pafs througli the apparent 

 fubftance of all material bodies witii tlie moll perfeft freedom, 

 and there would, therefore, be no occalion to apprehend any 

 difliculty from a retardation of the celeftial motion ; the 

 ultimate impenetrable particles of matter being, perhaps, 

 fcattered as tliinly through its external form as Itars arc 

 fcattered in a nebula, which has ftill tlie dlllant appearance 

 of an uniform light ; and tliere feems no reaion to doubt tlie 

 poffibility of the propagation of an undulation through the 

 Newtonian medium, v.ith the actual velocity of liglu. It 

 muft be remembered, that the difference of its preffure is not 

 to be ellimated from the aftual bulk of the earth, or any 

 other planet alone, bat from the effeft of the fphere of 

 repulfion of which that planet is the centre ; and we may 

 thus deduce the force of gravitation from a medium of no 

 tery enormous elallicity. 



Dr. Young oblervos, that a fimilar combination of a 

 fnnple preffure with a variable repulfion is alio obfervable 

 in the force of cohefion, and fuppoles that if two parti- 

 cles of matter, floating in fuch an elallic medium, capa- 

 ble of producing gravitation, were to approach each other, 

 their mutual attraftion would at once be clianged from 

 gravitation to cohelion, upon the exclulion of the portion 

 «f the mediiun intervening between them. The well known 

 experiment of the two exhaulled hemifpheres of Magde- 

 burg affords an illullratiou of this hypothelis, where we 

 Ice ap">arei.t cohelion derived from atmofplierical preffure, 

 aai if we place between them a thick ring of elallic gum, 



we may rcprefent the natural equilibrium between the forces 

 of cohefion and repulfion, for the ring would refill any 

 fmall additional preffure with the fame force as would 

 be required for feparating the hemilpheres fo far as to 

 allow it to expand in an equal degree ; and at a certain 

 point the ring would expand no more ; the air would be 

 admitted and the cohefion deilroyed, as when a folid of 

 any kind is torn afunder. Thefe fuppofiuons, however, 

 are direftly oppofite to the hypothefis which affigns to 

 the elallic medium the power of paffing freely througl; all 

 the interftices of the ultimate atoms ot matter, fince it 

 could never pafs between two atoms cohering in this 

 manner ; we cannot, therefore, at prefent affert the identity 

 of the forces gravitation and colielion fo ftrongly as theory 

 would allow us to do if ellablilhed. In (liort, the \\ Lole 

 of our enquiries refpefting the intimate nature of forces 

 rnufl; be confidered as merely fpeculative amufements, wliich 

 are of no farther utility than as they make our views more 

 general, and affill our experimental inveftigations. 



La Place, after having ftiewn how tlie law of gravita- 

 tion is deduced from the phenomena of the folar lyilem, 

 concludes his reflections on this fubjeft: by enquiring wlie- 

 tlicr the principle of gravitation is a primordial law of nature, 

 or if it may not be the general effeft of fome unknown 

 caufe ? Here, he oblerves, we are Hopped by our igno- 

 rance of the nature of the intimate properties of matter, 

 and deprived of every hope of anfwering this quc-llion in 

 a fatisfaftory manner. Inlcead, fays this great author, of 

 forming hypothefes on this fubjeft, let us content ourfelves 

 willi examining more particularly the manner in which 

 this principle lias been employed by pliilofophers. 



They have admitted tlie live following fuppofitions : 



1. That gravitation takes place between the moll mi- 

 nute particles of bodies. 



2. That it is proportional to the maffes. 



3. That it varies inverfely as the fquare of the diftance, 



4. That it is tranfmitted iiillantaneoufly from ©ne body 

 to another. 



5. And that it equally afts on bodies in a Rate of repofe, 

 and on thofe winch by their motion in tlie direftion of its 

 aftion fhould feeni likely to avoid a part of its influence. 



The firll of thefe propofitions is, as we have feen, a ne- 

 ceffary rcfult of the equality which exifts between aftion 

 and re-aftion, every particle of the earth attrafting it as 

 the particle itfelf is attrafted ; this fappofition is confirmed 

 by the meafures of the degrees of the meridian, and by 

 experim.ents on pendulums ; for amidll all the irregularities 

 of the meafured degrees, we may perceive the traces of a 

 regular figure, winch is conformable to the theory. The 

 great influence that the compreflion of Jupiter has upon 

 the nodes and perigees of the orbits of its fatellites, proves 

 to us that the attraftion of this planet is compofcd of the 

 attraftions of all its particles. The proportionality of the 

 attraftive force to the maffes is demonllrated in the earth 

 by experiments on pendulums, the ofcillations of which are of 

 the fame length of whatever lubllance they arc compofcd. 

 It is proved in the celeftial regions by the conllant relation 

 which exiils between the fquares of the periodic times of 

 bodies revolving about a common focus, to the cubes of the 

 greater axes of their orbits. 



We have feen with what precifion the almofl; abfolute 

 ftate of repofe of the perihelia of the planetary orbits 

 indicate that the force of gravity varies according to 

 the inverfe fquare of the dutance : and now that we 

 know the caule of the motions of thefe perihelia, we 

 may regard tliis law as rigoroufly exaft. It is the fame 

 with all emanations wliich proceed from a centre, fucli a.s 



litfht i 



