ATT 



ATT 



are brought nearer each other, which would be impofflble. than it renllv « Tf *V.n »l . i ir r ,_ • . . 



unk-fs they had been at fome diftance before the application TZ.\ )7 • / I r "'"' ^'^''7" "^ ^^^'^ '«li- 



• application tudcs l)c determined by meafuring the nillance between the 



two ftalions on the ground, the excels of the dilTcrcr.ce, 



of the cold. Almoll all bodies are diminiflied in'bulk by 

 prefTure, and confequently their particles are brought nearer 

 each other ; and the diminution of bulk is always propor- 

 tioned to the prefTure. Newton has alfo fnewn that it re- 

 quires a force of many pounds to bring two glalTes within 

 the Sooth part of an inch of each other ; that a much 

 greater force is neceflary to diminilh that diilancc, and that 

 no preiFure whatever is capable of diminilliing it beyond a 

 certain point. Confequently there is a force which impedes 

 the aftual contaft of bodies, which increafcs inveiftly as 

 fome power or fiin<Stion of the diftance, and which no 

 power whatever is capable of overcoming. Bofcovich has 

 likewife demon (Irated that a body in motion communicates 

 part of its motion to another body before it a6tually reaches 

 it. Hence we may conclude, that, as far as we know, 

 there is no fuch thing as actual contaft in nature ; and that 

 lo lies of courfe always act upon each other at a diftance. 

 Even impulfion, therefore, or preffure, is an inftance of 

 bodies afting on each other witliout being in contact, and 

 confequently this is no better an explanation of attraftion 

 than the fuppofition that it is an inherent power. We muft 

 therefore be fatisiied with confidering attraction as an un- 

 known power, by which all bodies are drawn towards each 

 other, and which afts conltantly and uniformly in all 

 times and places, fo as always to diminilh the diftance between 

 them, unlefs when they are prevented from approaching 

 each other by fome force equally powerful. But why it 

 diminifhes as the diftance increafes, it is impolTible to fay; 

 although the faft is certain, and is almoft incompatible 

 with the fuppofition of impulfion being the caufe of attrac- 

 tion. The truth is, that we muft not be too precipitate in 

 drawing conclullons, but muft wait, with patience, till future 

 difcoveries fliall enable us to advance farther ; fatisfying our- 

 felves, in the mean time, in arranging the laws of nature 

 which have been afcertained, without attempting to devc- 

 lope the caufcs upon which they depend. 



Attraction, in Chem'tftiy. See Affinity. 

 Attraction, Centre of. .See Centre. 

 Attraction of MouiUaiiis. According to the New- 

 tonian theory of attraftion, this principle per\'adcs the mi- 

 nuteft particles of matter, and the combined aftion of all 

 the parts of the earth forms the attradion of the whole. 

 For the fame rtafon, therefore, that a heavy body tends 

 downwards in a perpendicular to the earth's furface, confi- 

 dcred as fmooth and even, it muft be attrafted towards the 

 centre of a neighbouring mountain, by a force proportional 

 to the quantity of matter contained in it ; and the effeft of 

 this attraftion, or the accelerative force produced by it, 

 muft depend on the nearnefs or diftance of the mountain 

 from the gravitating body, becaufe this force increafes as 

 the fquares of the dillanccs decreafe. Upon thefe principles 

 it is obvious, that the plumb-line of a quadrant, or of any 

 other aftronomical inftrument, muft be dtflefted from its 

 proper fituation, by a fmall quantity, towards the mountain, 

 and the apparent altitudes and zenith diftances of the ftars, 

 taken with the inftrument, be altered accordingly : e. g. if 

 the zenith diftance of a liar on the meridian were obferved 

 at two ftations under the fame meridian, one on the foulh 

 fide of a mountain, the other on the north ; and the plumb- 

 line of the inftrument were attrafted out of its vertical po- 

 fition by the mountain, the ftar muft appear too much to 

 the north, by the obfervation at the foulhern ftatiou, and 

 too much to the fouth by that at the northern ftation ; and 

 confequently the difference of the latitudes of the two fta- 

 tions, refulting from thefe obfervations, would be greater 

 Vol. III. 



tound by the obfervations of the ftar above that found by 

 this meafurement, muft have been produced by the attrac- 

 tion of the mountain ; and the half of it will be the cffca 

 of fuch attraftion on the plumb-line at ea^h obfcr.ation, 

 provided that the mountain attraft-, equally on both fides. 



The fivft hint for determining the qtiantity of this attrac- 

 tion was fuggcftcd by Newton in his Trcatife of the Syftem 

 of the World ; and the fa ft experiment for this purpofc wa« 

 coiiduftcd by M. Bougiicr, and M.de la Condaminc, in the 

 year 1738. Whilft they were employed in meafuring three 

 degrees of the mendian, near (j^uito in Peru, thev endea- 

 voured to afcertain the effcdt of tlic attraftion of Chimbo- 

 raco, a mountain in that neighbourhood, which, by a rough 

 computation, they fuppofed to be equal to about the zoootU 

 part of the attraftion of the whole caitli. By obferving 

 the altitudes of fixed ftars at two ftations, one on the fouth 

 fide of the mountain, and the other on the north, they 

 found the quantity of 7^" in favour of the attraftion of the 

 mountain by a mean of their obfervations ; whereas ihe 

 plumb-line, according to the theoiy, ftiould have declined 

 from the true vertical line i' 43". However, though the 

 general refult is favourable to the Newtonian doftrinc, the 

 experiment was performed under fo many difadvanta^cj, 

 as not to afford the fatisfadlion which was to be wilhed ; 

 and M. Bouguer terminates his account of their obferra- 

 tions, with expreffing his hopes, that the experiment might 

 be repeated under more favourable circumftanccs either in 

 France or in England. Bouguer, Fi];ure dc la Terre. 



Nothing was afterwards done, till Mr. (now Dr.) Mafke- 

 lyne, the prefent aftronomer royal, madf a propofal to the 

 Royal Society for this purpofe in the year 1772 ; and ia 

 1774, he was deputed to make the trial, accompanied with 

 proper aftiftants, and furnifhed with the moft accurate in- 

 ftruments. He made choice of the mountain Scheha'licn, 

 iu Scotland, for the fcene of his operations ; the diieftion 

 of which is nearly from caft to weft, its mean height alx^ve 

 the furrounding valley about 2000 feet, and its higheft part 

 above the level of the fea 3550 feet. Two ftation^ for ob- 

 fervations were felcfted, one on the north, and the other on 

 the fouth fide of the mountain. Every circumlla.ice that 

 could contribute to the accuracy of the experiment was re- 

 garded ; and from the obfervations of ten ftars near t'.ie ze- 

 nith, Mr. Man<clyne found the apparent ditfcrence of the 

 altitudes of the two ftations to be 54.6"; and from a mra- 

 furenient by triangles, formed from twobafesond.fi" 

 fides of the mountain, he found the diftance of tlieir y 

 lels to be 4364 feet, which, in the latitude of Schehai., 

 viz. 56'' 40', aniwers to an arch of the meridian of 4^ , 

 which is lefs by 1 1.6" than that found by the feftor. it; 

 half, therefore, or 5.S" is the mean eflect of the attociion 

 of the mountain. From this experiment, conducted »;th 

 great afliduity and accui-acy, and tending to the cftab-ilh- 

 ment of the Newtonian theory, Mr. Malke'yne infers, ili.it 

 tlie mountain Schehallien exerts a fcnfible attraction ; an<l, 

 tlierefore, that eveiy mountain, and ever)- particle of tne 

 earth, is endued with the fame property, in proportion to 

 its quantity of matter. The law of the vanation of t -.is 

 force, in the inverfe ratio of the fquares of the diftances, is 

 likewife confirmed by it ; for if the force of the attniftion 

 of the hill had been only to that of the earth as the mat- 

 ter in the hill to that of the earth, and had not been greatly 

 incrcafed by the near approach to its centre, the attraction 

 muft have been wholly infenfible. He infers alfo, that the 

 mean deality of the earth is at leaft double of that at the 

 tj_q furface 



