A C C 



to the fine of ilie pbiit's inclination, and the fpaccs 

 kfs according to the fquare of the line. Sec Inclined 

 Plank. 



Acceleration of ihc Motion of Pendulums. See P e n - 



DVI.VM. 



Acceleration of the Motion of ProjeSiles. Sec Pro- 



JFCTItr. 



AccKLERATiON of thf Molion of Compicjfed Bodies, in 

 expanding or rclloring thcnifelves. See Compression, 

 Dilatation, and Elasticity. 



Acceleration, in ^IJIronomy, is a term applied to the 

 fixed ftars. Thus, the diunial "acceleration is the time by 

 wliich the ftars, in one diurnal revolution, anticipate the 

 mean diurnal revolution of the fun, which is 3"" ;Sts] pf 

 mean time, or nearly 3 minutes 56 feconds ; i. e. a ftar riies 

 or fets, or paffes the meridian, 3'" 56" fooner each day. 

 This appirent acceleration of the ftars is owing to the real 

 retard ition of the fun ; and this depends upon his apparent 

 motion towards the eaft, which is at the rate of about 59' 

 SI" of a degree every day. In confequence of this, the 

 ftar which palTed the meridian at the fame moment with the 

 fun vellerday, is to-day about 59' 8" beyond the meridian to 

 the wdli when the fun anives at it ; and this diftance it 

 will require about 3'" 6' for him to pafs over ; and therefore 

 the ftar will anticipate the motion of the fun at this rate 

 ever)' day. The true quantity of tliis anticipation or accelera- 

 tion, is found by the following proportion, via. 360° : 59' 

 8!" :: 24 hours : 3" 55 tV> 'he acceleration required. 

 This diurnal acceleration ferves to regulate the lengtlis and 

 vibrations of pendulums. If the pendulum marks f. ^. 

 8" 10"', when a fixed ftar fets or paftcs behind any inter- 

 vening ob'iecl to-day, and on the next day, the eye being 

 in the fame fituation, the fame appearance occur at 8'' 6" 4' 

 by the pendulum, it may be inferred that fuch a pendulum 

 is' truly regidated, or juftly meafures mean time. See 

 Clock. 



Acceleration of the moon, is a term ufed to exprefs 

 the increafe of the moon's mean motion from the fun, com- 

 pared with the diurnal motion of the earth ; fo that it is 

 now a little fwifter than it was formerly. Dr. Halley 

 (Phil. Tranf. No. 218) was the firft who made this dif- 

 coveiy ; and he was ltd to it by comparing the ancient 

 cclipfes obferved at Babylon with thofe obferved by Alba- 

 tegnius in the ninth century, and fome of his own time. 

 He was not able to alcertain the quantity of this accelera- 

 tion, becaufe the longitudes of Bagdat, Alexandria, and 

 Aleppo, where the obfervations were made, had not been 

 accurately determined. But fince his time, the longitude of 

 Alexandria has been afcertained by Chazelles, and Babylon, 

 according to Ptolemy's account, lies 50' eaft from Alex- 

 andria. From thefe data, Mr. Dunthome (Phil. Tranf. 

 No. 492. abr. vol. x. p. 84. &c. ) compared feveral ancient 

 and modern eclipfes, with the calculations of them by his 

 own tables, and thus verified Dr. Halley's opinion : for he 

 found, that the fame tables reprefent the moon's place in 

 the ancient eclipfes behind her true place, and before it in 

 later eclipfes : and thence juftly inferred, that her motion in 

 ancient times was flower, but in later times quicker than 

 the tables give it ; and therefore, that it muft have been 

 accelerated. But he did not content himfelf with merely 

 afcertaining the faft. He proceeded to determine the 

 quantity of the acceleration ; and by means of the moft 

 ancient edipfe of which any authentic account remains, 

 obferved at Babylon in the year before Chrift 721, he con- 

 cluded that the obferved beginning of this eclipfe was not 

 above an hour and three quarters before the beginning by 

 the tables ; and therefore the moon's true place could pre- 



A C C 



cede her place by computation but little more than 50' of a 

 degree at that time. Admitting the acceleration to be 

 uniform, and the agg'-egate of it as the fquare 6i the time, 

 it will be at the rate of about 10" in too years. M. de la 

 Lande makes it 9".886. In Mayer's tables it is 9'', be- 

 ginning from I 700. 



Dr. Long (Aftron. vol. ii. p. 436.) attributes the acce- 

 leration above dcfcribed to one or more of thefe caufes : 

 either, i. the annual and diurnal motion of the earth con- 

 tinuing the fame, the moon is really carried round the earth 

 with a greater v;locity than it was fonnerly : or, 2. the 

 diurnal motion of the earth, and the periodical revolution of 

 the moon continuing the fame, the annual motion of the 

 earth round the fun is a little retarded ; which makes the 

 fun's apparent mouon in the ecliptic a httle flower than it 

 fonnerly was ; and, confequently, the moon, in pafling from 

 any conjtmftion with the fun, fpends lef? time before fhe 

 again overtakes the fun, and forms a fubfcquent conjunc- 

 tion : in both thefe cafes, the motion of the moon from the 

 fun is really accelerated, and the fynodical month aftually 

 ihortcned : or, 3. the annual motion of the earth, and the 

 periodical revolution of the moon continuing the fame, the 

 rotation of the earth round its axis is a little retarded : in 

 this cafe, days, hours, minutes, feconds, &c. by which all 

 periods of time muft be meafured, are of a longer duration ; 

 and, confequently, the fynodical month will appear to be 

 fhortened, though it really contain the fame quantity of 

 abfolute time as it always did. If the quantity of matter 

 in the body of the fun he lefl'ened by the particles of light 

 continually ftreaniing from it, the motions of the earth 

 round the fun may become flower ; if the earth increafes in 

 bulk, the motion of the moon round the earth may be thus 

 quickened. M. de la Place (Mem. de I'Acad. Roy. des 

 Scienc. for 1786.) has evinced this acceleration of the 

 moon's motion to arife from the aftion of the fun upon the 

 moon, combined with the variation of the excentricity of 

 the earth's orbit. By the prefent diminution of the ex- 

 centricity, the moon's mean motion is accelerated ; but, 

 when the excentricity is arrived at its minimum, the acce- 

 leration will ceafe : after which, the excentricity will in- 

 creafe, and the moon's mean motion will be retarded. 

 M. de Lambre found, by comparing the modem obferva- 

 tions at about the diftance of a century, that the fecular 

 mean motion of the moon in the laft tables of Mayer was 

 too great by 25" ; and that the place of the moon cal- 

 culated by thefe tables ought to be corrected by the 

 quantity — 25" n -\- 2", 135 n^ + o", 04398 n', n being the 

 number of centuries from 1700. M. de la Lande, in his 

 tables of the moon, has thus con-efted Mayer's tables. 

 Hence it appears, that the 'prefent acceleration of the moon 

 is nothing more than an equation, _the period of which is 

 very long. It will be accelerated and retarded by the fame 

 quantity ; and therefore, if the mean motion be taken for 

 the whole time of acceleration or retardation, it will be 

 found never to vary. Vince's Aftron. vol. i. p 206. 



Acceleratiom, in Mufic. See Accelerando. 



ACCELERATORES Urin^f, called by Winflow 

 Bulbo-caverno/t, and by others Urime Jlimulatores and ejacu' 

 latores feminis, in Anatomy, a pair of mufcles whole oflice it is 

 to expedite the difcharge of the urine and of the femen. 

 Thefe mufcles may be faid to arife from juft before the 

 verge of the anus, where the fphinfter ani terminates. They 

 are fpread over the bulb and a fmall portion of the coi-pus 

 fpongiofum tirethrx ; having that appearance, which anato- 

 mifts have termed a doubly penniform mufcle. From the 

 anterior part of the mufcle a fafcicula of fibres proceeds on 

 each iide, by which the l>ody of the penis is encircled 

 I When 



