| XXX | (451) | XXX |
451 A S T R < ) N O M Y. we have two concurring appearances which, deceive our their projeftile forces are diminiftied almoft to nothing ; imagination, and caufe us to refer the fun and moon to and then they are brought back again by the fun’s attrac- a greater didance at their rifing or fetting about e, than tion, as before. when they are confiderably high, as at f: fird, their If the projeftile forces of all the planets and comets feeming to be on a part of the atmofphere at e, which is were deftroyed at their mean diftances from the fun, their really farther than f from a fpeftator at E; and, fegravities would bring them down fo, as that Mercury their being feen through a grofler medium when would fall to the fun in 15 days 13 hours; Venus, in condly, at e than when at f, which, by rendering them dimmer, 39 days 17 hours; the earth or moon, in 64 days 10 caufes us to imagine them to be at a yet greater didance. hours; Mars, in 121 days; Jupiter, in 290; and Saturn, in 767. The nearelt comet, in 13 thoufand days ; And as, in both cafes, they are feen much under the the middlemoft, in 23 thoufand days; and the outermod, fame angle, we naturally judge them to be larged when in 66 thoufand days. The moon would fall to the earth they feem farthed from us. in 4 days 20 hours: Jupiter’s firft moon would fall to Any one may fatisfy himfelf that the moon appears him in 7 hours; his fecond, in 15; his third, in 30; under no greater angle in the horizon than on the meriand his fourth, in 71 hours : Saturn’s firft moon would dian, by taking a large flieet of paper, and rolling it up form of a tube, of fuch a width, that'obferving fall to him in 8 hours; his fecond, in 12; his third, in inthethe 19; his fourth, in 68; and the fifth, in 336. A done moon through it when die rifes, Ihe may* as it were, would fall to the earth’s centre, if there were an hollow jud fill the tube; then tie a thread round it to keep it paflage, in 21 minutes 9 feconds. Mr Whidon gives of that fize ; and when the moon comes to the meridian, the following rule for fpch computations. “ It is de- and appears much lefs to the eye, look at her again “ mondrable, that half the period of any planet, when through the fame tube, and Ihe will fill it jud as much,. “ it is diminiihed in the fefquialteral proportion of the if not more, than fire did at her rifing. “ number 1 to the number 2, or nearly in the proportion When the full moon is in her perigeo, or at her lead of 1000 to 2828, is the time that it would fall to the didance from the earth, die is feen under a larger angle, • centre of its orbit.” This proportion is, when a and mud therefore appear bigger that when die is full at quantity or number contains another once and a half as other times : And if that part of the atmofphere where much more. die rifes be more-replete, with vapours than ufual, die The quick motions of the moons of Jupiter and Sa- appears fo much the dimmer ; and therefore we fancy turn round their primaries, demondrate that thefe two her to be dill the bigger, by referring her to an unplanets have ftronger attrardive powers than the earth ufually great didance, knowing that no obje&s which are has: for the dronger that one body attracts another, the very far didant can appear big unlefs they be really fo. greater mud be the proje&ile force, and confequently.thequicker mud be the motion of that other body to keep it from falling to its primary or central planet. Jupiter’s Chap. VII. life of the common Quadrant, and fecond moon is 124 thoufand miles farther from Jupiter the Method of finding the Diftances of the' than our moon is from us; and yet this fecond moon Sun, Moon, and Planets. goes almod eight times round Jupiter whild our moon goes only once round the.earth. What a prodigious at- To enable the young adronomer to underdand the traftive power mud the fun then have, to draw all the method of finding the didances of the .heavenly bodies, planets and fatellites of the fydem towards him; and we fliall here give a flrort defcription of the quadrant. . what an amazing power mud it have required to put all This inllrument (Plate XLV. fig, 6.) is chiefly ufed in thefe planets and-moons into fuch rapid motions af fird 1 taking altitudes. The altitude of any celedial phenomenon is an arc of dcy intercepted between the horizon and the pheno- Chap. VI. Renfons why the Sun, Moon, and the nienon. fig. 6. of Plate XLV. Jet HOX- bz a horiSigns, when rifing or Jetting, appear lar- zontal line,In fuppofed to be extended from the eye at X ger than when they rife higher in the Hea- 'to X, where the fliy and earth feem to meet at the end ; of a long and level plain ; and let S be the fun.. The vens. arc XY will be the fun’s height above the horizon at X,', The fun and moon appear larger in the horizon than and is found by the indrument EDC, which is a quadranat any confiderable height above it. . Thefe luminaries, tal board, or plate of metal, divided into 90 equal parts although at great didances from the earth, appear float- or degrees on its limb DPC-, and has a couple of little ing, as it were, on the furfaceof ouratmofphere, (Plate. brafs plates, as a and b, with a fmali hole in each of them, . XLI. fig. 1.) HGFftC, a little way beyond the clouds; of Jight-boles, for looking through, parallel to the : which, thofe about F, dire&ly over our heads atT, are. c.Az& of the (quadrant whereon they dand. To the cennearer us than thofe about if or e in the horizon HEz. edge tre E is fixed one end of a thread F, called the plumbTherefore, when the fun or moon appear in the horizon at line, which has a fmall weight or plummet P fixed to e, they are not only feen in a part of the Iky which is really end. Now, if an obferver holds the quadrant farther from us than if they were at any confiderable al- its other without inclining it to either fide, and fo that titude, as about f; but they are alfo feen through a upright, the horizon at X is feen thrdbgh the fight-holes a and b, , greater quantity of air and vapours at e than at /. Here the plumb-line will cut or hang over the beginning of the degrees
