A L T 



wliich fubtraficU fioni tlio natural fmo ui tli« fmi". meridian 

 altitiule, leaves^ tin; iiatinal fiiif of the akitiiJc at tlie i\:. 

 quirtd time. For, finding tlic altitude of the inoon or a liar, 

 he gives the foliowinjr nile. Tiom the tables above nieii- 

 lioned, take out the rifmg, oorrefpomlinjr to the lioraiy 

 angle in the dilhince of time from the liar's palling the me- 

 ridian ; add to it the iogarithmie eofine of tin; liar's decli- 

 nation, and the logarithmic cofme of the latitude of the 

 place ; the fiim, abating twenty from the index, is the lo- 

 garithm of a number, which fiihtradcd from tlie natural 

 line of the Rar's meridian altitude, leaves the natural line of 

 the altitude at the given time. Thefe rules are of great im- 

 portance in determining the longitude at fea. See Naut. 

 Aim. for 1778. 



In taking of altitudes from the vifible horizon, where 

 great exaftnefs is required, an allowance is to be made for 

 refraftion, and the height of the obferver's eye above the 

 furface of the fea. To find the altitude of the liars, ixc. 

 by the globe, fee Globe. 



An irregularity hasbeen obferved in the apparent altitudes 

 of the liars near tlie meridian. On fome ocealioiis, when 

 they are mounting towards the meridiaH, they appear to fall, 

 and after paffing the meridian, to rife. Hill. Acad. Scicnc. 

 1719. p. 75. 



M. Parent fuggefts a new method of taking altitudes at 

 fea, by a common watch. It is obvious, that in an oblique 

 fphcre, the difference between the riling and fetting of two 

 flars, in the fame meridian, is greater, as they are farther 

 diilant from one another. 



Now the allronomical table furnifhing us with tables of 

 the right afcenlions and declinations for the fixed itars, it is 

 eafy, atter obferving the difference of time between the 

 rifing of two ftars, to diftinguilli that part of the difference 

 which accrues from their different pofition from that which 

 arifes from the obliquity of the fphcre But fucli differ- 

 ence is the preeife height of the pole of the place of ob- 

 fervation. 



Indeed, the fhip not being immoveable, but changing 

 place between the two obi'ervations, feems to lay the me- 

 thod under fome difficulty ; but to this M. Parent anfwered, 

 that a fmall alteration either of the (hip's longitude or lati- 

 tude, will make no fenfible error ; and that if file have gone 

 a large dillance between the two obfervations, it is eafy 

 leckoning how much it is, and accordingly allowing for it. 

 See Sailing. 



Altitude, meriJian. The meridian being a vertical 

 circle, a meridian altitude, that is, the altitude of a point 

 in the meridian, is an arch of the meridian intercepted be- 

 tween it and the horizon. 



If HO [j'ljlronomy, Plate I. Jig. fi.) be the horizon, and 

 TIZ O the meridian, then the arc H E, or the angle HC E 

 -.vill be the meridian altitude of an objeil in the m^'ridian at 

 the point E. 



To obferve the meridian altitude of the fuo, of a flar, or 

 other phenomenon, by means of the quadrant, fee Meri- 

 dian /lllituile. 



To obftrvc a meridian altitude by means of a gnomon, 

 fee Gnomon. 



Altitude, or elevation of the pole, is an arc of the me- 

 ridian OP (Jig. 6.) intercepted between the pole P and the 

 horizon : or the angle OCP. 



The altitude of the pole coincides with the latitude of 

 the place ; and may be found by obierving the meridian alti- 

 tude of the pole flar, when it is both above and below the 

 pole, and taking half the fiim, after it has been correfted 

 ua account of refradion. Or the fame may be found 



Vol. I. 



A L r 



by means of tUc declination, and meridian altitude of the 

 fun. 



ALTiriDr, nt flnutfion 9/ the equator, \t the romplemcnt 

 of the altitude of the jwie t.) a quadrant of a circle. Or, 

 it ii the angle IICE (^/f^. r,.) „r arc H E of the mcridi.1.1 

 between tlie iiori/on and the equator at E, and equal tu 

 ZP, tlie co-latitudc of the place. 



Ai.Ti ruDE of the Irnfirt aiuounti, to the fame with \vh»t 

 is otherwife called the folllitinl aliitud.' of the fun, or his 

 meridian altitude when in the folllitial point*. 



Altitude of the h'iritoti, or of liars fccn in i(, ii va- 

 riablc by the retraftion, according to the quantity of which 

 the horizon is, more or kfs, cither elevated or dtprtlfcd. 



Altitude of the nonagejinuil, 'n the alliiuHi- of the 90th 

 degree of the ecliptic, counted upon it from the point 

 where it intcrfefts the horizon, or of the middle or highell 

 point of it which is above the horizon, at any time ; and it 

 is eiiual to the angle made by the ecliptic and horizon where 

 they intcrfcft at that time. See Nonagesimal. 



Altitude, rtfratiion of, isan arc of the vertical circle, 

 as S J {Ajlronomy, Pldlel. Jig. ";.) whereby the altitude 

 S E, of a liar or other celellial body, is incrcafcd by means 

 of the refraftion. This is different at different altitudes, being 

 nothing at the zenith, and greatcil at the horizon, where it 

 is about 33'. See Refraction. 



Altitude, parallax of, is the difference Cli {fig. 8.) 

 between the true and apparent place of a liar ; or the differ- 

 ence liC, between the true dillance of a flar A B, and flic 

 obferved dillance AC, from the zenith A. The parallax 

 dlminiflies the attitude of a flar, or increafes its dillance front 

 the zenith. This arc, or the angle miafured by it, is cvi- 

 dently lef;, as the celellial body is farther diilant from the 

 earth, and alio lefs, for the lame body, as it is higher above 

 the horizon, being greatell there and nothing at the zenith. 

 To find the parallax of altitude, &c. fee Parallax. 



Altitude of the cone of the earth' s or moon' .< Jhadotv denotes 

 the height of the fliadow of one or the other in an eclipfe, 

 and is meafured from the centre of the body. It is found 

 by this propofiLion : as the tangent of the angle of tlir fun'i 

 apparent femidlameter is to radius, fo is one to a fourth pro- 

 portion-al, which will be the height of the (liadow in fcmi- 

 dianaeters of the body. The greatell height of the earth's 

 fhadow is 217.S femidiameters of the earth, when tUe fun 

 is at his greatefl dillance, or his femidiameter fubtends an 

 angleof about 15' 47"; and the height of the fame is 210.7 

 femidiameters of the earth, when the fun is ncarcll the 

 earth, or when his fcmidiameter is about 16' 19": and be- 

 tween thofe limits it is proportional to the intennediate 

 dillances or apparent femidiameters of the fun. The alti- 

 tudes of the Ihadow of the earth and moon are nearly as 

 1 1 to 3, the proportion of their diameters. 



Altitude, ov exaltation, in AJlrology, denotes the fccoiid 

 of the five effenlial dignities, w hich the planets acquire by 

 virtue of the figns in which they are found. 



Altitude of motion, in Alechanics, is a ttrm ufed by Dr. 

 Wallis, for themeafureof any motion, ellimated according 

 to the line or direelion of the moving force. 



Altitude, thterminalive, is fomelinies ufed for the 

 height, whence a falling body acquires, by acceleration, a 

 certain velocity. Hcnnaii. Phoron. lib. i. 



ALTiTt;DE, in fpcakiug of fluids, is more frequently ex- 

 preffed by the term depth. 



The ingenious Dr. Hales, in his vegetable Statics, pro- 



pofed a method of meafuring unfathomable depths of the 



fea ; on the principles by which Dr. Dcfagnhers contrived 



an inllrument called a stA-r; age, which was tried before 



5 H tlic 



