ALT
Ai tit IDE of the horizon, or of ftars feen therein, Is variable by the retraction, according to the quantity of which the ho- rizon is cither elevated or depreflcd more or lefs. Hift. Acad Scienc. i 700 . p . I2g- It . I707 . p . ,„_
Altitude of the moon's atmofphere is thought, by fome to be much greater than that of the earth ; the former being not lefs than fixty-four French leagues. But the exiftence of this atmofphere is ffill in difpute. Hift. Acad. Scienc 17 tc p. 68. ' ' - 1 '
M. de In Hire propofed a method of difcovering the Altitude of the atmofphere, the hint of which was firft given by Kep- ler, viz. by the magnitude of the arch whereby the fun is funk below the horizon, when the erepufculum begins or ends. V. Hift. Acad. Scienc. 1713. p. 8. feq.
Altitude of the Aurora Btrealis in 1719, has bceh much contorted, viz. whether it were above the atmofphere, or within the limits of it : the former opinion being defended by Dr. Halley, the latter by Mr. Whifton. The name meteor, which is given it, feems to favour the latter.— Befides, it apl pears not by any obfcrvation to have been above thlrty-eieht miles high. V. Phil. Tranf. N°. 360. Bibl. And. T. 6 p. 443. feq.
Altitude, in fpeaking of fluids, is more frequently expreffed
by the term depth. Altitude of the fea's furface is not every where the fame, as appears from the drift or currents fating ft rong out f one fea into another. VHSrks of Learn. T. 4. p ,,, See Cur- rent, Sea, &c. Cycl. and Suppl. Altitude of the mercery, in the barometer, is marked by de- grees placed on the face of that Inftrument, the variations of which are the chief objedt of barometrical obfervations. See Barometer, Cycl.
The mean Altitude of the mercury at London is about 29 inches— The extreme Altitudes arc 27 § inches and 31 ? inches. Some fuggeftions have been made, as if the Altitude of the mercury were regularly greater in the morn- ing than in the evening; at leaft fomething of this kind was oblerved to hold for a confiderablc time at Berlin. Hift. Crit. Rep. Let. T. 14. p. 230. Altitude of the pyramids in Egypt was meafured fo long a»o as Thales, by means of their lhadow, which makes one of the firft geometrical obfervations we have any exad account of. 1 lutarch has given an account of the manner of this operation, which, according to this author, was done by eredtmg a ftaff perpendicular upon the end of the fhadow of the pyramid, and by two triangles made by the beams of the iun, he demonftrated, that what proportion there was be- tween the fhadows, the fame was betwixt the pyramid and the ftaff. Stanl. Hift. Philof. P. t. p. 9. Altitude of mountains may be found divers ways, befide thole already mentioned in the Cyclopedia, for acceflible and inacceffiblc heights; viz. by the plain table, theodolite, femi- circle, barometer, &c. Mathematicians have even found out ways for meafunng the mountains of the moon, as well as thofe of the earth ". Various obfervations have been made ot the height oi the French and Swift mountains above the level of the fea ". The Altitude of the higheft mountain in i- ranee nfes only to 1660 toifes. Mount Olympus, as meafured by the antients, was found ten ftadia, or furlongs '. But thefe are fmall Altitudes, in comparifon of that of the Cordelleras in Peru._[" Jour, des Scav. T. 70. p 352. » Mifc. Lipf. T. 8. p. 14.. Mem. de Trev. ,713. p. I2 k &1468. It. 1715. p. I345 . It. 17,2. p. 87c. Hift. Acad. ^ienc. 1708. p. 32. It. 1712. p. 67. = Works of Learn. 1 . 7. p. 663.] See Mountain.
The barometrical method of meafuning the Altitude of moun- tains is but of late invention. It is found very commodious . m praflicc, being done with a fmall apparatus, but is liable to great errors and irregularities, for which, however, cer- tain corre&ons have been contrived. To conceive the prin- ciples of this method, it is to be obferved, that the ordinary . or mean Altitude of the barometer by the Tea-fide is fuppofed to be 28 Pans inches, which are here equivalent to the weight of the wnole incumbent atmofphere. If the barome- ter be carried up higher, the mercury falls, as having a lefs depth of air to fuftam ,t. The proportion of this fall is commonly fuppofed a line for every 60 feet of air above the level of the fea. As the barometer varies according to the divers alterations of the a,r, efpecially as the weather is found fair, rainy, windy, or calm, it is evident the obfervations which are to determine the quantity, which the mercury falls for a given Altitude of place, ought to be made in the fame weather, that the alterations thereof may have no fhare in the event of the experiment.
If the Altitude of 60 feet always anfwered to a line of mer- cury, it would be eafy to meafure the height of a mountain above the level of the fea. All here neceffary would be, to find at what height the mercury ftands near the fca-fide, and how muc h , t f a l] s at the 6me time> o£ . under the fime difpo _
i '" on of *e air, when carried to the top of a mountain. But as the air is always more rare in proportion as it is further frctm the furface of the earth, that column of air, which, taken from the level of the fea, will fuftain a line of mercury, is denfer, and confequently fhortcr than a higher column Suppl. Vol. I. °
ALT
which will fuftain another line; and fo cm, according to i certain progreffion not hitherto well afcertained. Meffieurs Caffini and Maraldi, in continuing the meridian, made teveral experiments and obfervations of the barometer; at different Altitudes, which being compared with the geo- metrical meafures of the fame, and with the barometrical obfervations made at the obfervatory at Paris, which is known to be 46 fathom above the furface of the ocean, they have hence ventured to fix the progreffion wherein the feveral co- lumns of air anfwering to a line of mercury, grew higher and higher to be fuel, as that, fuppofing the firft column to be 61 feet high, the fecond will be 62 feet, the third 63 feet, and fo on, at leaft for the height of half a league; for their obfervations had not been made on any mountains at a greater height than 1 this By fuppofing this progreffion, they always found, by the fall of the mercury on a mountain, the moun- tain s height to be the fame as they had found by geometrical mcnfuration, at leaft within the difference of a few fathoms By fuppofing this progreffion, therefore, it will be eafy by carrying a barometer to the top of a mountain, to find how much that mountain is above the furface of the fea, pro- vided it may be known at what height the mercury flood at the fame time near the edge of the fea, or in a place whofe height above the fea is known.
This method will even fucceed ordinarily, though the moun- tain be at a very great diftance from the fea, unlefs it be ap- prehended, that at two places very remote the different Al- titudes of the mercury may arifc, in fome meafure, from the different ftates of the air, as well as from its different Alti- tudes. Hift. Acad. Scienc. 1703. p. 13. feq. Mem. ejufd. p. 274.
Suppohng the progreffion above-mentioned to obtain through- out the whole atmofphere, it would be eafy to find the 'Al- titude of it, fince the 28 inches of mercury, which are equi- valent to the weight of the whole atmofphere, producing 336 lines, we have hereby an arithmetical progreffion, confid- ing of 336 terms, the difference whereof is one, and the firft term 61, which will eafily give us the Turn of the whole, viz. 6f leagues for the Altitude of the whole atmofphere Fonten. Hift. Acad. Scienc. 1703. p. 16. The defects of this method are, that we are obliged to fup- pofe the barometer to vary at the fame time, and in the fame manner, in places at a confiderable diftance, which will not always hold true, befides the uncertainty of the ratio of the dilatation of the air at different Altitudes of the atmofDhere Id. ib. 1708.
The firft experiment of this kind was made in France in 1648, by M. Pe'rier, on the high mountain Puy de Dome in Auvergne. Others were afterwards made in 1666, by Sin- clair, in Scotland ; others by the undertakers of the great meridian line drawn through France. M. Mariotte, from thefe obfervations, drew rules for the conftrufliori of tables, to fhew the different Altitudes of places from the different Altitudes of the mercury, and the Altitude of air anfwering to each line of mercury in the barometer, from 28 inches, at which the mercury was fuppofed, at a medium, to ftand ncaf the fca-fide. Dr. Halley, in 1686, made another cal- culation, partly from the fame principles with thofe of Ma- riotte, and partly from the proportion of the fpecific [gravi- ties of mercury to air, which he found to be 1080O to" one. On which footing, a cylinder of air of 10800 inches will be' equal to one inch of mercury. Agreeable to this, the fame author calculated two tables, one to fhew the Altitudes of air correfponding to the obferved Altitudes of mercury, the other the Altitudes of mercury, correfponding to given Alti- tudes of 1\k stir. In 1703, M. Caffini, the younger, com- paring feveral obfervations which had been made in the fouthern parts of France, in the profecution of the new me- ridian, with Mariotte's rules, found a difagreement between them ; the Altitudes of the mountains meafured generally fin-palling thofe which were given by the rules. On this he calculated new tables, wherein the refults were confidcrably greater than according to the rules of Mariotte. In 1709, Dr. Scheuchzer made new experiments on the mountains of the Alps, by Which he found 71, or, in other cafes, 69 Palis feet of air equal to one line of mercury. On the whole, ac- cording to this author, the tables made by the rules of Ma- riotte were found preferable, as coming much nearer to the truth than thofe of Caffini. Yet father Lavat, by other ob- fervations made on the mountains of St. Baume in 1 708, found Caffini's tables to hold with great exadtnefs, beyond what could have been had from the rules of Mariotte. The brother of Dr. Scheuchzer, however, thought it neceffary to calculate a new table, from the experiment at Pfeffers, which was made under fuch circumftances as feemed to render it in fome meafure, a decifive one. V. Phil. Tranf. N°. 40c p c'a2 fen. Mpm Acs,! RmoAV.- rwvo- ~ C.4 c:..i-j ,,-/,' v 3 Ti*
„, „ W.U.. , M v,re. . . . ..... x .am, j_, .405.P.C42
feq. Mem. Acad. Scienc. 1705. p. 61. Ejufd. Hift 170S p. 33. feq. Sec alfo Afl. Phyf. Med. Acad. N. C T 2 App. p. 52.
Several authors have written exprefs on the fubjea of Alti- tudes-, Boiiger ', on the taking Altitudes a t fea ; de Louville % on the folftitial Altitude of the fun ; de la Hire f , on tlft Al- titude of the atmofphere ; Halley e, Scheuchzer h , de Lavat ', and- the acadcmifts of Paris k , on the method of finding Altl'- 2 G tudts
