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in 1736, to appear 10" more northerly than it did in the year 1727.
As this appearance in y Draconis indicated a diminution of the inclination of the earth's axis to the plane of the eclip- tic, and as feveral aftronomers have fuppofed that inclination to diminifh regularly ; if this phenomenon depended upon fuch a caufe, and amounted to 18" in nine years, the ob- liquity of the ecliptic would, at that rate, alter a whole mi- nute in thirty years ; which is much fafter than any obfer- vations before made, would allow. The Doctor had there- fore rcafon to think that fome part of this motion at leaft, if not the whole, was owing to the moonV action upon the equatorial parts of the earth ; which he conceived might caufe a libratory motion of the earth's axis. But as he was unable to judge, from only nine years obfervations, whether the axis would entirely recover the fame pofition that it had in the year 1727, he found it necefiary to continue his ob- fervations through a whole period of the moon's nodes ; at the end of which he had the fat is faction to fee, that the Jiars returned into the fame pofitions again, as if there had been no alteration at all in the inclination of the earth's axis : which fully convinced him that he had guefl'ed rightly as to the caufe of the phasnomenon. This circumftance proves likewife, that if there be a gradual diminution of the obliquity of the ecliptic, it does not arife only from an alteration in the pofition of the earth's axis, but rather from fome change in the plane of the ecliptic itfelf : hecaufe the Jiars, at the end of the period of the moon's nodes, appeared in the fame places, with refpect to the equator, as they ought to have done, if the earth's axis had retained the fame inclination to an invariable plane. The Doctor having communicated thefe obfervations, and his fufpicion of their caufe, to the late Mr. Machin, that excellent geometer foon after fent him a table, containing the quantity of the annual preceflion in the various pofiti- ons of the moon's nodes, as alio the correfponding nutations of the earth's axis ; which was computed upon the fuppofi- tion that the mean annual preceflion is 50", and that the whole is governed by the pole of the moon's orbit only : and therefore Mr. Machin imagined that the numbers in the table would be too large ; as, in fact, they were found to be. But it appeared that the changes which Dr. Bradley had ob- ferved, both in the annual preceflion and nutation, kept the fame law, as to increasing and decreafing, with the numbers of Mr. Machin's table. Thofe were calculated upon the fuppofition, that the pole of the equator, during a period of the moon's nodes, moved round in the periphery of a little circle, whofe center was 23 29' diftant from the pole of the ecliptic : having itfelf alfo an angular motion of 50" in a year, about the fame pole. The north pole of the equator was conceived to be in that part of the fmall circle, which is fartheft from the north pole of the ecliptic, at the time when the moon's afcending node is in the beginning of Aries : and in the oppofite point of it, when the fame node is in Libra.
If the diameter of the little circle, in which the pole of the equator moves, be fuppofed equal to 18"; which is the whole quantity of the nutation, as collected from Dr. Brad- ley's obfervations of thenar y Draconis ; then all the phe- nomena in the feveral jiars which he obferved will be very nearly folved by his hypothefis. But for the particulars of his folution, and the application of his theory to the practice of aftronomy, we muft refer to the excellent author him- felf b ; our intention being only to give the hiftory of the invention. — { b See Phil. Tranf. N e 4?5.] The corrections arifing from the aberration of light, and from the nutation of the earth's axis, muft not be neglected in aftronomical obfervations; fince fuch neglects might pro- duce errors of near a minute in the polar diftances of fome Jiars. See Phil. Tranf. loc. cit. p 26. As to the allowance to be made for the aberration of light, Dr. Bradley aflures us, that having again examined thofe of his own obfervations, which were molt proper to determine the tranfverfe axis of the ellipfis, which czch Jtar feems to defer ibe, he found it to be ncareft to 40"; and this is the number he makes ufe of in his computations relating to the nutation. Ibid. p. 23.
Monfieur d'Alembert has pubiifhed a treatife, intitled, Re- chercbes fur la preceffion des equinoxes et fur la nutation de la tern dans le fyjhme Newt omen, 4W. Paris, 1749. The cal- culations of this learned gentleman agree, in general, with Dr. Bradley's obfervations. But Monfieur d'Alembert finds, that the pole of the equator defcrihes an ellipfis in the hea- vens, the ratio of whofe axes is as 4 to 3'; whereas, ac- cording to Dr. Bradley, the curve defcribed is either a circle, or an ellipfis, the ratio of whofe axes is as 18 to 16 d . — [ c Journal. Britan. par Monfieur Maty, Jan. 1750, p. 93.
- Phil. Tranf. ibid. p. 35.]
Dr. Bradley e fays, in genera!, that experience has taught him that the obfervations of {\ich Jiars as lie neareft the ze- nith, generally agree beft with one another, and are there- fore the fittelf, to prove the truth of any hypothefis. — [' Phil. Tranf. loc. cit. p. 29.J
From the refult of the comparifon of our befi modern ob- Aptend, Vol. II.
fcrvationsj with fuch as were formerly made with arty tole- rable degree of exactnefs, there appears to have been a real change in the pofition of fome of the fixed Jiarsy with re- fpect to each other ; and fuch as feems independent of any motion in our fyftem, and can only be referred to fome mo- tion in the Jiars themfelves. Arcturus affords a ftrong proof of this: for if its prefent declination be compared with its place, as determined either by Tycho or Flamfteed, the dif- ference will be found to be much greater, than what can be fufpected to arife from the uncertainty of their obferva- tions.
It is reafonable to expect, that other inftances of the like kind muft alfo occur among the great number of viCible Jiars ; becaufe their relative pofitions may be altered by various means. For if our own folar fyftem be conceived to change its place with refpect to abfolute fpace, this might, in pro- cefs of time, occafion an apparent change in the angular diftances of the fixed Jiars: and in fuch a cafe, the places of the neareft Jiars being more affected than of thofe that are very remote, their relative pofition might feem to alter, though the Jiars themfelves were really immoveable. And, on the other hand, if our fyftem be at reft, and any of the Jiars really in motion, this might likewife vary their apparent pofitions ; and the more fo, the nearer they are to us, or the fwifter their motions are, or the more proper the directi- on of the motion is to be rendred perceptible by us. Since then the relative places of the Jiars may be changed from fuch a variety of caufes, confidering the amazing diftance at which it is certain fome of them are placed, it may require the obfervations of many ages, to determine the laws of the apparent changes, even of a fingle _/?#;•: much more diffi- cult, therefore, muft it be to fettle the laws relating to all the moft remarkable Jiars.
When the caufes which affect the places of all the Jiars In, general are known ; fuch as the preceflion, aberration, and nutation ; it may be of Angular ufe, to examine nicely the relative fituations of particular Jiars; and efpecially of thofe of the greatcft luftre, which, it may be prefumed, lie neareft to us, and may therefore be fubject to more fenfible changes, either from their own motion, or from that of our fyftem. And if, at the fame time that the brighter_/fan" are compared with each other, we likewife determine the relative pofitions of fome of the fmalleft that appear near them, whofe places can be afcertained with fufficient exactnefs ; we may perhaps be able to judge to what caufe the change, if any be ob- fervable, is owing. The uncertainty that we are at prefent under, with refpect to the degree of accuracy wherewith former aftronomers could obferve, makes us unable to de- termine feveral things relating to this fubject : but the im- provements, which have of late years been made in the me- thods of taking the places of the heavenly bodies, are fo great, that a few years may hereafter be fufficient to fettle fome points, which cannot now be fettled, by comparing even the earlieft obfervations with thofe of the prefent age. See Dr. Bradley, in Phil. Tranf. N° 485.
St AR-apple, the Englifh name of a genus of plants, called by botanifts cainito and chryfophyllum. See the article Chry- sophyllum, Snppl.
$>TAR-thiflle, the Englifh name of a fpecies of centaury, called by fome calcitrapa. See the article Centaure a, Append.
STARCH (Cycl.)-ln the hiftory of the royal Academy of Sciences 1739, p. 24, Edit. Paris, a kind of Jl 'arch is men- tioned, made of potatoes and red truffles. By the fhort ac- count there given, this Jiarch feems not to be fo good as the common, but might be of ufe in cafe of a fcarcity of wheat.
STARLING, the Englifh name of a genus of birds, called by authors Jlurnu s. Some, crroneoufly, account it a fpecies of turdusy or thrufh.
Of this genus there is only one fpecies, which is the Jiarling^ the beak of which is of a fubulated figure, depreffed in an angulated manner, and obtule at the extremity ; and it's tongue is marginated and acute. It is of the fize of the common black-bird. The beak is near half an inch long, and is yellow in the male, and brown in the female. The general colour is black, but variegated with fpots of grey; and the tips of the feathers of the neck and back are yel- lowifh. It has alfo various other tinges, according to the light it is fecn in. The wing and tail-feathers are brown, with fome yellow at their edges. Hilly Hift. of Animals, p. 496.
STEEL (Suppl.)— -The manner of hardening Jicel for magne- tical bars, which Mr. Canton ■ fays proved better than any other he could meet with, is as follows: Cut a fufficient quantity of the leather of old fhoes into very fmall pieces ; provide an iron pan, a little exceeding the length of a bar, and wide enough to lay two fide by fide without touching each other or the pan, and at leaft an inch deep. This pan is to be nearly half filled with the bits of leather, upon which the two bars are to be laid, a fmall wire being fattened to the end of each to take them out by. Then fill the pan quite with the leather, and place it on a gentle flat fire, co- vering and furrounding it with charcoal. The pan being brought to fome what more than a red heat, and kept fo for C c about
