had been paffed immediately before. 4 Carta de Foresta (1300). This is alfo a Charter of Infpeximus, and is copied from an entry on the Charter roll. There are like Charters under the Great Seal at Durham and Oriel College. They were granted on the fame occafion as the laft mentioned Charters of Infpeximus of Magna Carta. 5. Carta Confirmationis (1301). This Charta, which is a general confirmation of the two, is taken from an original in the Bodleian Library, from which the copy in Blackstone was likewife printed. It was made fhortly after a general perambulation of the forefts had been finifhed.

Thus we have enabled our readers to form fome judgment with respect to the most important part of the volume before us. The value of this collection, which "exhibits a complete feries of the Charters of Liberties granted by the Kings of England, in which their original and progress may be traced until their final and complete eftablishment in the 29th year of King Edward I," will be fully appreciated by the future hiftorian, whom it will materially affift in the illuftration of a very interefting portion of his fubject, and in the correction of many erroneous notions which have found their way into our hiftories, from the difficulty of confulting those authentic fources of information, which are here opened for our ufe. The antiquary will obferve with equal pleasure, that the skill of the engraver has been employed to perpetuate faithful representations of thofe curious reliques, which were long in danger of perifhing amidst the wrecks of time.

We fhall referve our further obfervations on this work till the appearance of the fucceeding volumes fhall bring it again under our review. At prefent we fhall only add the names of the Gentlemen commiffioned to execute it; who are Thomas Edlyne Tomlins, John France, and William Elias Taunton, Efqrs.. Barristers at Law. We must not omit to mention at the fame time, that the zeal, and indefatigable diligence of the Speaker of the House of Commons have been eminently ufetul in promoting "a work highly expedient to be undertaken for the honour of the nation, and for the benefit of all his Majesty's fubjects."

ART. II. Philofophical Tranfactions for 1809. Part I. (Concluded from p. 356.)

IV.

N Account of a Method of dividing aftronomical and other Inftruments, by ocular Inspection; in which the

ufual

ufual Tools for graduating are not employed; the whole Operation being fo contrived, that no Error can occur but what is chargeable to Vifion, when affifted by the best optical Means of viewing and meajuring minute Quantities. By Mr. Edward Troughton. Communicated by the Aftronomer Royal.

This account is preceded by fome obfervations on the comparative merits of the different methods of dividing, employed by Bird, Smeaton, and other artifts, and by a relation of the fteps by which the ingenious author was led to the invention and gradual improvement of the method of dividing here described.

The furface of the circle to be divided, as well as its inner and outer edges, but efpecially the latter, must first be turned in the molt exact and careful manner, a roller 1's part of the diameter of the circle, is then applied to its outward edge, the circumference of the roller is divided into fixteen parts, each of which are equivalent to part of the circuit of the circle, but as it is almoft impoflible to turn a roller fo accurately as precifely to measure the circumference of the circle, it is made flightly conical, and contrived fo as to flide up and down in the direction of its axis, by which means it may be adjusted to measure the circumference accurately. This adjustment is afcertained by means of two microfcopes, one placed over the circle, the other over the roller. An inftrument for making dots is now fixed at a convenient diftance from the edge of the circle, and one divifion of the roller being brought exactly under the wire of its microfcope, the pointer is pushed down, which makes the first dot on the circle. The fecond divifion of the roller is then brought under its microscope, and a second dot made, and in this way the whole 256 dots are completed.

The next process is to difcover the errors of the divifions, and in this examination confifts the peculiar excellence of Mr. Troughton's method.

The roller and apparatus for making the dots being taken off, the circle is to be mounted in the fame manner that it will be in the obfervatory, two microfcopes, one provided with a micrometer, the other with only a crofs wire, are fixed to a frame on oppofite fides of the circle, fo that the dot at zero is accurately bifected by the former, and the dot at 180° by the latter. The circle is now turned half round, and the dot at the zero bifected by the oppofite microscope, the dot at 180° is then examined, and if not found bifected,

the

the apparent error is meafured by the micrometer, and half the quantity noted in a table for the real error of that dot, with the fign of + or affixed, as the point is too forward or too backward, according to the order of divifions. The micrometer micrófcope is now removed to only 90° from the other, and the four quadrants or bifections of the femicircle examined, and half the difference between the two quadrants taken as the real error, and noted in the table as before, this being done, the microfcopes are fucceffively placed at the distance of 45°, 22° 30′, 11° 15′, 5°37′ 30′′, and 2° 48′ 45′′ asunder, and the error of each dot examined and inferted in the table.

In this manner a table is conftructed, fhowing the error of each dot, compared with the two contiguous ones, fuppofing them to be rightly placed.

This table, which the author calls a table of apparent errors, (we think a table of comparative errors would have been a better title,) furnishes data for computing the table of real errors, for let q be the error of the preceding dot, b that of the following one, and the apparent error of the dot under investigation, then a+b +the real error of the pofition of

that dot.

2

The original dots being laid down, and the error of each correctly afcertained and inferted in the table, it only remains to cut the ultimate divifions on the inftrument; for this purpofe the circle is again placed horizontally, and the roller applied to it as before, but to the axis of the roller, and in contact with its upper furface is now centered a small inftrument, called a fubdividing sector, confifting of an arc of thin brafs four times the radius of the roller. This arc, which ferves as a micrometer head to the roller, is divided on the engine into eighteen fpaces, fixteen of which are equal to 1° 20′ on the circle. The two extreme spaces, which properly speaking are arcs of excefs, are each fubdivided into eight parts.

The apparatus invented by Mr. Hindley of York, and afterwards applied by Mr. Ramfden to his dividing engine, is employed for cutting the divifions.

This apparatus being properly fixed, the firft dot on the circle, and the zero point on the fector, are brought under their respective microfcopes; the firft divifion is then cut; after which the circle is moved forward, until the fecond divifion of the sector is bifected, when the second divifion is cut, and fo on to the fixteenth division.

But

But the fixteen divifions on the sector being equal to only 1° 20′, while the dots are 1° 24′ 23′′ from each other, it is evident that there must remain a space equal to of a divifion between the last cut divifion and the fecond dot, in removing the fector, therefore, to the adjacent dots, regard muft be had to this circumftance, and inftead of the zero point of the sector being adjufted to its microfcope, the point arc of excefs must be adjufted to it for the fecond dot, for the third dot, &c.

The first fixteen divifions being cut, the wire of the microfcope belonging to the circle must be adjusted to the error of the fecond dot, and the circle moved forward, until the fecond dot is bifected by it, then the fector being applied and adjufted, according to the foregoing obfervation, the next fixteen divifions must be cut, and fo on from each dot, until all the divifions are completed.

The paper concludes by obferving, that this method of dividing is applicable to the divifion of right lines, as well as circles.

The method of dividing defcribed by Mr. Troughton, no doubt, will be found the most accurate yet invented; it moft effectually guards against the errors of expanfion, and enables the artift to examine and correct his work as he proceeds in it. We cannot help thinking, however, but that it would be better to proceed in the bifectional method throughout, by dividing the circle into 4096 parts, inftead of endeavouring to accommodate it to the vulgar divifion of 360°. The procefs would thus be rendered more fimple, for the artift and the aftronomer would thereby be furnished with the ineftimable advantage of being able to examine and estimate the error of every ultimate divifion by the very fame procefs which the artift had before employed in correcting the pofition of the primary dots.

Too much praife cannot be bestowed upon Mr. Trough ton for making public fo valuable an improvement in the art of dividing. In doing which, we fear he has served the public more than himfelf. But true genius is always as ready to communicate as to invent, while fordid avarice is myfterious and imitative; hence it too often happens, that inferior fpirits reap the harveft of fruits fown and cultivated by ment of far fuperior talents, and much nobler minds; but we hope this will not prove the cafe with Mr. Troughton.

V. A Letter on a Canal in the Medulla Spinalis of fome Quadrupeds, in a Letter from Mr. William Sewell to Everard Home, Efg. F.R.S.

2

The canal here defcribed is found in the horse, bullock, fheep, hog, and dog. It takes its rife from the extremity of the fixth venticle of the brain, which corresponds to the fourth in the human fubject, continuing, by a direct course throughout the whole length of the fpinal marrow, and terminating on the cauda equina; its diameter is fufficient to admit a large fized pin; the canal is lined by a membrane, resembling the tunica arachnoidea, and is moft eafily diftinguished where the large nerves are given off in the bend of the neck and facrum. It was proved to be a continued tube throughout, by pouring mercury into it, which, paffed in a fmall fream with equal facility either way.

VI. A Numerical Table of elective Attractions, with Remarks on the Sequences of double Decompofitions. By Thomas Young, M.D. For. Sec. R.S.

Nothing can be more ufeful to the chemift than accurate tables of affinities, and until we are able to estimate the precife force with which one fubstance attracts another, we cannot be faid to poffefs any real knowledge of chemistry. Confiderable advances have been made of late towards determining which of two fubftances have the strongest affi. nity for either, but the precife force with which the particles of one body attract thofe of another ftill remains to be de termined; without this datum chemistry is mere guess work, but with it would be a fcience as much the fubject of cal. culation, as mechanics or aftronomy.

From twelve hundred cafes of double decompofition, enumerated by Fourcroy, Dr. Young has obtained a feries of numbers, expreffing the attractions of acids with different bafes, &c. This table, though by no means fo perfect as might be wifhed, is perhaps as complete as our prefent knowledge of chemiftry will allow. For whatever fome fuperficial pretenders to fcience may imagine, we cannot help thinking, that chemiftry is at prefent a fcience of which we talk much, and know but little.

VII. Account of the Diffection of a Human Fatus, in which the Circulation of the Blood was carried on without a Heart. By Mr. B. C. Brodie. Communicated by Everard Home, Efq. F.R.S.

Defcriptions of monflers, though of little ufe in other refpects, ferve to fhow the wonderful refources of nature, and in fome inftances furnilh us with negative information. respecting the functions of different parts of the animal frame.

There