angle may be taken in between the parallel threads, if need be. And it must always be remembered that the moveable parallel thread should be set either north or south of the other, according as the following star is expected to be really south or north of the preceding. In making an observation, either the inner or the outer edges, or the middle of the wires, may be brought to touch the objects; but then it must be remembered to allow something for the thickness of the wire, in case the observation be not made from that part to which the index is adjusted. In observing the diameters of the sun, moon, or planets, it may perhaps be most convenient to make use of the outer edges of the threads, because they will appear most distinct when quite within the limb of the planet, &c.; but if there should be any sensible inflection of the rays of light in passing by the wires, this would be best avoided by using the inner edge of one wire and the outer edge of the other. And in taking the distance or difference of declination between two stars, &c. the middle of the threads may perhaps be most convenient: but, however the observation is made, due correction must be allowed for the thickness of the wire, if requisite. The difference of declination of two stars, &c. may be observed with great exactness, because the motion of the stars is parallel to the threads; but in taking any other distance, the motion of the stars being oblique to them, is a great impediment, because if one star be brought to one thread, before the eye can be directed so as to judge how the other thread agrees to the other star, the former must be somewhat removed from its thread, so that in this sort of observations the best way of judging when the threads are at the proper distance, is by frequently moving the eye backwards and forwards from one to the other: this method must chiefly be made use of when the distance of the objects is pretty large, and the motion or rolling of the eye great. The micrometer is so contrived that it may be applied to telescopes of different lengths; but then there must be a table for each telescope, by which the revolutions of the screw may be turned into minutes and seconds of a degree. In order to this, it is necessary that the threads of the micrometer should be placed exactly in the common focus of the object-glass and eye-glass, that is, where images of objects seen through the telescope are distinctly formed. The readiest way of doing this, is, first to slide the micrometer into the grooves fixed to the short brass tube, which carries the whole apparatus of eye-glass, &c. and then to draw the eye-glass out by means of its sliding work, till the threads of the micrometer be in its focus, which is known by their appearing most distinct, &c. Then thrust the short tube before mentioned into its proper place, as far as the shoulders of the brass work will admit, and place the object glass in its cell, and looking through the telescope at some very distant object, slide the wooden tube in or out till you make the object appear most distinct, or till it has the least motion on the threads when the eye is moved to and fro; for then the threads of the micrometer will be in the common focus of both glasses, and that will be the proper distance that the object-glass ought always to be at from the threads; and there should be made some mark or ketch in the wooden tube, in order to set it always at the same distance. The proper distance of the threads from the object-glass being thus settled, the table for turning the revolutions, &c. of the screw into angles, or minutes and seconds of a degree, may be made several ways; but as good and easy a mcthod as any is, carefully to measure how many inches and parts of an inch the object glass is distant from the threads, and with the same scale to find also how many inches and parts of an inch 100, &c. revolutions or threads of the screw of the micrometer are equal to: then, making the first distance radius, the last will be the sine or tangent of an angle answering to 100 revolutions. And having the angle answering to 100 revolutions, the angle for any other number will be easily known and set down in the table, as also the parts of a revolution: for in small angles, such as can be observed with the micrometer, their sines, tangents, or chords, are nearly in the same proportion with the angles themselves. The distance before mentioned, to be used as radius, ought strictly to be taken from the threads, to a point within the object-glass, about one third of its thickness, from that surface which is towards the wires, if the glass be, as usual, equally convex on both sides; but if the focus of the object glass is pretty long, and its thickness not great, the error that can arise by measuring from any part of the object-glass will become insensible as to the alteration in the angle. The table for the micrometer may also be made by setting up two marks at a distance on the ground, and observing with the micrometer the revolutions, &c. which they subtend when seen through the telescope, and then computing the angles those objects subtend at the object-glass, by measuring their distance from each other and from the object-glass. The like may also be done by opening the threads to any number of revolutions, and then making a star move exactly on the perpendicular thread, and noting the time it is passing from one parallel thread to the other; for that time turned into minutes and seconds of a degree, by allowing for the star's declination and going of the clock, &c. will be the angle answering to the number of revolutions; from which the whole table may be made. This method perhaps might be most advantageously practised in stars near the pole, where the apparent motion being slow, a sccond in time will answer to a much smaller angle than towards the equator. But he believes, on trial, the first method will be found most easy and practicable, especially if the scale made use of be well divided. VII. Of the Roots used by the Indians, in the Neighbourhood of Hudson's Bay, to dye Porcupine Quills. By Mr. John Reinhold Forster, F. R.S. p. 54. Among the curiosities presented by the Hudson's Bay Company to the R. S., is a small parcel of porcupine quills, dyed by the wild natives, some red and some yellow, with the roots of some plants they use for that purpose. Mr. F. examined them carefully, and found that they are probably of the same kind with those mentioned by Prof. Kalın, vol. 3, p. 14, and 160 of the English translation. The one root, dying yellow, is called by the French in Canada, Tisavoyanne jaune; the other, dying red, has the name of Tisavoyanne rouge. Prof. Kalm declares the latter to be a new plant, belonging to the genus of galium, and received by Dr. Linnæus in his Species Plantarum, p. 153, by the specific name of tinctorium, on account of its dying quality. It grows in woody, moist places, in a fine soil. Kalm observes, that the roots of this plant are employed by the Indians in dying the quills of the American porcupine red, which they put into several places of their work: air, sun, and water, seldom change this colour. The French women in Canada sometimes dye their cloth red with these roots, which are but small, like those of the galium luteum or yellow bedstraw.' Dr. Linnæus describes this plant, as having 6 narrow linearleaves at each knot of the stem, and 4 at the branches; commonly 2 flowers are on each stalk, and its seeds are smooth. The roots, when dry, are of the thickness of a crow quill, brown on the outside, and of a bright purple red, when broken, on the inside. ́ p. The 2d plant, or the Tisavoyanne jaune, is, according to Prof. Kalm, vol. 3, 160, the three-leaved hellebore (helleborus trifolius Linn.); grows plentifully in woods, in mossy, not too wet, places. Its leaves and stalks are employed by the Indians to dye yellow several kinds of their work, made of prepared skins. The French learned from them to dye wool and other things yellow with this plant. Among the roots sent as a specimen from Hudson's Bay, Mr. F. found several leaves, which he separated, and found the plant undoubtedly to be the three-leaved hellebore. In the 4th vol. of Dr. Linnæus's Amoenitates Academicæ, is a figure of this plant, which on comparison Mr. F. found by no means. to be accurate for the leaves in our specimens, and in those collected by a gentleman who favoured him with the sight of the plant, are far more pointed, than in the engraved figure. The stalks have constantly but one flower. The dyed porcupine quills sent along with the roots from Hudson's Bay, are of the brightest red and yellow: and this circumstance suggested the thought of trying whether these roots might not be usefully employed in dying. For this purpose, he boiled a piece of flannel in a solution of half salt of tartar and half alum: the wet flannel was put into the decoction of the three-leaved hellebore roots, and boiled in it for the space of about 12 or 15 minutes; the flannel, when taken out, was dyed with a bright and lasting yellow dye. A white porcupine quill, boiled in the same decoction, became nearly of as bright a yellow, as those sent over from Hudson's Bay. This experiment made him believe, that he had hit upon the right method of dying with the three-leaved hellebore,; and will, he hopes, prompt the directors of the Hudson's Bay company to order larger quantities of this root from their settlements, as it will no doubt become a useful article of commerce. The flannel, boiled in salt of tartar and alum as above mentioned, was likewise immersed and boiled for nearly the same space of time as in the former experiment, in a decoction of the root of the galium tinctorium, but it would dye only a dull and faint red. A porcupine quill boiled with it became yellow, but by no means red. This operation convinced him, that the Indians must certainly have some method to extract the bright and lasting colour, which he could not do. They use perhaps the root quite fresh, which circumstance probably makes them succeed in their dying process. If it could be brought about, to extract and afterwards to fix on wool the dye of this root, it would, no doubt, on account of its bright colour, be a valuable acquisition for our manufactures, and he does not in the least doubt of the probability of succeeding in the attempt, ast the woollen stuffs are animal substances as well as the porcupine quills, and therefore easily susceptible of any dye. The Spaniards of Mexico have but lately learned of the inhabitants of California, the art of dying the deepest and most lasting black that ever was yet known. They call the plant they employ for that purpose cascalote; it is arboreous, with small leaves and yellow flowers; its growth is still slower than that of an oak; it is the least corrosive of all the known substances employed in dying, and strikes the deepest black: so that, for instance, it penetrates a hat to such a degree, that the very rags of it are thoroughly black. The leaves of the cascalote are similar to those of the husiaoke, another plant likewise used for dying black with, but of an inferior quality. The latitude of California gives us hope that the country near the Mississippi, or one of the Floridas, contains this cascalote, the acquisition of which would be of infinite use in our manufactures. VIII. Of a Subærated Denarius of the Platorian Family, adorned with an Etrus- This piece exhibits on one side a female head, representing the goddess Libera, in the exergue, appear in Etruscan letters FIR, or rather FVR, ANTIE, i. e. FORS, FORTVNA, OF SORS, ANTII, OF ANTIAT, equivalent to the Latin inscription above it. The Etruscan elements seem rather better preserved than the Latin. The coin is however in but indifferent conservation, though pretty much of the thin silver plate remains still upon it. The symbol on the reverse here is the same that occurs on the reverses of 2 or 3 other consular coins of the Plætorian family, with the word SORS attending it. The Latin inscription on this piece is extremely similar to one on a denarius of the Rustian family. The symbol there is a double Fortune, or rather 2 galeated Fortunes, which were considered as deities by the Romans. The Etruscan inscription, on the reverse of this denarius, in the exergue, seems to allude to a passage in Tully, relative to the origin of those deities denominated SORTES by the Romans, and to be illustrated by, as well as to throw some light upon, that famous passage: and as the inscription formed of those characters, mentioned by Tully, cannot well be supposed to have contained any other word than FIR, or rather FVR, applicable to the deity, or deities, so called, and worshipped, both at Antium and Præneste; we may fairly suppose the Etruscan inscription to have glanced at that celebrated passage. The medals of the Platorian family similar to this, Havercamp takes to have been struck in the time of the civil war that succeeded Julius Cæsar's death; in which perhaps he may not be very remote from truth. If it should however be allowed probable by the learned, this coin, which must be nearly of the same date with that war, will seem to have preceded about 40 years the birth of Christ. Who P. Cosinius was, whose name seems to have been handed down to us by the denarius, cannot at present be known. But that the Cosinian family was of some note in Rome, we may infer, not only from this curious denarius, but likewise from 2 or 3 ancient Roman inscriptions, which have preserved the name of that family. As for M. Plætorius, mentioned on this, and other similar coins, he was, according to M. Havercamp, questor to Brutus, one of Cæsar's murderers; and this piece appeared a little after that emperor's death. The Etruscan letters were not then entirely out of use: nay, they were not totally disused in some parts of Italy, and particularly at Falerii, a considerable number of years after that tragical event. This we learn from Strabo, who flourished when Tiberius sat upon the imperial throne. IX. A Deduction of the Quantity of the Sun's Parallax from the Comparison of the several Observations of the late Transit of Venus, made in Europe, with those made in George Island in the South Seas. Communicated by Mr. Euler, Jun., Sec. of the Imp. Acad. of Petersburg. From the Latin. p. 69. This is a brief account of a calculation of the sun's parallax, given by Mr. |