A series of observations is then given, of the right ascensions of the comet, from which were calculated the following table of the places and elements, with the differences between the observed and computed places. By the help of these observations, which were made by the Rev. Mr. Professor Bliss, the transits excepted taken at Sherborn, Mr. Betts, by the method delivered in the 3d book of the Principia, determined the comet's parabolic trajectory; and found the place of the ascending node to be in 8 15° 45′ 20'; the logarithm of the perihelion distance 9,346472; the logarithm of the diurnal motion 0,940420; the place of the perihelion 17° 12′ 55′′; the distance of the perihelion from the node 151° 27′ 35"; the logarithm sine and co-sine of the inclination of the orbit to the ecliptic, 9,865138, 9,832616; and thence the time the comet was in the vertex of the parabola, or the time of the perihelion, Feb. 19d 8h 12m; the motion of the comet, in its orbit thus situated, was direct, or according to the order of the signs. From these elements, by the help of Dr. Halley's general table, to which they are adapted, Mr. B. computed the comet's places for the times of observation, exhibited in the following table: to which are added the comet's longitudes and latitudes deduced from the observed right ascensions and declinations; with the errors between the observed and computed places; the observations being all reduced to Oxford mean time. Equal Time at Longit. Comet North Latit. Longit. Comet North Latit. Diff. in | Diff. in observed. observed. computed. computed. Long. Latit. 1743, Dec. 23d 5h 32m 14° 10′ 2′′ 17° 33′ 11′′ 14° 10′ 3′′ 17° 33′ 37′′ 27 5 7 Υ 12 2 25 17 51 29r 12 2 26 17 51 47 28 5 1 11 32 11 17 55 54 | |+++ | ++++++ | | | | | 13 6 25 5 9 14 10 17 40 X 11 32 14 17 56 8 19 Perhaps it may not be thought improper to remark, that the nodes of the comet, and the planet Mercury, are situated within less than half a degree of each other; which probably gave rise to a report, that the comet had carried Mercury from its orbit. In order therefore to find how nearly they approached each other, 1 Mr. B. brought the matter to calculation; and presently found, that there was above a week's difference in the times of their coming to the nodes; the comet passing its descending node, Feb. 22, about 2h in the morning; and Mercury not coming to his till Feb. 29; the comet moving all that time southwards with a prodigious velocity. Again, computing their heliocentric conjunction, which happened Feb. 18, about 1h in the afternoon, the comet was at that time distant from Mercury nearly part of the semidiameter of the orbis magnus; being almost twice as near to the sun as the planet ; and having then 31° 30′ of north latitude; Mercury's not exceeding 3° 58′ to an eye in the sun: whence it is easily collected, that the comet could have no sensible influence on 's motion. The elements above-given cannot possibly differ much from the true. For, after an interval of 2 months, in which time the comet had gone through almost + part of its orbit, it is surprising to find the observed and computed places agree so ac-curately, that the difference no where amounts to a minute. In some parts of the orbit, the agreement is still greater; particularly in the observations made at Sherborn, which come within half that quantity. The comet was in conjunction with the sun, Feb. 15, about midnight; and its perigee, Feb. 16, about 1h in the afternoon; at which time it was somewhat nearer the earth than the sun is at its perigee; the comet's distance being them 83, and the sun's 98, such parts, as the semidiameter of the magnus orbis is 100; from which we may have some idea of the comet's magnitude; and therefore may suppose it at least equal to the earth. Of a Scirrhosity of the Cerebellum. By Albert Haller, F. R. S. Professor of Physic at Gottingen, &c. N° 474, p. 100. An Abstract from the Latin. Professor H. describes in this communication the appearances observed on opening the head of a beggar-girl who was supposed to be about 6 years old. Her body was much emaciated, and the mesenteric, inguinal and bronchial glands were found to be in a scirrhous state. On examining the head, nearly the whole of the left lobe of the cerebellum was found to adhere strongly to the dura mater which lines the occiput. On making an incision into the tentorium, Professor H. discovered a very large scirrhous tumour, into which the whole of the medullary or of the cortical substance had degenerated. It was of an uniform density throughout, and of a fibrous appearance, not unlike the texture of the kidney. No blood vessel could be seen on cutting into it, nor was there any vestige of the cortical or medullary structure. Whatever might be the cause of this morbid affection, it was evident that, in this girl, the middle portion of the cerebellum had been rendered useless, and H VOL. IX. that for a considerable length of time. Nevertheless she had continued to exist, and had been capable of going about, begging from door to door. A parallel case of a scirrhous cerebellum is recorded in the Memoirs of the Parisian Academy of Sciences for 1705. An Essay on the Causes of the different Colours of People in different Climates. By John Mitchell, M. D. N° 474, p. 102. The cause of the colour of negroes being a subject so little known, but so much inquired after, and so curious and useful, as to excite the particular attention and inquiries of the learned in Europe, particularly the Academy of Bourdeaux, in their prize-problems, Dr. M. therefore offered his thoughts on that subject, having had frequent opportunities to make the proper and necessary observations. This problem supposes the knowledge of the causes of colours in general; so that if he can deduce the colour of the skin from its structure, &c. in the same manner, and for the same reasons, from which the great Newton deduces the colours of other substances, it is all he pretends to, which will be as much as that branch of philosophy will permit: and as this problem will include the cause of the colour of the skin in general, he first inquires into the cause of the colour of white people; with a change from that colour in some preternatural affections, whose causes seem not well understood. PROP. 1.-The Colour of white People proceeds from the Colour which the Epidermis transmits; that is, from the Colour of the Parts under the Epidermis, rather than from any Colour of its own.-The truth of this proposition wi plainly appear to those who consider, that the colour of white people is always more or less clear or vivid, as the skin is thinner or thicker, finer or coarser; that is, as it is more or less adapted to transmit the colour of the white parts below it. These parts are the parenchyma of the skin, corpus reticulare, papillæ nervosa, the limpid and clear juices contained in the vessels, and perhaps the inner epidermis itself may appear through its outer porous coverlet; all which parts we know are white, and are what appear so in white people. But this will be better confirmed, from the following considerations: 1. The palms of the hands, lips, &c. where the epidermis and skin are so thin as to transmit the colour from any thing below them, appear red, or of the colour of the red blood under them; especially in those in whom the skin is fine and thin; but where the skin is thick and coarse, those parts appear almost of the same colour with the rest of the body. 2. The blushings of the cheeks, and their redness in fevers, seem to be another proof of this cause of their colour; for, in a moment, they change from a pale to a deep red; but no one will imagine, that the epidermis then changes its colour, or power of reflecting the rays of light; but that it transmits the colour of the blood; which at such times is more forcibly driven into the capillary subcutaneous vessels, and shines through the epidermis; but before, these vessels contained only a serous liquor, and accordingly the skin appeared of that colour: which will further appear on squeezing such red parts, which drives the blood out of them, and makes them appear white; whereas, on removing such pressure, they recover their colour, as the blood does its place. 3. The yellow colour of the skin in the jaundice is a further proof of this assertion; where the yellow bile is diffused through the vessels of the cutis, and appears through the epidermis; but no one will imagine, that the epidermis itself receives this viscid bile into its vessels; which are so small, that many accurate anatomists, as Morgagni, have denied it to have any vessels at all; and the most accurate could never show them. 4. The pale look of those in whom the blood is viscid, or circulates with little force, shows that the epidermis then transmits the colour of the juices and fibres below it, which are then unmixed with red blood. 5. The same is manifest in those whose blood is poor and serous, as the leucophlegmatic, &c. in whom the epidermis transmits the colour of the water or serum under it. Hence it appears, that the epidermis is a transparent membrane, which easily shows the colour of the parts under it, in the same manner as the cornea of the eye transmits the colour of the iris. And this will appear more plainly from some considerations below; where we shall assign the cause of this pellucidity; and show that the numbers of pores in the epidermis necessarily make it tranparent; and that the smallness of the particles, into which it is divided by them, make it unfit to reflect any rays of light, and consequently to manifest any colour of its own. PROP. 2.-The Skins of Negroes are of a thicker Substance, and denser Texture, than those of white People, and transmit no Colour through them.-For the truth of the first part of this proposition, we need only appeal to our senses, and examine the skins of negroes when separated from the body; when not only the cutis, but even the epidermis, will appear to be much thicker and tougher than in white people. But because the substance and texture, especially of the epidermis, is not a little altered in anatomical preparations, and that in such a measure as to alter the texture perhaps, on which the colour depends, by boiling, soaking, peeling, &c. let us examine the skins of negroes on their body; where they will appear, from the following considerations, to have all the properties assigned: 1. In bleeding, or otherwise cutting their skins, they feel more tough and thick, than in white people. 2. When the epidermis is separated by cantharides, or fire, it is much tougher and thicker, and more difficult to raise, in black, than white people. 3. Negroes are never subject to be sun-burnt, or have their skins blistered by any such degree of heat, as whites are. 4. Though their skins, in some particular subjects, should not be so very thick in substance, yet in winter, when they are dry, and not covered with that greasy sweat which transudes through them in summer, their skins feel more coarse, hard, and rigid; as they do in ardent fevers, with a dry skin. 5. Their exemption from some cutaneous diseases, as the itch, prickly heat or essere, which no adult negroes are troubled with, but those of fine and thin skins are most subject to, show the thickness or callosity of their skins, which are not easily affected from slight causes. 6. And not only the thickness, but also the opacity of their skins, will appear, from their never looking red in blushing, or ardent fevers with internal inflammations, nor in the measles, nor small-pox; where, though the blood must be forcibly impelled into the subcutaneous vessels, yet it does not appear through the epidermis. The like may be said of their veins; which, though large and shallow, yet do not appear blue, till the skin is cut. 7. In the jaundice, anasarca, &c. the skin of negroes never shows the colour of the parts under it; though visible enough in the eyes: of which Dr. M. saw a more convincing proof in some negroes labouring under a bilious fever, in whom the serum of the blood, when let, was of a deep bilious yellow, but no yellow colour appeared on the skin, though plain enough to be seen in the eyes. Corollary.-Hence might be deduced one plain cause of the blackness of negroes: for if the colour of the skin depends on what it transmits, and the skins of negroes transmit no colour through them, they must needs appear black; according to the known doctrine of light and colours, that wherever there is a privation of light or colour, there of course ensues darkness or blackness. But as most solid bodies, which are not pellucid, do generally reflect some colour, which we know no black body does, we shall next inquire into the particular make of their skins, by which they are rendered incapable to reflect, as well as to transmit, the rays of light. PROP. 3.-The Part of the Skin which appears black in Negroes, is the Corpus Reticulare Cutis, and external Lamella of the Epidermis: and all other Parts are of the same Colour in them with those of white People, except the Fibres which pass between those Two Parts. For a proof this proposition, we must examine the structure of the skins of negrces more narrowly, which may be done after blistering with cantharides, or after a scald or burn; when their skins appear in the following manner: the cuticle, which is separated, appears nearly of the same colour on the outside, as before such separation from the body; but on the inner side is almost as white as the same part in white people. This cuticle is almost always, in blistering with cantharides, divided into two lamella; especially on the thighs, where it is as thick almost as both the skin and scarf-skin of white people: the surfaces, by which these two parts or lamella of the epidermis cohere, are partly white, and partly black; for you may see many black fibres pervading the inner lamella, and perforating the upper one, which appear like so many black spots on these two surfaces, when separated from each other; but these black spots do not |