rius, &c. These indeed, as well as all other heavenly bodies, cannot be said to be entirely free from the influence of the stars surrounding them; but the character assigned to them is, that the attraction in one direction is so counteracted by a contrary influence of the same nature, as to be retained for many ages in a state almost equal to undisturbed rest. Dr. Herschel suspects that we are to look for solar systems only among those insulated stars. 2. Binary sidereal systems, or double stars.—It is sufficiently obvious that these are not stars seen nearly in the same visual ray, for these rays may be an immense distance from each other; but by these are meant two stars that are connected together by the influence of attraction. It is easy to prove, by the doctrine of gravitation, that two stars thus connected, and sufficiently distant from the influence of other celestial bodies, will perform revolutions round a common centre of motion; that hence they will always move in directions opposite and parallel to each other; and that their system, if not destroyed by some foreign cause, will remain permanent. This kind of rotation is exemplified by the instance of our earth and the moon. Dr. Herschel proposes, on a future occasion, to communicate a series of observations made on double stars, whereby it will be seen that many of them have actually changed their situation with regard to each other, in a progressive course, denoting a periodical revolution round each other, and that the motion of some of them is direct, while that of others is retrograde. 3. More complicated sidereal systems, or treble, quadruple, and multiple stars.-From the combination of two stars, it is easy to advance a step further, and allow that three or more stars may be connected in one mutual system of reciprocal attraction; and the computation for determining the common centre of their respective orbits is here exemplified by a variety of hypothetical cases. The author at the same time asserts, that there is not a single night when in passing over the zones of the heavens by sweeping, he does not meet with numerous collections of such multiple stars, apparently insulated from other groups, and probably joined in some small sidereal system of their own. 4. Clustering stars.-These are described as great collections of small stars that are profusely scattered over the milky way, by no means uniformly, but unequally dispersed in many separate allotments. An instance of one of these aggregates is given, which in a space of about 5° between ẞ and y Cygni, contains above 331,000 stars. A more particular account of the milky way, we are promised, will be the subject of a future communication. 5. Groups of stars.-These differ from the preceding class by being collections of closely, and almost equally compressed stars, of any figure or outline; and from the next following, by showing no particular condensation that seems to point out any ideal centre of attraction. 6. Clusters of stars.-These are generally round, and the compression of their stars indicates a gradual accumulation towards their centre, where they are sufficiently condensed to produce the appearance of a nucleus. These we are told are the most magnificent objects that can be seen in the heavens. 7. Nebula.-These, it is thought, may be resolved into the three last-mentioned species, only removed to such a distance that they can only be seen by means of the most powerful telescopes. 8. Stars with burrs, stellar Nebula.-These are thought to be clusters of stars, at great distances, the light of which is gathered so nearly into one point, as to leave but just enough of it visible to produce the appearance of burrs. 9. Milky nebulosities.-These phænomena are probably of two different kinds, one of them being deceptions; namely, such as arise from extensive regions of closely connected clustering stars contiguous to each other, like those that compose our milky way: the other, on the contrary, being real, and possibly at no very great distance from us. The milky nebulosity of Orion, discovered by Huygens, is given as an instance of this singular appearance. 10. Nebulous stars.-Whether these be the effect of the atmospheres of certain stars remains yet to be determined; and indeed every thing respecting the nature of these appearances is still involved in much doubt and obscurity. 11. Planetary Nebula; and 12. Planetary Nebula with centres.— These also, though objects manifestly distinct from the former ones, are as yet so imperfectly known, as to baffle all reasoning concerning their nature and habits; and Dr. Herschel contents himself for the present with merely inserting the few he has observed in his catalogue. Here follows the copious catalogue of Nebulæ, &c., which being a continuation of two preceding papers of the like nature, and arranged in the same manner, requires no further explanation. The Bakerian Lecture. Observations on the Quantity of horizontal Refraction; with a Method of measuring the Dip at Sea. By William Hyde Wollaston, M.D. F.R.S. Read November 11, 1802. [Phil. Trans. 1803, p. 1.] In a communication on this subject, published in the volume of the Philosophical Transactions for the year 1800, Dr. Wollaston accounted for various singular phænomena of horizontal refraction by certain gradual changes in the density of the refracting medium. Having since perused what M. Monge has published in the Mémoires sur l'Egypte, concerning the appearance known to the French by the name of Mirage, where it is ascribed to permanent rarefied strata of air near the surface of the earth; our author, having reconsidered the subject, and finding that the facts related by the French philosopher accord entirely with his own theory, declares here that he still adheres to his former opinion, and assigns his reasons for not departing from it. The chief of these reasons is, that the definite reflecting surface, which M. Monge supposes to take place between two strata of air of different density, is by no means consistent with that continued ascent of rarefied air which he himself admits; and that the explanation founded on this hypothesis will not apply to other cases, which may all be satisfactorily accounted for, upon the supposition of a gradual change of density, and successive curvature of the rays of light by refraction. The subject being of far greater importance than may at first sight appear, since the variations in the dip of the apparent horizon, on which all observations of altitude at sea necessarily depend, must be influenced by this variable refraction, our author has been vigilant in availing himself of every incident that might serve to throw some light on the subject: among these, the first that occurred was an appearance he saw on the river Thames; when being seated in a boat, with his eye about half a yard above the surface of the water, he perceived the oars of barges at some distance, bending inwards, the point of curvature or angle taking place at a small height above the sensible horizon. He now recollected that the warmth of the summer having been very considerable, the temperature the water had acquired, and still retained when the atmosphere had become cooler, must occasion a rarefaction of the stratum of air above its surface greater than those at higher elevations. This led him to a series of further observations, which he has collected in a table, from which we learn that, taking in likewise the hygrometrical changes in the atmosphere, the depression of the horizon is greater the higher the temperature of the water is above that of the air; but that this depression is materially diminished by the increasing dryness of the air. That these refractions (which in the above-mentioned observations were by no means at all times consistent,) must be affected by the vicinity of land influencing the temperature of the air, will be easily admitted; and hence the observations at sea may, it is thought, afford some more accurate conclusions, though the quantity of depression may not be so great. Thus much however is evident, that the error in nautical observations, arising from a supposition that the horizon is invariably according to the height of the observer, stands greatly in need of correction. How to apply this correction is the object of the close of this paper. This consists in measuring, by a back observation, the whole vertical angle between any two opposite points of the horizon, either before or after taking an altitude, and calculating half the excess of this angle above 180°, which will of course be the dip required. A few cautions are lastly given for correcting some inaccuracies in the instruments, especially the index error in the back observations, which it is owned had been some years since suggested by Mr. Ludlam. A chemical Analysis of some Calamines. By James Smithson, Esq. F.R.S. Read November 18, 1802. [Phil. Trans. 1803, p. 12.] The uncertainty that has till now prevailed concerning the nature and composition of the ores of zinc called Calamine, has induced our author to enter upon the investigation now before us. In the first part of the paper, we find the analysis of four kinds of calamines ; the first from Bleyberg in Carinthia, the second from the Mendip hills in Somersetshire, the third from Derbyshire, and the fourth an electrical calamine from Regbania in Hungary. Referring to the paper for the detail of the four processes there circumstantially described, we must content ourselves with reciting here the results deduced from each of them. 1000 parts of the Bleyberg ore were found to consist of 714 calx of zinc, 135 carbonic acid, and 151 water. Some carbonate of lime and lead were likewise found in it; but these appeared to be merc accidental admixtures, and in too small quantities to deserve notice. 1000 parts of the Mendip ore consisted of 648 parts of calx of zinc, and 352 of carbonic acid, and yielded no water. In the Derbyshire ore were found 652 of calx of zinc, and 348 of carbonic acid. And in the Hungarian ore, 683 of calx of zinc, 250 of quartz, 44 water and here there moreover appeared a loss of 23, owing, no doubt, to some defect in the manipulation. The water was by no means considered as an essential part of this ore; and hence the proportions of the two other ingredients were as 739 to 261. In a second part of the paper, the author communicates some observations to which he was led by the uncertainty that still prevails in our chemical researches, and the want of uniformity in the results of the multitude of experiments that are daily made, which appear to him to clash essentially with the simplicity of nature. When we consider, he says, the simplicity found in all those parts of nature which are sufficiently known to come within the reach of our observation, it appears improbable that the constituent parts of bodies, which we consider as endowed with reciprocal affinities, should be so loosely united as is often indicated by the most accurate analysis. Hence he is led to conjecture, that in all chemical combinations, those ingredients which are really essential to the compound are but few in number; that they are by nature certain aliquot parts of the whole compound; and that as the aliquot may be expressed by fractions, the denomination of these fractions will always be a small quantity, perhaps never exceeding the number 5. The author applies this theory to the above-mentioned experiments on calamine; and finding that, with a trifling correction, the results coincide with this theory, he entertains sanguine hopes that future investigations will finally establish it. If so, he thinks that the discovery will introduce in chemistry a rigorous accuracy, of which it has not hitherto been thought susceptible; that it will enable the mist, like the geometrician, to rectify by calculation the unavoid able errors of his manual operations, and authorize him to climinate from the essential elements of a compound those products of an analysis whose quantity cannot be reduced to any admissible proportion, and may therefore be considered as extraneous. The author, at the close of his paper, controverts the opinion of those who think that crystallization requires a previous state of solution in the matter crystallized; and contends, that as long as any quantity of fluid is present in a solution, no crystallization can possibly take place. Experiments on the Quantity of Gases absorbed by Water, at different Temperatures, and under different Pressures. By Mr. William Henry. Communicated by the Right Hon. Sir Joseph Banks, K.B. P.R.S. Read December 23, 1802. [Phil. Trans. 1803, p. 29.] After a short recapitulation of what has of late been done by Mr. Cavendish, Dr. Priestley, Dr. Nooth, and others, respecting the impregnation of water with different gases, our author observes, that the circumstance of the different degrees of temperature and pressure had not been as yet sufficiently attended to. Dr. Priestley, indeed, had long since remarked, that, in an exhausted receiver, Pyrmont water will actually boil at a common temperature, by the copious discharge of its air; and that hence it is very probable, that by means of a condensing engine, water might be much more highly impreg nated with the virtues of the Pyrmont spring: but this conjecture remained as yet to be proved by experiments; and this is the task our author has undertaken in the present paper. This paper consists of two sections; the first treating of the quantities of gases absorbed by water under the usual pressure of the atmosphere; and the second, of the influence of pressure in promoting the absorption of gases. The apparatus contrived for these experiments may be described as a siphon, of which one side, or leg, is a glass vessel of comparatively a considerable diameter, and the other a long glass tube of about a quarter of an inch bore; the junction of these two parts at the bottom being a short pipe of India rubber, well secured by proper integuments of leather, thus forming a joint, which admits of the vessel being briskly agitated. This vessel has a stop-cock both at top and bottom, in order to insert and emit fluids and gases; and both the vessel and tube are accurately graduated. It may now be understood, that a known quantity of water and of a certain gas being put in the vessel, and the tube being filled to a certain extent with mercury, the absorption of the gas will be accurately measured by the column of mercury in the tube. Those who are particularly interested in this inquiry will find in the paper various precautions and additional contrivances, all tending to insure the success and accuracy of the investigation. The first experiments were made on the absorption of carbonic acid gas by water: and here a singular disagreement was observed in the first trials made under exactly the same circumstances |