Dr. Herschel, having made these preliminary experiments, proceeds to apply them to investigate the nature and magnitude of the star lately discovered by Mr. Harding. A regular series of observations on this star are detailed, beginning on the 29th of September, and ending on the 11th of October. Of these we must necessarily confine ourselves to mention merely the general result with the conclusions deduced by the author from the whole of the investigation. These conclusions are as follows:

1. A ten-feet reflector will show the spurious or real discs of celestial and terrestrial objects, when their diameter is one fourth of a second; and in favourable circumstances that diameter may be divided, by estimation, into two or three parts.

2. A disc of the above diameter, whether spurious or real, to be seen as a round well-defined body, requires a magnifying power of 500 or 600, and must be sufficiently bright to bear that power.

3. A real disc of half a second in diameter will be so magnified by the above-mentioned power, that it may be easily distinguished from a spurious one of equal size, the latter not being affected by magnifying power in the same proportion as the former.

4. The different properties of the inside and outside rays of a mirror, with regard to the appearance of a disc, will show whether it is real or spurious, provided its diameter is more than one-fourth of a second.

5. When discs, either spurious or real, are less than one fourth of a second in diameter, they cannot be distinguished from each other, because the magnifying power is not sufficient to make them appear round and well defined.

6. The same kind of experiments are applicable to telescopes of different sorts and sizes, but will give a different result for the quantity here stated at one fourth of a second, being more when the instrument is less perfect, and less when it is more so.

The general results of Dr. Herschel's observations on Mr. Harding's newly discovered celestial body, to which the name of Juno has been given, are,—

1. That it is in every respect similar to those discovered by Mr. Piazzi and Dr. Olbers, so that Ceres, Pallas, and Juno, are certainly three individuals of the same species.

2. That these bodies (the last of which appears to be the smallest,) are incomparably smaller than any of the planets; for a telescope that will show a diameter of one fourth of a second, will not determine whether their discs are real or spurious, although a power of more than 600 has been applied to each of them.

3. That the criterion of the apertures of the mirror has, on account of the smallness of the object, been equally unsuccessful; every method that has been tried only proving their resemblance to small stars. 4. That the definition of the term asteroid, formerly given by Dr. Herschel, will equally express the nature of Juno, which, on account of its similar situation between Mars and Jupiter, and its departure from the general condition of planets, by the smallness of its

disc, and the great inclination and eccentricity of its orbit, be considered as a true asteroid.

may also

Dr. Herschel concludes by observing, that the specific difference between planets and asteroids appears now, by the addition of a third individual of the latter species, to be more fully established; and that circumstance, he thinks, has added more to the ornament of our system, than the discovery of another planet could have done.

An Essay on the Cohesion of Fluids. By Thomas Young, M.D. For. Sec. R.S. Read December 20, 1804. [Phil. Trans. 1805, p. 65.]

Dr. Young's principal objects in this paper are to reduce the phenomena of the capillary action of fluids to the general law of an equable tension of their surfaces; to investigate the properties of the curves resulting from this law; to determine the magnitude of the apparent adhesion of solids to fluids, and the cohesion of moistened solids; and to show in what manner the law itself is probably derived from the fundamental properties of matter.

Dr. Young observes, that a fluid which is not capable of wetting a given solid, forms with it an angle of contact which is constant in all circumstances; that the curvature of the surface of a fluid, or the sum of the curvatures, where the curvature is double, is always proportional to the elevation or depression with respect to the general surface, and that the curve itself admits, in all cases, an approximate delineation by mechanical means, but is not always capable of being easily subjected to calculation. When, however, the curvature is simple, the direction of the surface, at any given height, admits a correct determination. Hence the elevation of a fluid in contact with a given surface, whether vertical, horizontal, or inclined, is deduced from its ascent between plates, or in a tube, of the same substance; and the result is shown to agree with experiments. Thus, taking th of an inch for the diameter of a tube, in which water rises to the height of an inch, it is inferred that the apparent adhesion of water, to a square inch of any horizontal surface capable of being wetted by it, must be 50 grains, which is only half a grain more than the result of Taylor's experiments. The adhesion of alcohol, and of sulphuric acid, as measured by Achard, are also found to agree with the ascent of those fluids in capillary tubes. Lord Charles Cavendish's table of the depression of mercury in barometer tubes, is compared with the same principles by means of diagrams constructed for each particular case; and the apparent adhesion of the surface of mercury to glass, as well as the depth of a portion of mercury spread on a plate of glass, is deduced from these measures, and is shown to agree with experiments. The observations of Morveau, on the attraction of the different metals to mercury, are also discussed; and some remarks are made on the magnitude of drops of various substances.

The hydrostatic actions of these elevations and depressions of fluids are such as to afford a ready explanation of the attractions

and repulsions of floating bodies: these attractions are found to vary ultimately in the inverse ratio of the squares of the distances; and they appear to be the same as are found to cause an apparent cohesion between any moistened surfaces nearly in contact: the magnitude of this cohesion, as measured, in a particular case by Morveau, being found to agree with the calculation of the effect of capillary action.

The attraction of a drop of a fluid towards the line of contact of two plates of glass, which was found by Hawkesbee to vary nearly in the inverse ratio of the square of the distance of the plates, was supposed by Newton to indicate an immediate cohesive force, varying in the simple inverse ratio. But Dr. Young has shown that the fundamental law of the equable tension of the surface is sufficient to explain this phenomenon, and to remove the apparent irregularity in the laws of cohesive forces.

The equable tension of the surface is shown to be a consequence which may be mathematically deduced from the existence of a stable equilibrium between the forces of repulsion and of cohesion, which is a necessary condition of liquidity, as the repulsive force always varies more rapidly than the cohesive force. The mutual attractions of solids and fluids are then considered; and Dr. Young agrees with Clairaut, although upon different grounds, in affirming that a fluid will be elevated when in contact with any solid of more than half its attractive density. The tension of the common surfaces of a solid and a fluid, or of two continuous fluids, is supposed to be proportional to the difference of the attractive densities; and this supposition is confirmed by some observations, with which the paper is concluded, on the phenomena of oily substances floating on water.

Concerning the State in which the true Sap of Trees is deposited during Winter. In a Letter from Thomas Andrew Knight, Esq. to the Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read January 24, 1805. [Phil. Trans. 1805, p. 88.]

This paper may be considered as a continuation of Mr. Knight's former communications respecting the motion of the sap in trees. Du Hamel, and other subsequent naturalists, have shown that trees contain two kinds of sap; and the chief purpose of Mr. Knight's paper is to prove that one of them (called by Du Hamel suc propre, and by Mr. Knight the true sap,) is generated in the leaf; and that this fluid, in an inspissated state, or some concrete substance deposited by it, exists during the winter in the alburnum, from which substance, dissolved in the ascending aqueous sap, is derived the matter which enters into the composition of the new leaves in the spring. To the above-mentioned deposition, Mr. Knight attributes the well-known superiority of winter-felled wood, which superiority has generally been supposed owing merely to the absence of the sap at that season.

Du Hamel has remarked, that trees perspire more when the leaves

are full grown, and when the annual shoots have ceased to elongate, than at any earlier period. This energy in the powers of vegetation must certainly, Mr. Knight thinks, be employed in some very important operation. He has observed that the produce of his meadows has been greatly increased when the herbage of the preceding year had been left till the end of autumn, on ground that had been mowed early in the summer; from which he has been led to imagine, that leaves are employed during the latter part of the summer in the preparation of matter calculated to afford food to the buds and blossoms of the succeeding spring.

In order to determine whether the foregoing opinions were well founded or not, Mr. Knight made the following experiments.

Having made incisions in the trunks of sycamore and birch-trees (some of these incisions being close to the ground, others at the elevation of seven feet), he found that the sap obtained from the sycamore close to the ground, was of the specific gravity of 1·004, while that obtained at the height of seven feet had a specific gravity of 1.008. The sap of the birch was somewhat lighter; but the increase of specific gravity, at different elevations, was comparatively the same. The sap of both these trees, when extracted near the ground, was almost void of taste; but when obtained at a greater height, it was sensibly sweet. In one instance it was extracted from the sycamore-tree at the height of twelve feet; it was then very sweet, and its specific gravity was 1·012.

Mr. Knight then made an experiment to compare the sap obtained from a recent incision with that obtained from an old one. He found that the sap from an old incision was reduced in specific gravity to 1002, while that from the recent incision continued at 1·004, as before. These incisions were made in a sycamore-tree, and were close to the ground.

Some observations then follow on the variation in the specific gravity of the alburnum at different seasons. After taking every precaution to avoid error, the author found the specific gravity of winterfelled oak to be 0.679, and that of summer-felled oak to be 0.609, after they had both been immersed five minutes in water. This difference appearing to Mr. Knight very considerable, he repeated the experiment several times, but found no reason to suspect any error in it; and upon measuring pieces of both kinds of wood, which were equal in weight, it appeared that the winter-felled pieces were much less than the others. The more recently formed layers of winterfelled wood had a specific gravity of 0.583; that of the summer-felled wood was only 0.533. In another experiment the former was 0.588, the latter 0-534.

On pouring boiling water on equal quantities of summer- and of winter-felled wood, it appeared that the latter communicated a much deeper colour to the water than the former; it also raised the specific gravity of the water to 1·002; the specific gravity of the other infusion was 1.001.

Mr. Knight thought he had reason to believe that the matter de

posited in the alburnum sometimes remains unemployed during several successive years; he therefore cut off, in the winter, all the branches of a large and very old pear-tree, at a small distance from the trunk, and pared off, at the same time, all the lifeless external bark. No marks of vegetation appeared till the beginning of July following, when numerous buds and leaves, of large size, appeared; and in autumn every part was covered with very vigorous shoots. The number of leaves appeared to Mr. Knight to exceed very much the whole of those the tree had borne in the three preceding years.

Mr. Knight says that he has repeated, with success, the experiments of Bonnet and Du Hamel, and that he is in possession of many other facts which, like those experiments, tend to prove that seedling trees depend, at first, entirely on the nutriment afforded by the cotyledons and that they are greatly injured, and often killed, by being put to vegetate in rich mould. He thinks there is very decisive evidence that bulbous and tuberous-rooted plants contain within themselves the matter which subsequently composes their leaves; also that it appears extremely probable, that the blossoms of trees receive their nutriment from the alburnum, particularly as the blossoms of many plants precede their leaves.

Mr. Knight also thinks the existence of a vegetable circulation, though denied by many eminent naturalists, must be admitted. He supposes that when a sced is placed in a proper situation for vegetation, water is absorbed by the cotyledons, and a young radicle is emitted. This increases in length, by the addition of new matter to its apex, not by any general distension of its vessels or fibres; which new matter appears, from the experiments of Bonnet and Du Hamel, to descend from the cotyledons. The first motion, therefore, of the fluids is downwards, towards the point of the root; and the vessels which carry those fluids are similar to those which are subsequently found in the bark. In support of this opinion, he mentions some observations he has made on the progressive changes which take place in the radicle of the horse-chestnut. From these it appears, that when the roots were considerably elongated, and not till then, alburnous tubes were formed, and that as soon as these tubes had acquired a sufficient degree of firmness, they appeared to begin their office of carrying up the aqueous sap; at which time, and not sooner, the leaves of the plunula expanded. When the leaf has attained its proper growth, it seems to perform precisely the office of the cotyledon, being fed by the alburnous tubes and central vessels; and the true sap is discharged from the leaf, as it was previously from the cotyledon, into the vessels of the bark. Here one part of it produces the new layer of wood (or new epidermis when that is to be formed), and the remaining part enters the pores of the wood already formed, and mixes with the ascending aqueous sap.

The author thinks it probable that the true sap undergoes a considerable change on its mixture with the ascending aqueous sap, as in the sycamore; it was found to become more sensibly sweet in its progress in the root, in the spring, although he could never detect