meridional distance is 1,686,595 feet, and 60,825 fathoms is the length of the degree in latitude 51° 9'.

Experiments on Trees. By Mr. KNIGHT.-[1804.]

THE sap, having been absorbed by the bark of the root, is carried up by the alburnum, or white wood, of the root, the trunk, and the branches; it passes through what are there called the central vessels, into the succulent part of the annual shoot, the leaf-stalk, and the leaf; and it returns to the bark through the returning vessels of the leaf-stalk.

Mr. Knight begins with the leaf, from which, he assumes, all the descending fluids in the tree are derived. The efforts which plants always make to turn the upper surfaces of their leaves to the light, have induced naturalists to conclude, that each surface has a totally distinct office. Mr. Knight has detailed a number of experiments to support that conclusion. From these he infers, that in the vine, the perspiratory vessels are confined to the under surface of the leaf, and that they, like the cutaneous lymphatics of the animal economy, are probably capable of absorbing moisture when the plant is in a state to require it. The upper surface seems, from the po-> sition it assumes, either formed to absorb light, or to operate by the influence of that body; and if any thing exhale from it, it is probably vital air, or some other permanently elastic fluid. It is known, that perpendicular shoots grow with greater vigour than others, and Mr. Knight imputes it, from some experiments, to a more minute and rapid circulation.

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To prove the effects of motion on the circulation of the sap, he selected a number of young seedling apple-trees, whose stems were about an inch in diameter. By means of stakes and bandages of hay, not so tightly bound as to impede the progress of any fluid within the trees, he deprived the roots and lower parts of the stems of several trees of all motion, to the height of three feet from the ground, leaving the upper parts of the stems and branches in their natural state. the succeeding summer, much new wood accumulated in the parts which were kept in motion by the wind, but the lower parts of the stems and roots increased very little in size. Removing the bandages from one of these trees, he fixed a stake in the ground, about 10 feet distant from the tree, on the east side of it, and attached the tree to the stake, at the height of six feet, leaving it liberty to move towards the north and south, but in no other direction. Thus circumstanced, the diameter of the tree from north to south, in

that part of its stem which was most exercised by the wind, exceeded that in the opposite direction in the following autumn, in the proportion of 13 to 11.

The principal office of the horizontal branches, according to Mr. Knight, in the greatest number of trees, is to nourish and support the blossoms, and the fruit or seed; and, as these give back little or nothing to the parent tree, very feeble powers alone are wanted in the returning system. No power at all would have been fatal; and powers sufficiently strong wholly to counteract the effects of gravitation, would probably have been in a high degree destructive, and it is Mr. Knight's opinion, that the formation of blossoms may, in many instances, arise from the diminished action of the returning system in the horizontal or pendent branch.

Experiments and Calculations relative to physical Optics. By Dr. YOUNG. [1804.]

THE proposition which Dr. Y. intended to establish was, "that fringes of colour are produced by the interference of two portions of light." From the experiments and calculations we may infer, that homogeneous light, at certain equal distances in the direction of its motion, is pos sessed of opposite qualities capable of neutralising and destroying each other; and of extinguishing the light where they happen to be united; that these qualities succeed each other alternately in successive concentric superficies, at distances which are constant, for the same light passing through the same medium. From the agreement of the measures, and from the similarity of the phenomena, we may conclude that these intervals are the same as are concerned in the production of the colours of thin plates; but these are shown by the experiments of Newton to be the smaller the denser the medium; and since it may necessarily be sumed that their number must remain unaltered in a given quantity of light, it follows, of course, that light moves more slowly in a denser than in a rarer medium: and this being granted, it must be allowed that refraction is not the effect of an attractive force directed to a denser medium.

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Since we know that sound diverges in concentric superficies, and that musical sounds consist of opposite qualities capable of neutralising each other, and succeeding at certain equal intervals, which are different according to the difference of the note, we are fully authorised to conclude, that there must be some strong resemblance between the nature of sound and that of light.

An Enquiry concerning the Nature of Heat, and its Mode of Communication. By BENJAMIN Count RUMFORD.

THE principal object of this essay is to obtain a more intimate knowledge of the nature of heat, and of its mode of action, by which the author thinks that we may be enabled to excite it with greater economy, confine it with greater facility, and direct its operations with more precision and effect.

From the first series of these experiments, he concludes that all the heat which a hot body loses when it is exposed in the air to cool, is not given off to the air which comes into contact with it, but that a large proportion of it escapes in rays, which do not heat the transparent air through which they pass, but, like light, generate heat only when and where they are stopped and absorbed.

As the results of various other experiments made with a view to determine the relative quantities of rays emitted from the surfaces of different substances, from living animals, dead animal matter, &c. we are informed that those substances which part with heat with the greatest facility are those which also acquire it most readily or with the greatest celerity; and also, that the greater the power is which an animal possesses of throwing off heat from the surface of his. body, independently of that which the surrounding air takes off, the less will his temperature be affected by the occasional changes of temperature which take place in the air; and the less will he be oppressed by the intense heats of hot climates.

The warmth of any kind of substance used as clothing, or its power of preventing our bodies from being cooled by the influence of surrounding colder bodies, depends much on the polish of its surface; for, upon careful examination, it will be found that those substances which supply us with the warmest coverings, as furs, feathers, silk, &c. are not only smooth, but highly polished; it will also be found, other circumstances being equal, that those substances are the warmest which are the finest, or which are composed of the greatest number of fine polished detached threads or fibres. The fine white shining fur of a Russian hare is much warmer than coarse hair; and fine silk, as spun from the silk-worm, is warmer than the same silk twisted together into coarse threads.

Formerly Count Rumford considered the warmth of natural and artificial clothing as depending principally on the obstacle it opposed to the motions of cold air by which the hot

body is surrounded; but by a patient examination of the subject, he is now convinced that the efficacy of radiation is much greater than he had supposed it to be. Only a very small part of the heat which a hot body appears to lose, when it is cooled in the air, is, in fact, communicated to that fluid; a much greater portion of it being communicated to other surrounding bodies at a distance.

Count Rumford supposes that cold as well as hot bodies emit rays, which he denominates frigorific and caloric; and that the intensity of the rays which hot and cold bodies emit, in a medium perfectly transparent, follows the same law. He also informs us, that there are so many striking analogies between the rays of light, and those invisible rays which all bodies at all temperatures appear to emit, that there can hardly be a doubt of their motions being regulated by the same principles.

Perhaps there may be no other difference between them than exists between those vibrations in the air which are audible, and those which make no sensible impression on our organs of hearing. If the ear were so constructed that we could hear all the motions which take place in the air, we should be stunned with the noise; and if our eyes were so constructed as to see all the rays which are emitted continually by day and by night, by the bodies which surround us, we should be dazzled and confounded by that insupportable flood of light poured in upon us on every side.

In all cases where it is designed to preserve the heat of any substance which is confined in a metallic vessel, it will contribute to that end, if the external surface of the vessel be clean and bright. But if the object be to cool any thing quickly, in a metallic vessel, its external surface should be painted or covered with substances which have been found to emit calorific rays in great abundance.

Polished tea-urns may be kept boiling with a much less expence of spirit of wine than such as are varnished; and the cleaner and brighter the dishes, and covers for dishes, which are used for bringing victuals to table, and for keeping it hot, the more effectually will they answer that purpose.

Saucepans and other kitchen utensils, which are very clean and bright on the outside, may be kept hot with a smaller fire, than such as are black and dirty; but the bottom of a saucepan or boiler should be blackened, in order that its contents may be made to boil quickly, and with a small expence

of fuel.

When kitchen utensils are used over a fire of sea-coal or

of wood, there will be no necessity for blackening their bottoms, for they will soon be made black by the smoke; but, when they are used over a clear fire of charcoal, they should be blackened with the smoke of a lamp or coal-fire.

It has been thought that brewers' flats would answer the purpose of cooling liquors better, if made of metal, than of wood: but a metallic surface is ill calculated for expediting the emission of calorific rays. The thickness of the timber of which these tubs are commonly made, is favourable to a speedy cooling of the wort; for, when they are empty and cold, a great part of the heat of the liquor is absorbed by the wood.

Where metallic tubes, filled with steam, are used for warming rooms, the external surface of them should be painted, or covered with some substance which facilitates the emission of

calorific rays. A covering of thin paper will answer the purpose very well, if it be black, and closely attached to the surface of the metal with glue.

Tubes designed for conveying hot steam from one place to another should be covered up with a warm covering, or should be kept clean and bright. It might be worth while to gild them, or to cover them with gilt paper or tin-foil, or some other metallic substance which does not easily tarnish with the air. The cylinders and principal steam-tubes of steam-engines might be covered, first with some warm clothing and then with sheet-brass, kept clean and bright. The expence of this covering would be repaid by a saving of fuel.

If garden walls painted black acquire heat faster when exposed to the sun's direct rays, they will likewise cool faster during the night, and gardeners must best determine whether these changes of temperature are or are not favourable to fruit trees.

Black clothes are known to be warm in the sun; but they are far from being so in the shade, especially in cold weather. No coloured clothing is so cold as black, when the temperature of the air is below that of the surface of the skin, and when the body is not exposed to the action of calorific rays from other substances.

The warmth of clothing depends much on the polish of the surface of the substance of which it is made; hence, in choosing winter garments, those dyes are to be avoided which tend most to destroy that polish; and as a white surface reflects more light than an equal surface equally polished, of any other colour, there is reason to think, that white garments are warmer than any other in cold weather. They are uni