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and as it follows from this view of the subject that actual impregnation takes place in the ovarium, many hypotheses have been formed to account for the manner in which the semen can pass along the Fallopian tube. The present writer, on the contrary, supposes that the corpus luteum is a compact glandular substance in which the ovum is formed, and that, from certain causes, it may pass into the uterus, where it is impregnated.

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The small ovum was given to Mr. Bauer, of Kew, in order that he might examine it by his microscope, and we are presented with a very minute account of its appearance. It is described as consisting of a membrane, comparatively speaking of considerable thickness and consistence, forming a kind of bag of an oval form, nearly of an inch long, and about of an inch broad: on one side it has an elevated ridge down its longest diameter, and on the other side it appears open for nearly its whole length, the edges of the membrane being rolled inwards, so as to give it something of the shape of a shell of the genus voluta. The outer bag contained an interior smaller bag, one end of which was nearly pointed, the other obtuse; in the middle it was slightly contracted, so as to leave two protuberances, which, it is conjectured, were the rudiments of the heart and head. These protuberances were formed by two little corpuscles, which were contained in the interior bag, and were enveloped in a slimy substance like honey. The paper is accompanied by some characteristic engravings from Mr. Bauer's drawings.

The object of Mr. Knight's paper on the expansion and contraction of timber is to show that this effect is principally produced by means of what is called the silver grain of the wood, a series of cellular processes, which are extended in the form of radii, from the central medulla of the tree to the bark. In a paper which was inserted in the Philosophical Transactions for the year 1801, he endeavoured to prove that the motion of the sap depends upon the action of these processes, as they are affected by the different degrees of heat and moisture to which they are exposed; and he has been confirmed in this opinion by many experiments and observations, which he has had an opportunity of making since that period.

The first set of experiments which he relates consisted in taking thin boards of oak and ash, which were cut from the tree in different directions with respect to the silver grain, "so that the convergent cellular processes crossed the centre of the surfaces of some of them at right angles, and lay parallel with the surfaces of others." When both these pieces of wood were placed under similar circumstances, those which had been formed by cutting across the convergent cellular processes soon changed their form very considerably, the one side becoming hollow, and the other raised; and in drying, these contracted nearly 14 per cent. relatively to their breadth, The others retained,

with very little variation, their primary form, and did not contract more than 34 per cent. in drying."

Mr. Knight's second experiment consisted in taking a transverse section, of about an inch in thickness, from the stem of a tree that was just felled. An incision was then made with a saw from the bark towards the central medulla, in the direction of the convergent cellular processes, when they were found almost entirely to prevent the action of the saw in consequence of their expansion; and when a second incision was made from the bark to the medulla, about an inch from the first, leaving a triangular wedge, the expansion of the silver grain kept the piece closely retained in the stem. When incisions were made in the other part of the block, but in such a direction as to cut the processes across, the saw was found to move with perfect freedom. From these facts the author was led to infer that the medullary canal must be subject to have its diameter considerably affected by variations in the quantity of moisture contained in the wood; and this conjecture seemed to be confirmed by an experiment, in which a plug of metal forced into the central space, which had been occupied by the medulla of a young stem, while this was in a dry state, was found too small to fill the cavity, when the stem was saturated with moisture. Mr. Knight conceives that the internal clefts which are frequently met with in timber may be produced by this kind of expansion and contraction; a cause which he conceives more likely to operate than either winds or frosts, to which they have generally been attributed. Another cause by which timber becomes warped in drying is pointed out, which has probably no connexion with the power by which the sap is raised in the living tree, but which arises from the greater or less solidity of the different parts of the trunk, according as they are nearer or more remote from the centre, the former being more compact, and of greater specific gravity, and therefore being less affected by the evaporation of its moisture.

Dr. Davy's observations, which were made during his voyage to Ceylon, were principally confined to three topics: "the specific gravity of the water of the ocean, and its temperature, and the temperature of the atmosphere." He first presents his principal results in the form of a table, and he afterwards informs us how they were obtained, and offers various remarks concerning them. The table consists of 13 columns: the first contains the date; the second, the latitude by observation; the third, the longitude by the chronometer; the fourth is the specific gravity of the sea water; the three next columns relate to the temperature of the air, its maximum in the course of the 24 hours, its minimum, and its mean; the next three columns give us the maximum, the minimum, and the mean temperature of the sea water; the 11th column contains the register of the

barometer; the 12th, of the winds; and the last, the account of the weather generally. The observations were continued, without much interruption, from the middle of February to the middle of August, when the author arrived at Ceylon, commencing in the 49th degree of north latitude, and 6 degrees west longitude, and proceeding round by the Cape of Good Hope and the Isle of France. The experiments on the density of the sea water were made on portions of water drawn from the surface of the ocean, its temperature being reduced by calculation to 80°, a number which was fixed upon because it is nearly the mean annual température of Ceylon, and of the sea generally in the intertropical regions. The results of these experiments show that the ocean resembles the atmosphere with respect to the general uniformity of its composition, the specific gravity of the water being very nearly the same in all the different trials. The number of observations recorded is 36; the highest specific gravity is 10277, and the lowest 12051. These variations seem to have no connexion with the temperature, or at least not to bear any regular proportion to it. The differences seemed rather to depend upon what may be regarded as incidental circumstances, as the roughness of the surface, a heavy fall of rain, and a succession of tropical squalls. Dr. Davy's observations controvert an opinion which has been adopted, that the different zones of the sea have each their peculiar specific gravity.

With respect to the temperature of the air and water, the observations were made every two hours with delicate thermometers. Dr. Davy conceives that the temperature of the atmosphere in hot climates has been frequently overrated from the thermometer not being sufficiently protected from the radiation of caloric by neighbouring bodies. The highest temperature that is noted is 82°; this occurred at 2° 10′ north latitude, about five days before they arrived at Ceylon; the uniformity of the temperature in these regions is very remarkable, the maximum and minimum not differing more than 3° or 4° in ordinary cases, and seldom more than 5° or 6°. The author has made an observation on the diurnal variation of the temperature of the atmosphere at sea which had not been before noticed, which, when the weather is fine, and the wind steady, appears to have few exceptions: the air was "at its maximum temperature precisely at noon, and at its minimum towards sun-rise." But many circumstances were found to disturb the regular progression: on the one hand, in a perfect calm the accumulation of heat, not only in the ship, but in the water itself, cause the greatest heat to occur some time after the hour of noon, and by showery and unsettled weather the regular variation was still more disturbed. Contrary to what is commonly asserted, the diurnal change of temperature in the sea is very nearly as great as in the atmosphere. When there were the fewest disturbing causes, the weather fine, the surface smooth, and no land near, the

maximum temperature of the sea is about three, p.m. and the minimum towards sun-rise. One of the most considerable of the causes that disturb the temperature of the ocean is currents, and these currents affect it in different ways, according to the cause which produces them. Superficial currents often depend uon prevailing winds, and these currents are then warmer or colder than the other parts of the sea, according to the quarter from which the wind blows. Currents often depend upon inequalities in the bottom of the ocean; and it is now admitted as a fact, established by many observations, that when the sea is shallow its temperature its diminished. The author gives us the result of his observations on the currents which he encountered during his voyage, which all accord with the general principles stated above. He gives us the particular account of the effect produced by the well-known current flowing from the south-east coast of Africa. In crossing this current the temperature of the ocean suddenly varied as much as 10 degrees, which is probably occasioned by a sudden transition from the water which lies over the bank of Lagullas, along which the current rapidly flows, into the stream itself.

A certain conjunction of circumstances, connected with the warm streams of water, and cold winds blowing over them, is employed by our author to explain a phenomenon which has been often described by travellers who have visited the Cape, commonly called the "Table-cloth." It consists in a cloud or mist, which covers the upper part of the Table Mountain, but which does not descend to the plain below. The phenomenon only occurs when the south-east wind blows, which is there a cold wind, and, passing over the warm currents in its way to the land, condenses a portion of aqueous vapour, and produces a mist, which is carried along the eastern side of the mountain, and covers the top, but does not descend on the western side, in consequence of the heat of the plain below, but is suspended over it, in the form of a sheet, whence it has derived its name. (To be continued.)

Traité de Physique expérimentale et Mathématique, par J. B. Biot, Membre de l'Académie des Sciences, &c. &c. 4 tom. 8vo.

IN the history of the proceedings of the Royal Academy of Sciences, which appeared in our last volume, some account was given of this treatise; but as it is a work which, both from its own merits, and from the celebrity of the author, must excite considerable interest with our readers, we conceive that a somewhat more detailed analysis of it will not be unacceptable to them. It will indeed be impossible, in the limits of a few pages, to enter into an examination of the manner in which M. Biot treats the various topics that pass under his review, or to pronounce upon the merits of the reasoning which he employs in

the discussion of the controverted questions which necessarily forms a considerable proportion of all publications of this description. But to those who are not in possession of the work it may be important to know what are the subjects on which M. Biot treats, although we may be able to give little more than a mere table of contents.

This work contains about 2300 closely-printed octavo pages in a small type It abounds much in theoretical discussions, and in refined speculations; and, on all occasions where they could be introduced, the author employs mathematical reasoning and algebraical notation. Some extracts have already been given from the dedicatory epistle to Berthollet, in which he enters into a formal defence of the propriety of this method of proceeding. He admits that it is useless to employ an algebraical notation to express results which are so simple that they may be announced, comprehended, and appreciated, in simple and direct terms. It is still worse, or rather positively objectionable, to combine in this way parts or elements which are in themselves vague or hypothetical; for by doing so, "we only realize uncertainty, and give a body to error." But when we have observed with sufficient precision the different modes of the same phenomena, and have obtained correct numerical expressions of them, what inconvenience, he asks, can there be in uniting them by a formula, which may embrace the whole? When they are capable of being reduced to a simple law, but when this cannot be immediately perceived, is not this the best method of discovering it? Whereas, on the contrary, if the nature of their relations be essentially compound, which is commonly the case, is it not the only means which we possess to connect them into one whole, "and to obtain a common expression, which may be afterwards introduced, with all the generality of its indeterminateness, into the analysis of other phenomena, in which the first may bear a part?" To these remarks every one must assent; but they do not decide the point respecting the propriety of M. Biot's method, because it is a question of degree, rather than one of an absolute nature. No one will deny the propriety of introducing mathematics into all the departments of natural philosophy: in some they form the necessary foundation of the whole superstructure; and in all of them there are parts which, by this means, can be placed in a clearer light, and have their relations better illustrated, than by any other mode of expression. We must, however, confess that we are among the number of those who think that the sparing and cautious introduction of mathematical expressions into general physics is favourable to the progress of knowledge; for, although we may gain something on the score of accuracy and conciseness, we place science out of the reach of many who might profit by it, and might in their turn contribute to its ad

vancement.

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