obvious resemblances to various other kinds of marine productions, as well as from the chemical analysis of sponges in general, (see MULLER in Zool. Dan. T. 85,) we are amply justified in referring them to the class of animal productions. Dr. Grant, Professor of Zoology in the University of London, has even succeeded in discovering the eggs of sponges.

..

DECEMBER.

Curious Fact in the Economy of Bees.-M. de Jonas de Gelieu, pastor of the churches of Colombier and Auvernier in the principality of Neufchatel, Switzerland, in a work translated into English, under the title of The Bee Preserver, or Practical Directions for Preserving and Renewing Hives," affirms a very important and singular fact with regard to the economy of bees. It is, that when two or three distinct hives are united in autumn, they are found to consume together scarcely more honey during the winter than each of them would have consumed singly if left separate. In -proof of this remarkable result, the author states a variety of experiments to which he had recourse, and all of which led uniformly to the same conclusion. And, indeed, he shews positively, by a reference to upwards of thirty hives, six of which had their population thus doubled, that the latter do not consume more provisions during winter than a single hive does, and that, so far from the bees suffering from this, the doubled hives generally send forth the earliest and best swarms.

It is stated that the translator of M. Gelieu's work has practised in Scotland most of the plans recommended in the original publication, with the same results as the author.

Luminous Sea-weed.-Captain Home, R.N., in a paper addressed to the "Quarterly Journal of Science," communicates the discovery that the cause of the brilliant light observed in the sea-weed thrown on the beach at Lancing, on the coast of Sussex, is the animalcule Sertularia volubilis of Ellis, described by him in his Corallines and Zoophytes (the Clytia volubilis of Lamouroux), but not mentioned to be luminous. On the 8th of December, and three following days, a great quantity of weed had been thrown up by a hard-blowing south-west wind, so that the beach was covered with it to more than two feet deep in many places. After dark a small quantity was collected of the most brilliant, and this was always found to be that which had been left at the first of the ebb, and was only moist, rather than what had been just washed up. Picking out a single spark, and removing from it every extraneous matter, Captain Home ascertained, by the aid of a microscope, that the light was caused by the insects adhering to the sea-weed. The light would remain sometimes steady for about five seconds, often less, and when it ceased, was renewed by touching it with the finger. In a darkened room, by day, no light whatever was emitted; yet the same weed, kept till the evening, was as brilliant as any that had been found.

THIS Table is computed upon principles explained by Dr. Young in the Philosophical Transactions for 1819; and it appears to agree more perfectly with the latest observations than any other table before published.

The apparent altitude being found in the first column, the second shews the refraction when the barometer stands at 30 inches, which is its mean height on the level of the sea, and the thermometer at 50° of Fahrenheit. The third column contains the difference to be subtracted or added for every minute of altitude, reckoned from the nearest number in the first colunu.-Nautical Almangc.

IX. THE TIDES.

THE Ocean is continually agitated by winds and tempests which disturb its equilibrium; but superficial observation is sufficient to discover that these causes are inadequate to produce all the phenomena which its motion presents. It rises and falls alternately; its depth is observed to be greatest at any given place a certain time after the moon has passed the meridian of that place, after which it decreases until it reaches a certain point, when it again gradually rises. These phenomena recur after nearly the same intervals of time, and are called the tides.

The interval of time between low water and the following high water is called flood tide, and that between high water and the following low water is called ebb tide. If the interval of time between two successive passages of the moon over the same semimeridian be called a lunar day, there are generally, or on an average, two high tides in one lunar day.

The time of the moon's synodic revolution, or period of her mean conjunctions, is 29.530588716 days.-See the Exposition du Système du Monde, p. 23. Hence the mean interval between successive high tides is 12 h. 24 m. 22 s.

The interval, however, which elapses between the moon's southing and the time of high water is subject to great irregularities, which depend principally upon her angular distance from the sun.-See the Table, p. 66. These variations depend also upon the distances of both luminaries from the earth, and upon their declinations. Other circumstances being alike, the variations of this interval from the mean interval are greatest when the sun's distance from the earth is greatest, and the moon's distance is least; and when the moon's declination is greatest, and the sun's declination is least: they are greater therefore in the equinoxes than in the solstices. The heights of the tide at high water also vary; they are greatest soon after the moon is in syzygy, and least soon after she is in quadrature; the one are called spring, the other neap tides. These heights depend also upon the declinations and distances of the luminaries. The time of high water at any port when the moon is in syzygy (new or full), is called by French writers l'établissement du port.

The evident connexion between the period of the tides and that of the phases of the moon led philosophers to attribute these phenomena to her action long before their true theory was understood. There is a very remarkable passage in Pliny, lib. ii. c. 97, which is quoted by Lalande in the 4th vol. of his Astronomy, p. 8. It begins thus, "Estus maris accedere et reciprocare maxime mirum est, pluribus quidem modis accidit, verum causa in sole lunaque." Pliny proceeds to describe very accurately the principal phenomena of the tides. Kepler says, in his Epist. Ast. p. 555,"quemadmodum igitur ut magnes magnetem aut ferrum trahat cognatio corporum efficit, sic etiam de luna non est incredibile ut illa moveatur a terra cognato corpore,

Ꭰ

licet nec hic nec illic intercedat aliquis contactus corporum. Adeoque quid mirum lunam a terrâ moveri, cum videamus vicissim et lunam transitu suo super vertices locorum causare fluxum Oceani reciprocum in tellure? Nonne satis evidens hoc est documentum communicationis motuum inter hæc duo corpora?" The principle of gravitation is here clearly indicated, but the law is wanting. There is also a passage in his work De Stella Martis, relating to the same phenomena, which is written in the peculiar style of Kepler's fancy. "Dissolvitur discessu lunæ concilium aquarum seu exercitus qui est in itinere versus torridam, &c." Galileo, in his Dialogues upon the System of the World, expresses his regret that so ingenious a philosopher as Kepler should have given such an explanation. He, on the contrary, attributed them to the rotation of the earth combined with its revolution about the sun. Wallis, in 1666, in letters to Mr. Boyle, derives the phenomena of the tides from the consideration that it is the common centre of gravity of the moon and the earth which describes an orbit about the sun, while they revolve about this common centre. These letters are published in his works, and in the Phil. Trans. vol. i. p. 263. Wallis appears to have conjectured the principle of gravitation. In answer to an objection which was made to him in the form of a query, how two bodies which have no tie can have one common centre of gravity? Wallis says, "It is harder to shew how they have, than that they have it-that the loadstone and iron have somewhat equivalent to a tie, though we see it not, yet by the effects we know. And it would be easy to show that two loadstones at once applied in different positions to the same needle at a convenient distance, will draw it not to a point directly to either of them, but to some point between both. As to the present case, how the earth and moon are connected, I will not undertake to show, nor is it necessary to my purpose, but that there is somewhat that does connect them, as much as what connects the loadstone and the iron which it draws, is past doubt to those who allow them to be carried about the sun, as one aggregate or body, whose parts keep a respective position to one another, like as Jupiter with his four satellites, and Saturn with his one. Some tie there is that makes those satellites attend their lords and move in a body, though we do not 'see that tie nor hear the words of command.'

66

In the Philosophical Transactions, vol. xiii. p. 10, Flamsteed gives a correct tide-table, shewing the true times of the high waters at London Bridge to every day in the year 1683." It appears from the remarks of Flamsteed, that the earliest tide-tables which had been calculated for the port of London, had been made on the supposition that the tide always followed three hours after the time of the moon's southing. Mr. Philips, a writer on navigation, appears to have been the first who introduced an empirical correction, varying as the sine of twice the moon's angular distance from the sun, by which the tables were made to agree better with observation. Flamsteed's attention was turned to the subject, from

his having frequently occasion to go to Greenwich by water. Mr. Philips had placed the greatest and least differences between the moon's true southing and the high waters at the full or new and quarter moons, the error of which Flamsteed detected. Flamsteed was the first who gave tide tables, in which the time of high water is calculated, corresponding to the moon's second passage of the same meridian. A more detailed notice of the authors who endeavoured to explain the phenomena of the tides may be found in the 4th vol. of Lalande's Astronomy, in the Encyclopædia Brit., Art. Tides; or in the life of Galileo, which forms part of the Library of Useful Knowledge.

Before Sir Isaac Newton, these explanations were, at best, but vague surmises; to him was reserved the glory of discovering the true theory of these most remarkable phenomena, and of tracing in all its details the operation of the cause which produces them. This theory is contained in the Philosophic Naturalis Principia Mathematica, Prop. 66. Lib. i. Cor. 19 and 20, and Lib. iii. Prop. 24, 36 and 37, not 26 and 27, as is said in the Méc. Cél. vol. v. p. 146. In Prop. 66. Lib. i. Newton shews that in a system of three bodies S, P, T, the motion of P about T, relative to T, is disturbed by the difference of the attractive forces of S upon P and T, and he infers that if a sphere be covered by a fluid, and this system be attracted by a body (S) the equilibrium of the fluid will be disturbed by the difference of the attractive forces of S upon the centre of the sphere, and upon each particle of which the fluid is composed. . So that if S revolve about the sphere, the fluid will be constantly in motion relatively to the centre of the sphere.

In the 24th Prop. Newton describes accurately the different phenomena of the tides; but he assumes that the ocean takes the form of a spheroid, in consequence of the attraction of the luminary, which required a proof.

In the 36th Prop. he finds the force which the luminary exerts upon a particle of the ocean; and in the 37th Prop. he determines, from observations made by Sturm, at the mouth of the river Avon, near Bristol, of the heights of the tide when the moon was in conjunction and opposition, the ratio of the force of the moon to

'm'

that of the sun (mm) to be 44815: 1, not 6¡ : 1, ļas La

place has it; and from this determination he deduced the mass of the earth to be to that of the moon as 390788 : 1, the density of the sun to be to the density of the earth as 1:4, and the density of the moon to that of the earth as 11 : 9.

About the beginning of the eighteenth century many observations of the tides were communicated to the Academy of Sciences at Paris from different parts of France. See Mém. de l'Acad. 1710, 1713, &c. In the volume for 1713, Cassini discusses at great length the observations which had been sent to him from Brest; but although they agree with the theory of Newton in their minutest details, he never once mentions the name of that great man, but contents himself with giving empirical tables of