tions of the moon are quicker at present than they were in the time of the ancient Chaldeans, who have left us some observations of eclipses. But as we measure the length of years by the number of days, and parts of a day, which are contained in each of them, it is a new question, whether the days, or the revolutions of the earth round its axis, have always been of the same length.

At present we measure the duration of a day by the number of oscillations which a pendulum of a given length makes in this space of time. But even though the ancients had actually made such experiments, we could draw no inferences from them, without supposing, that gravity, on which the time of an oscillation depends, has always been of the same force; but who will ever be in a condition to prove this invariability in gravity? Thus, even supposing that the days had suffered considerable changes, and that gravity had been altered suitably to them, so that the same pendulum had always completed the same number of vibrations in a day, it would, nevertheless, be still impossible for us to perceive this inequality were it ever so great. And yet I have some reasons, deduced from Jupiter's action on the earth, to think, that the earth's revolution round its axis continually becomes more and more rapid.

New Discoveries relating to the History of Coral. By Dr. VITALIANO DONATI.-[1751.]

CORAL is a marine vegetation, in shape nearly resembling a shrub stripped of its leaves. It has no roots, but is supported on a broad foot, or basis, which adapts itself like wax, and sticks to any body in all its parts, so firmly, that it is impossible to disengage it. The shape of this foot is not always the same; but it mostly approaches to rotundity. Its use is to hold the coral fixed, and support it; not to nourish it since there are found pieces of coral, with their feet broken off, which nevertheless continue to live, to grow, and to propagate, at the bottom of the sea. From this foot arises a trunk, generally single, the greatest thickness of which seldom exceeds an inch.

The substance of the coral gives way to the cellule by small cavities: yet these are not very visible in the old thick branches, but they are pretty easily seen in the young and slender. Thus the cellule does not end at the coralline

substance; since the white pellicle is between it and the said substance. The hollow of the cellule grows narrow into a sort of cone, with an obtuse apex; the belly of which is greater in diameter than the basis. The bottom of such a cellule faces the foot of the coral, and its mouth the branchy or most distant part from the foot. In this cellule is lodged the polypus, which is visible to the naked eye, but its exact shape is only to be seen by the microscope; and it was by this means, that a drawing has been made of it.

From each cellule a white, soft, and somewhat transparent polypus comes forth, or extends itself; which in shape resembles a star with eight equal rays, nearly conical, and furnished with other conical appendices, a a, which issue out of it on both sides. The two rows of these have their direction nearly on the same plane. The rays are somewhat flattened, and a trough rises out of their centre, somewhat widened at its beginning, with an opening or great mouth at top, n. In its sides there are eight upright ridges, broad and elevated, and as many wrinkles or fur-rows; and each ray is inserted between every two wrinkles, a, a. This trough is placed on a smooth part, g, which we may call the belly of the animal.

It is in this position that the polypus is seen the moment the coral is drawn out of

the sea. The polypus, in this contracted state, seen without a microscope, resembles a drop of milk; and this is what all the good

coral-fishers take for the real milk of the coral; the rather, because, by pressing the bark of the coral with the fingers, the polypus is forced out, and in coming forth it always retains the appearance of

milk.

While the first cellule is shut up, or the egg of the coral is in its substance, we do not find any one hard part in it like bone or marble; it is all soft: but afterwards, when the cellule opens, we begin to observe some hard lamellæ ; and when it is grown larger, and arrived at the height of about a

line and a half, it widens at bottom and at the top, and grows narrower in the middle, assuming the proper consistence and hardness of coral. And as

this grows, the polypi are multiplied, and new branches of coral are formed. Here then we see the vegetation of a plant, and the propagation of an animal.

The madrepora is entirely like the coral, as to its hardness, which is equal to bone or marble. Its colour is white, when polished. Its surface is lightly wrinkled, and the wrinkles

run lengthwise of the branches. Its all inside is of a particular organisation;

having in the centre a sort of cylinder, which is often pierced through its whole length by two or three holes. From this cylinder are detached about 17

laminæ, which run to the circumference in straight lines.

In every one of the cellules is found a polypus here represented, but considerably magnified; the mechanism of which is this: three different parts, unlike each other, compose this animal; viz. the feet, a trough, and a head.

On the Class of the Phoca Marina. By JAMES PARSONS, M.D. F.R.S.-[1751.]

ALL the species of phocæ, this being the generical name, have among them a very great likeness to each other, in the shape, not only of their heads, but also of their bodies and extremities. They are webbed nearly alike, are alike reptile,

viviparous, bringing forth, suckling, and supporting their young alike; and, in fine, all have the same title to these appellations, phocæ, vitulus marinus, sea-cow, sea-lion, &c.; and these names are vulgarly given to them, as their size happens to be greater or smaller.

The different species of this class, or rather genus, of animals, are distinguishable, by their proportion, their size, as to their full growth, their teeth, webbed feet, and whatever other parts in some may not be proper to others.

Their size, as to the utmost growth of an adult, is every different. I have seen one which was 7 feet in length; and, being very young, had scarcely any teeth at all. One now in London is about three feet long, is very thick in proportion, and has a well-grown set of teeth; which, in a great measure, shows this to be about its full growth. The manati is also a phoca, and is one of those species which grows to a prodigious size.

A governor, in the province of Nicaragua, had a young manati, which was brought to him, to be put into the lake Guanaibo, which was near his house; where he was kept during 26 years, and was usually fed with bread, and such-like fragments of victuals as people often feed fish with in a fish-pond. He became so familiar, by being daily visited and fed by the family, that he was said to excel even the dolphins, so much celebrated by the ancients, for their docility and tameness. The domestics of this governor named him Matto; and at whatever time of the day they called him by that name, he came out of the lake, took victuals out of their hands, crawled up to the house to feed, and played with the servants and children; and sometimes 10 persons together would mount upon his back, whom he carried with great ease and safety across the lake.

All that is here mentioned of the docility of the manati does not much surpass that of a seal now in London. He answers to the call of his keeper, and is observant of his commands; takes meat from his hand, crawls out of the water, and stretches at full length, when he is bid ; and when ordered, returns into the water; and, in short, stretches out his neck to kiss his keeper, as often and as long as required. These are marks of a tractableness which one could hardly expect from animals whose mien and aspect promise little, and indeed whose place of abode, being for the most part inaccessible, prevent their being familiarised to any commerce with men, except by mere chance.

The walrus, or mors, is another species of phoca, and differs

very little in shape and parts from the other species of this genus; except that the two canine teeth of the upper jaw are of a prodigious size, like the great teeth of an elephant.

Letter, dated May 2. 1750, from Mr. FREEMAN at Naples, relating to the Ruins of Herculaneum. — [1751.]

ABOUT seven or eight years ago, the discovery of Herculaneum was much spoken of, which was reported to have been swallowed up by a violent eruption of Mount Vesuvius, according to the last accounts, in the first year of the reign of Titus, 79 years after Christ.

You are first conducted down a narrow passage, scarcely wide enough for two persons to pass; and in a gradual slope, to the depth of about 65 feet perpendicular. Here is shown a great part of the ancient theatre, a building in the form of a horse-shoe. That part where the spectators sat is visible, and consists of 18 rows of broad stone seats, one above another, in a semicircular form. At proper distances within the circuit of the seats, through the whole range, from bottom to top, are little narrow flights of steps, by which the spectators might come to or go from their seats commodiously, without crowding. These steps or stairs also lead up, in a straight line, to a sort of gallery, several feet wide, which ranges all round the outside of the theatre, and is called the precinct; above which there are other stairs, which lead to a second. By this precinct it is judged that the theatre, with the orchestra, must be about 52 or 53 feet diameter.

There is another opening, distant from that which leads to the theatre, by which they have made a way into some houses. Here they seem to have dug infinitely more than about the theatre; for one may ramble, as in a labyrinth, for at least half a mile. Among the things that have been dug out of either of the two places are many parts of broken horses, with part of a triumphal car or chariot, all of gilt bronze; and which, they say, were placed over one of the gates of the theatre.

Two equestrian statues were found on each side of one of the said gates, and they suppose fronting a street that led to the theatre. One of these statues cannot be repaired; the other, which happened to be better preserved, is well repaired, and is set up under the piazza in the gateway of the king's palace at Portici.

Of busts, there are some very beautiful, as that of Jupiter Ammon, Neptune, Mercury, Juno, Ceres, Pallas, &c. In the apartments of the palace is a vast number of little statues,