ART. I.-Philosophical Transactions of the Royal Society of London, for the Year 1808. Part 11.

XII. Observations of a Comet, made with a View to investigate its Magnitude, and the Nature of its Illumination. To which is added,an Account of a new Irregularity lately perceived in the apparent Figure of the Planet Saturn. By William Herschel, LL.D. F.R.S.-This memoir is introduced by a journal of Dr. Herschel's observations on the comet which appeared in the latter end of 1807, and the beginning of 1808. The observations were made on the nucleus of the comet; on the head, on the coma, on the tail, and on the nebulous appearance of the comet. They are well calculated to give more distinct ideas of the structure of these astonishing bodies, and to correct some erroneous assertions of former astronomers. The observations are followed by a succinct and well digested account of their results.

The nucleus of this comet is a small well-defined round point; this is surrounded with very bright rays; the whole seen through an ordinary telescope seems a single body about the size of the planet Jupiter. The nucleus, with its surrounding rays is the head of the comet. The coma is the nebulous appearance surrounding the head. The nucleus of this comet appeared in the shape of a disk which was experimentally found to be a real one. Its apparent diameter was not (on the 19th of October) quite so large as that of the third satellite of Jupiter. At the time of this observation it was about 1. The distance of the comet from the earth at the time of the observation was 1.69192 the mean distance of the earth being 1.

Now, since the disk of the, comet was observed to subtend an angle of 1", which brought to the distance of the earth gives 1,169, and since we also know, that the earth's diameter, which according to Mr. Dalby is 7913.2 miles, subtends at the same, distance an angle of 17,2, we deduce from these principles the real diameter of the comet, which is 528 miles."

CRIT. REV. Vol. 16. February, 1809.

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Dr. Herschel next examines the question, whether the light of the comet be from reflection from the sun, or it be self-luminous. On the 4th of October and the 19th by a calculation of its phases, the illumination was found to be 119° 45'9" and 124° 22′ 40′′, respectively. Both these phases appear to Dr. Herschel sufficiently defalcated to prove that the comet did not shine by reflected light alone. The disk at the time of these observations appeared perfectly circular, which the doctor thinks could not have happened under so great a deviation from the circular form as was presented at this time. The vivacity of the light had a much greater resemblance to the radiance of the stars than to the mild reflection of the sun's beams from the moon. This is an additional support of the inference from the calculation of its phases.

It has been said by many authors that the tails of comets are of so rare a texture as not to affect the light of the smallest stars that are seen through them. But Dr. Herschel's observations are in direct contradiction to this assertion. When small stars were immerged in the coma or in the tail of the comet, they suffered a very sensible diminution of their brightness, so that they are sufficiently dense to obstruct the free passage of star-light.

The brightness,' he observes, of the head, coma, and tail alone, will sufficiently account for the observed changes, if we admit that they shine not by reflection, but by their own radiance; for a faint object projected on a bright ground, or seen through it, will certainly appear somewhat fainter, although its rays should meet with no obstruction, in coming to the eye. Now, as in this case we are sure of the bright interposition-of the parts of the comet, but have no knowledge of floating particles, we ought certainly not to ascribe an effect to an hypothetical cause, when the existence of one, quite sufficient to explain the phenomena, is evident.

Dr. Herschel applies the same reasoning to the head, coma, and tail. The head and coma were very bright, and the remains of the tail were still visible, when the distance of the comet from the earth was 240.000.000 miles; a distance, the doctor conceives, too great to admit the hypothe sis of its light being reflected.

The magnitude of the tails of some comets makes it more probable that their light is caused by radiation, like the aurora borealis, than by reflection. The tail of the late comet was expanded over a space of more than nine millions of miles.

As the distance of the comet increased, it put on the ap

pearance of a nebula. It is possible then that some comets have been actually seen under a nebulous form.

The concluding part of this paper is unconnected with the

first. In a former number of the transactions Dr. Herschel had noticed a flattening of the polar regions of the planet Saturn, and also an apparent small flattening of the equatorial parts. In attempting to continue these observations, a new irregularity of the Saturnian figure was observed, which the doctor was assured had no existence at the time of the former observations. The northeru polar regions were observed to be flattened (as before); but the southern to be more curved or bulged outwards. This observation was confirmed by the doctor's son, John Herschel, and by Dr. Wilson without any communication from the doctor. Its reality then cannot be doubted. But he conceives the appearance not to be owing to a real irregularity in the figure of the planet, but to be the effect of an optical illusion. The situation of the ring was such, that it passed before the planet towards the south pole, and, consequently, behind it at the north. If then the ring has an atmosphere, the rays proceeding from one of the poles will pass through it, but those from the other traverse a space clear of every' object which might divert their course. The doctor apprehends, then, the appearance to be caused by refraction through the atmosphere of the ring. This phenomenon gives additional support to the hypothesis that the ring of Saturn is possessed of an atmosphere.

XIII. Hydraulic Investigations, subservient to an intended Croonian Lecture on the Motion of the Blood. By Thomas Young, M.D. For. Sec. R. S

-XIV. A Letter on the Alterations that have taken place in the Structure of Rocks, on the Surface of the basaltic Country in the Counties of Derry and Antrim. Addressed to Humphrey Davy, Esq. Sec. R.S. By William Richardson, D.D.

The celebrated giant's causeway has been often described. In this interesting paper Dr. Richardson has described some of the beautiful and astonishing scenery of the country contiguous; and has stated the conclusions which result from his observations.

The basaltic area comprehends the greater part of Antrim, and the east side of Derry to a considerable extent. It is on the northern side of the area, that the regular basaltic structure is most conspicuous. There are four façades particularly distinguished for their grandeur and beauty. These

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are Magilligan rock, Cave rock, Bangore, and Fairhead. Bangore is in this paper the principal object of the doctor's attention. It is a promontory occupying about four English miles of the coast. The façades of the basaltic columns at the point of their greatest height have their perpendicular part 170 feet, and the precipitous part, from the bottom of the pillars to the sea, 200 feet. The promontory itself and the strata composing it, ascend to the northward, but not at the same angle, the strata being more inclined to the horizon than the surface of the promontory. Its western side is cut down perpendicularly by eleven chasms called Whyn Dykes, reaching from the top of the precipice to the water. When the last of these is past, a new and curious circumstance is discovered: viz. that the western half of the promontory has sunk or subsided between thirty and forty feet without the slightest concussion or derangement of the parallelism of the strata. Two other similar depressions appear farther west; but they are far less considerable in thickness than the preceding, neither of them exceeding five feet.

still

Having taken a general survey of the whole line, the doctor proceeds to examine the intimate structure of the façades. He enumerates no less than 16 different strata, commencing with that which forms the summit, and descending gradually to the basis; and very distinctly points out the characteristic differences of each stratum. Some are fifty or sixty feet in thickness, others only five or ten. It is impossible for us to follow him in this enumeration. The following facts are some of the general results of these observa

tions.

Every stratum is nearly of the same thickness through its whole extent, with very few exceptions. Each surface preserves an exact parallelism, except the stratum is superficial, when the upper surface may have been scolloped or sloped away.

Nature,' says the doctor, has never acted upon an extensive scale in our basaltic area, but changes her materials or arrangement or both, every two or three miles, and often at much smaller intervals; and in these cases the change is always made per saltum and never per gradus, the lines of demarcation always distinct and well defined; yet the different materials pass into each other without interrupting the solidity and continuity of the whole

mass.'

The bases of the precipices commonly extend a considerable way into the sea; and between the water and the foot

of the precipice fragments are scattered in the most wild and irregular manner. But all these fragments once formed part of the original mass of coast, stratified like it, and their strata still correspond in material and inclination, with those in the contiguous precipice. The vertical basaltic columns are not confined to the sea coast. Several of them are scattered over a great part of the basaltic area, especially on the ridges of the hills and mountains. The last observation is very important. It is this:

'Whenever the strata are thus suddenly cut off, whether it be a mass of accumulated strata, as in the façades on our coast, or 'solitary strata in the interior; the materials on one side of the abruption are completely carried away, without a fragment being left behind, while on its other side the untouched stratum, remains intire and undisturbed.'

Having laid down the facts, Dr. Richardson proceeds to examine the theories to which the observation of these stupendous phenomena has given rise. It is natural enough to suppose that the perpendicular façades being found on the coast have been broken off by the action of the sea.

But it

is observed, that the sea would rather excavate the basis; whereas, the columns are found on the highest part of the cliff, and receding from the water. The inland columns have precisely the same characters, and are equal in magnificence to those on the coast; as at Bienbraddoch thirteen miles in land, and Monyneeny, which is still farther. Some other theories the doctor briefly examines, and shows satisfactorily how little they are adapted to the solution of the question.

His own hypothesis is shortly that these columns are the remains of the original crust or covering of the earth, and that in some stupendous operation of nature, the parts which were contiguous to them, have been wholly and completely carried away. What was the agent which effected such a great revolution, the doctor, with becoming modesty, does not hazard even a conjecture. His theory is supported by many concurrent circumstances in the present state of the whole basaltic area. The hypothesis is little suited to the pride of man, who thinks his own faculties powerful enough to dive into all the mysteries of nature. We do not know that it is for this reason more remote from the truth.

XVI-A Letter on the Differences in the Structure of Calculi; which arise from their being formed in different