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Every singular phenomenon in the structure of animals ought, to be recorded : for though they may not at first sight furnish any useful improvements in medical practice, or explain any of the unknown parts of the animal economy, yet, from a comparison of them with each other, and from a general view of several facts taken together, considerable light may be thrown on many obscure parts of phyfiology; and perhaps some of the anomalous fymptoms in uncommon diseases might be accounted for in a satisfactory manner.

ASTRO'N OMICAL PAPER. On the Georgian Planet and its Satellites. By William Herschel,

LL. D. F.R. S. In the last volume of the Philosophical Transactions, Dr. Herschel gave an account * of bis having discovered two fatellites, revolving round the Georgian planet. The present me. moir gives not only a detail of his observations on these fatel. lites, but also the deductions which he has made from them, in order to ascertain their orbits.

To determine the orbits of secondary planets, is an astrono. mical problem of no little difficulty ; and in the present cafe, this difficulty is increased, by the want of observations of the eclipses of the satellites; and by the great nicety of making even such observations as the present situation of the satellites afford.

The result of Dr. Herschel's observations, and the calculations which he has made from them, are,

The period of the ist satellite, 84 17. 1' 19". Its distance 33" : and on the 19th October 1787, at 1961 28", its polition was 76° 43' North, following the planet.

The period of the 2d satellite, isd 11 51.5. Its distance 44":23: and on the 19th October 1787, at 171 22'40" was 76° 43' North, following the planet.' The orbit is inclined to the ecliptic 91° 1' 32".2 or 89° 48' 27":5; its ascending node in 18° of Virgo, or 6 of Sagittarius. The situation of the orbit of the first differs not materially from that of the second. There will be eclipses of these fatellites about the year 1799, or 1818, when they will appear to ascend through the fhadow, in a direction almoft perpendicular to the eclipric.

The diameter of the new planet is to the diameter of the earth, as 4.31769:1 ; its bulk, 80.49256:1; its denfity, 0.220401:1; its quantity of matter, 17.740612:1; and heavy bodies fall on its surface 15 feet 3} inches in a second.

From this recapitulation of the contents of this valuable Paper, our astronomical readers will easily perceive that D:

* See Rev. yol, lxxvii. p. 179.

Herschel's

Herschel's calculations have been intricate and laborious. It were to be wilhed, that tables of the planet's motions were confructed from Dr. Herschel's own observations of its places. Those given in the Connoissance des Temps of 1787 mult, necessarily, not be so correct as others that might now be formed, fince the planet has been longer observed, and more frequent opportunities have occurred for asceraining the times and places of its oppositions and stations. The oppositions indeed seem at present the moft eligible observations for determining the planet's orbit ; few of the astronomers of the present day have a chance of seeing it in the node, and human life will not suffice for the same observer to see it owice in the same place ; for which reason it is a duty incumbent on our present observers to record their observations.

NATURAL HISTORY. Observations on the Natural History of the Cuckow. By Mr. Ed

ward Jenner. The fingularity of the cuckow has engaged the attention of several naturalifts; who have made these general conclusions, viz. that the cuckow is a bird of passage, that it does not build its own neft, nor hatch irs young,-ihat it depofits its eggs in the nests of other birds, who become the folter parents of the young cuckows. Mr. Jenner relates various facts which he hath observed, respecting the time of the cuckow's coming into England, the manner of its living, the nests which it chufes for depoficing its eggs, the size of the egg, the time of incubation, the manners of the young one, how it is fed, and the time of its continuance with us.

These facts are all particularly related with great precision ; and are the result of a long and careful observation of the bird. We mutt however except the first and the last circumstance ; which are difficult to be ascertained, even admitting the hypothesis of migration. The notion has been generally adopted, without, perhaps, fufficiently attending to nature. Batts and swallows, we believe, do not migrate, though they remain invisible during many months of the year. The disappearance therefore of the cuckow cannot alone be admitted as a proof of its migration; and other concomitant circumstances must neceso farily be adduced in its confirmation. Have the flights of the cuckow, either in coming or going, been goticed? What countries does the cuckow frequent when invisible in England ? Satisfactory answers to one or both of these questions, are required to complete ihe natural history of this fingular bird; and from the specimen which this memoir contains of Mr. Jenner's inclination and abilities for observing natural phenomena, and afcertaining the laws deducible from them, the naturalist is en

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couraged

Rev. Feb. 1789.

couraged to hope for some account of the cuckow's manner of pafling his time, and of his actual residence, while invisible in England.

MUSIC.
Of the Temperament of those musical Inst, uments, in which the

Tones, Keys, and Frets are fized, as in the Harpsichord, Organ, Guitar, &c. By Mr. Tiberius Cavailo, F.R.S.

In reading the later volumes of the Philosophical Transactions, we have had frequent opportunities of admiring the extent of Mr. Cavallo's philosophical knowlege, the ingenuity which he displays in many mechanical inventions, and the improvements which the arts have received from his labours. The subject of the present memoir has been attentively considered by many able mathematical musicians; their speculations however, alehough sublime and highly ingenious, have neither afforded any practical rules to inftrument makers, nor facilitated the methods of tuning ihe inftruments.

In the beginning of this paper, Mr. Cavallo gives a short description of the octave, and adds tome remarks concerning the nature of sounds and the properties of founding bodies, such as ftrings or pipes; and in this introductory part, we meet with an error. Mr. Cavallo, fuppeting the strings in every other respect equal, fays, p. 239, the number of vibrations, which they [ftrings) perform in a given time, is simply in the proportion of their lengths.' According to our system of music, founded on the Newtonian doctrine of gravitation, and mathematical principles, the square of the time of vibration of any musical Itring is as its length and weight directly, and its tension reciprocally. Hence, the weight and tension being the same, the time of vibration is as the length; for the matter of the string being the same, the weight is as its length and the square of its diameter: and the time of vibration is reciprocally as the number of vibrations in a given time; therefore the number of vibrations in a given time, is reciprocally as the length.

He then describes temperament, and thews the neceffity for using it in such inftruments as have their tones or keys permanently fixed. Of this necessity there has never been the leaft doubi, and various have been the efforis of the learned in order to ascertain what division of the octave would be the best ; different writers proposing different temperaments, not one of which wholly removes the imperfection of these instruments. Mr. Cavallo thews that the best division is that of 13, by equal ascents, called by other writers the isotonic scale ; of the lengths of the ftrings forming the octave, he gives the following table : 1C0000, 91,87, 89090, 84090, 79370, 74915, 70710, 66743, 62997, 59462, 56123, 52973, 50000; which we thould not have

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transcribed but for the sake of correcting 74915 to 74914, 70710 to 70711, 66743 to 66742, 62997 to 62996, and 59462 to 59460.

To what do all these investigations tend? or, has Mr. Cavallo applied them to practice? In some respect he has ; for a monochord being accurately made with the divisions jutt mentioned, is recommended as a help in tuning the harpficord; but to ascertain the divisions and fix the moveable frers on the monochord, will be found to be a work of confiderable difficulty.

The advantages attending this scale are many; Mr. Cavallo particularly mentions one, which indeed is of considerable 'importance, viz. that, on an instrument thus tuned, in whatever key the performer plays, the harmony will be perfectly equal throughout. He does not however recommend this scale for tuning instruments that are to serve for solo playing, or for a particular kind of music; but advises to tune in the usual manner, viz. so as to give the greatest effects to those concords which more frequently occur.

CHRONOLOGY.
On the Era of the Mahometans, called the Hejerà. By William

Marsden, Elg. F.R.S. and A.S.
The flight of Mahom med from Mecca to Medina, was (eighteen
years after it is said to have happened) eftablished, by the Caliph
Omar, as an epoch to which the dates of all the transactions of
the faithful should have reference. The year of the Mahommedans
consists of 12 lunar months, each containing 29 days 12 hours
and 792 scruples *; so that the year contains 354 days 8 hours and
864 fcruples. In order to reduce this year to an iniegral number
of days, a cycle of 30 was chosen as the most convenient period,
because 30 times 8 hours and 864 scruples is exa&ly 11 days ;
and in this cycle there are 19 years of 354 days, and 11 of 355;
the intercalary day is added at the end of the 2d, 5th, 7th, 10th,
13th, 16th, 18th, 2ift, 24th, 26th, and 2gth years of the cycle.
The commencement of each year of the Hejerà will never fali
on the same day of the month according to our calendar, but
will anticipate about a days. Mr. Marsden has added a very
valuable table, exhibiting the correspondence of the years of the
Hejerà with those of the Christian era. The first year of the
Hejerà began Ann. Dom. 622, July 16th. The 1201st of the
Hejerà, which is the first of the cycle, began Ann. Dom. 1787,
Oá. 24. So that the table may be easily formed, or extended
to any length, either backward or forward.

In the perusal of this memoir, we could not but remark the precifion, with which the Mahommedans, in 622, fixed the lunar

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* 1080 scruples make an hour,

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Pye's Poetic of Aristotle. month at 294 12" and 792", being only 3" 2" too little. The Chaldeans however were wonderfully near the truth, for they made the lunar month 29° 13793, being only jo of a second too much.

[ To be continued.] R......

Art. XI. The Poetic of Aristotle, translated from the Greek, with

Notes. By Henry James Pye, Esq. 8vo. 45. Boards. Stockdale,
1788.
F Aristotle's Poetic, fo much talked of, and so little read,

this is the first good translation that hath appeared in the English language. That of Dacier, in French, is not in any degree to be compared with the present version, in point of neatness and precision. The French critic, it must be acknowleged, bas enriched his work with copious and learned notes, long held in esteem by the literary world. There is room, however, to hope that this country will have to boast a work of equal excellence, fince Mr. Pye has promised a continued commentary, illustrated by examples from the modern, and particularly the English drama. By the bill of fare which he gives us, a rich banquet may be expected, and for this reason :- because the particulars, set forth in the preface, bespeak a mind prepared for the true beauties of the dramatic art, and the essentials of true criticism. He proposes to draw a comparison of the advantages and defects of the ancient and modern drama : this is a wide field, and the zealots, who consider the chorus as the established religion of the drama, will, it is presumed, see, in this part of the work, reason to read a recantation of their prejudices. Mr. Pye means to treat at large the question concerning poetic justice, and to examine Aristotle's reasons for preferring the unhappy catastrophe, where all are involved in one common distress, to that, where vice is punished, and virtue rewarded. A discussion of this kind will go deep into the subje&, and may serve to render the gentlemen, who write for the stage, acquainted, before hand, with the nature and the forft principles of their art. A differtation like that which is announced, will be of this further service: it may teach the critic not to adopt, with superftition, rules of the drama, merely because they are in Aristotle ; for in the Greek writer many rules are to be found, which are not fundamental, but adapted entirely to the ftructure of the ancient drama. We have, indeed, in the Poetic, many eflential rules from which we ought never to depart, for they are founded in nature, Mr. Pye is aware of this distinction; and the performance of this part of his promise will, probably, help to open the eyes of tuch as have, hitherto, been bigotted to antiquity. Another advantage will arise from such a work as Ms. Pye has project. ed, and we will give it in his own words :

: The

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