In our anxiety last Saturday to lay before our numerous readers the earliest and most authentic information respecting the ' Australian Comet,' as it has been not inappropriately christened, and from the
shock of whose unexpected advent the colony is scarcely yet recovered, we were betrayed into one or two slight inaccu- racies of statement. A respected correspondent has since
handed to us the subjoined paper, the re- sult of his more mature observations, which, in the absence of any astronomical data, will be read with deep interest. We may just state that the inquiries we have made, and the attention we have been able to bestow upon the subject, confirm us in the opinion already ex- pressed, that the comet which is at pre- sent hovering within the limits of our system has never before visited these re- gions of space, or, at all events, that it has never been tbe subject of astronomi- cal observation. On Thursday evening last, immediately the sun disappeared below the horizon, an unusually bright streak in the west attracted general attention. Although the nucleus was hidden by a heavy bank of cloud, there could be no doubt that the effulgence was that of a comet. The tail was then considerably above the horizon, but from the circumstance of the few previous nights on which it might have been visible having been cloudy, it had escaped observation. As compared with the comet of 1811, and that of Halley on its reappearance in 1835, our present visitant is far more splendid. It was not, however, until Sunday evening that one of our fine cloudless skies permitted us fully to appreciate tbie magnificent object. The Comet is on its return from the sun, having passed its perihelion. It becomes visible directly the sun sets, about five minutes after seven, and from the rapidity of its motion will, each evening, appear higher above tbe horizon, and be visible for a longer period each night, until we lose it altogether. Its nucleus ap- pears at a bright speck, and tbe nebulous matter usually surrounding the nucleus is either absent or so diaphanous as to be invisible to the naked eye. The tail springs immediately from tbe head, gradually expanding, and with the cur- vature common to these bodies on their return from the sun, extending so far as can be estimated without instrument!, from 42 to 45 decrees of space. The motions of comets are either direct or retrograde, in the former case moving, like the planets, from west to east, in the latter from east to west. The motion of this splendid body is retrograde, Comets, although they traverse all parts of the heavens, are only visible when near their perihelia, when, from their velocity, they re main but a short time within tbe planetary orbits. They are all presumed to move in ex- tremely eccentric ellipses ; three only having their regular periodicity established. Halley's has a period of about 75 years, Encke's of 1207 days, and Biela's of 6¾ years. A hundred and forty comets have appeared within the earth's orbit during the last century that have not again been aeen. M. Arago considers that, as thirty comets are known to have their perihelion distance within the orbit of Mercury, and assuming their distribution generally in space, it is more than probable seven millions of comets frequent ».ht planetary orbits.
A vast number, especially of telescopic comets, are only like clouds or masses of vapour without tails. Such were those of 1795, 1797, and 1798. Encke's comet of 1907 days has no tail, and is a small object. The tails of comets do not attain full magnitude until the comet has left the sun. The magnificent ob- ject now visible to us has a tail of enormous length preceding its head, and it is probable it may yet increase. Those of 1618, 1680, and 1769, had tails which extended respectively over 104, 90, and 97 degrees of space, so that when their heads were set a portion of the ex- tremity of their tails was still in the zenith. The tail of the comet of 181 1, which was visible many weeks, covered thirty four millions of leagues. It may interest some of your readers to know how they may acquire a tolerably correct idea of the extent of space expressed by any number
of degrees. For a common observer a ready measure exists in the belt of Orion, the three stars of which are known to every one. The line uniting these three stars measures exactly three degrees. By applying this to any other object the number of degrees between two points, or the distance covered by the tail of the comet, may be estimated. Orion is visible nearly over our heads, a little to the north, at the time of the comet's becoming visible. The number of degrees, however, will give no idea of the actual length of the tail, as they express the apparent, not the real distance. A minute of a degree on the moon's surface is about 70 miles, while a minute on the sun's is 28,000 miles , the real diameter, therefore, of any object in the heavens can only be determined by first ascertaining its distance. Should leisure permit, I may trouble you witb aome further remarks for your next number. D.
The Criminal Sittings of the Supreme Court commenced yesterday, at the Court house, Whitmore-square. The Grand Jury hating been sworn his Honour the Judge de livered his charge, for which we regret we have not space to-day. In the course of his address he laid considerable stress on tbe proposal of the Home Government to inoculate the Colony with a portion of the viru« of the English jailx and penitentiaries for juvenile thieves his Honour's object, we presume, being to induce the jury to make a presentment in favour of that ne'arious scheme to cheat the Colonists, out of the only remaining healthy and salutary pro vision of ike Act by which the Colony itself was constituted, and which is perpetuated in the subsequent Act for Us (tetter Government and administration. He also adverted to the operations of the Savings' Bank, and to the beneficial results it had etercised on tbe con dition of the labouring population. The Grand Jury afterwards returned true bills agaiast »e»*ral prisoners. Two cases were tried and the Court adjourned to to-day. Mr John Stephens has, we understand, kindly consented to give a lecture on Comets to the Members and friends of the Mechanics' Institute, in the Queen's Theatre, Gilles arcade, on Tuesday evening next, of which further notice will be given by advertisement in the public papers. Our Colonial contemporaries appear to have as much trouble as their transatlantic bre thren of the broad sheet to mikt their subscri bers 'cash up.' A recent Sydney paper con tained an intimation that ihwr subscriber* Wre forthwith to pay their outstanding accnoort to an authorited agent ; and, rabseipwetlr, the agent is advertised ai a defaulter and requested to disgorge.