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Kept at Croom's Hill, Greenwich, by Mr. W. Rogerson, of the Royal Observatory.
From the 21st of May to the 20th of June, 1841.
On considering the letter of Veritas in all its bearings, we must decline publishing it, in preference to the great quantity of useful matter with which our pages are crowded.
We have received MR. LIDDELL's letter, but far too late for our present number,
THE VARIATION OF THE COMPASS.
Trte variation of the compass must ever be a subject of interest to seamen, and an authentic register of it, from an infallible source, as time marches on, will form useful matter for reference. The subject of magnetism has been pursued lately with more than common energy on the continent, and has also employed the attention of some of our own men of science. Magnetic observations have been made at various places by them, in Great Britain and Ireland, and an expedition under the command of Capt. J. C. Ross, is now investigating the subject abroad, by order of the government, at the recommendation of a committee of the Royal Society. In connexion with this measure, also, magnetic observatories have been established in various parts of the British dominions abroad, as well as at the Royal Observatory at Greenwich; and as it will no doubt interest our readers, our present purpose is to lay before them the description of the latter, as we find it recorded in the introduction to the Greenwich observations for 1839, by Mr. Airy, the Astronomer-royal.
Observations of the Free Magnetic Needle, &c. In consequence of a representation of the Board of Visiters to the Lords Commissioners of the Admiralty, an additional space of ground on the southeast side of the existing boundary of the Observatory grounds was inclosed from Greenwich Park for the site of a Magnetic Observatory, in the summer of 1837. In the spring of 1838, the Magnetic Observatory was erected. Its nearest angle is about 230 feet from the nearest part of the Astronomical Observatory, and about 170 feet from the nearest outhouse. It is built of wood; iron is carefully excluded. Its form is that of a cross with four equal arms, nearly in the direction of the cardinal magnetic points : the length within the walls, from the extremity of one arm of the cross to the extremity of the opposite arm, is forty feet: the breadth of each arm is twelve feet. The height of the walls inside is ten feet, and the ceiling of the room is about two feet higher. The northern arm of the cross is separated from the central square by a partition, so as to form an ante-room. The meridional magnet is mounted in the southern arm : it is intended to mout a bifilar magnet in the eastern arm, and a magnetometer for variations of vertical force in the western arm.
The theodolite with which the meridional magnet is observed is by Simms : the radius of its horizontal circle is 8.3 inches : it is divided to 5', and read to 5" by three verniers, carried by the revolving frame of the theodolite. The fixed frame stands upon three foot-screws, which rest in brass channels let into a stone pier, that is firmly fixed in the ground and unconnected with the floor. The revolving frame carries the Y's (with vertical adjustment at one end) for a telescope with transit axis : the length of the axis is ten inches and a half: the length of the telescope twenty-one inches : the aperture of the object-glass two inches. The Y's are not carried immediately by the T head which crosses the vertical axis of the revolving frame, but by pieces supported by the ends of that T head and projecting horizontally from it ; the use of this construction is, to allow the telescope to be pointed suffiently high to see ở Ursæ Minoris above the pole. The eye-piece of the telescope carries only one fixed horizontal wire, and one vertical wire moved by a micrometer-screw. The stone pier is fixed nearly in the line which divides the southern arm of the cross from the central square : in the roof of the building an opening is made (closed by shutters), in the direction of the astronomical meridian of the pier, through which circumENLARGED SERIES - NO. 8.-VOL. FOR 1841.
polar stars can be observed, as high as 8 Ursa Minoris above the pole, and as low as ß Cephei S. P.
For supporting the magnet, a braced tripod wooden stand is provided, resting on the ground and unconnected with the floor. Upon the cross-bars of the stand rests a drum (having a covering of glass), within which the magnet vibrates. On the southern side of the principal upright piece is a moveable upright bar, turning in the vertical E. and w. plane, upon a pin in its centre, which is fixed in the principal upright : this moveable upright piece carries at its top the pulleys for the suspension of the magnet: and this construction is adopted in order to give an E. and W. movement to the point of suspension, by giving a motion to the lower end of the bar. The top of the upright piece carries a brass frame with two pulleys: one of these pulleys projects beyond the north side of the principal upright, and from it depends the suspension skein : the other pulley projects on the south side : the suspension skein being brought from the magnet up to the north pulley is then carried over it and over the south pulley, and is then attached to a string, which passes downwards to a small windlass, that is carried by the lower part of the moveable upright. The intention of this construction is, to make it easy to alter the height of the magnet without the trou. ble of climbing to the top of the frame. The height of the two pulleys above the floor is about eleven feet nine inches, and the height of the magnet is about three feet : so that the length of the free suspending skein is about eight feet nine inehes.
The magnet was made by Meyerstein, of Gottingen : it is a bar two feet long, one inch and half broad, and about a quarter of an inch thick : it is of hard steel throughout. The suspension piece was also made by Meyerstein, but it has since been altered under my direction by Simms. The magnet is not now inserted endways in its support, but sideways; a double square hook being provided sustaining it: and (by an alteration very lately made) the upper part of the supension-piece is simply hooked into the skein.
The suspending skein is of silk fibre, in the state in which it is first prepared by silk-manufacturers for further operations ; namely, when seven or more fibres from the cocoon are united by juxtaposition only (without twist) to form a single thread. It was reeled for this purpose at my request by Mr. Vernon Royle, of Manchester. The skein is strong enough to support perhaps six times the weight of the magnet, &c. : I judged this strength to be necessary, having tound that a weaker skein (furnished by Mr. Meyerstein) broke ultimately even with a smaller weight.
Upon the magnet there slide two small brass frames, firmly fixed in their places by means of pinching-screws. One of these contains, between two plane glasses, a cross of delicate cobwebs : the other holds a lens, of thirteen inches focal length and nearly two inches aperture. This combination, therefore, serves as a collimator without a tube : the cross of cobwebs is seen very well with the theodolite telescope, when the suspension bar of the magnet is so adjusted as to place the collimator object-glass in front of the theodolite objectglass, their axes coinciding. The wires are illuminated by a lamp and lens in the night, and by a reflector in the day.
In order to diminish the extent of vibrations of the magnet, a copper bar, about one inch square, is bent into a long oval form, intended to contain within itself the magnet (the plane of the oval curve being vertical). A lateral bend is made in the upper half of the oval, to avoid interference with the suspensionpiece of the magnet. The effect of this copper bar is very striking : it appears from rough experiments, that every second vibration of the magnet (that is, when a direct and reverse swing have been finished) is reduced in the proportion of 5 : 2 nearly. Two such bars were mounted, for convenience of use, in different positions of the magnet. Experiments made in the year 1840 have given reason to think that one of them (that which was used in 1839 with the collimator west) is not entirely free from magnetic action: and the determinations, therefore when the collimater was west, may be liable to an unknown constant error from this cause.
But in laying before our readers the foregoing account of the place, in which the magnetic observations are made, it was no less our object to place on record the following, which have been kindly supplied to this journal by the Astronomer-royal.
Royal Observatory, June 25th, 1841. Mean Variation. 1840-November
23 23 36 December
23 21 11 1841-January
23 11 46 February
23 17 35 March
23 19 16 April
23 11 48
23 17 45 (Signed) G. B. AIRY, Astronomer-Royal.
It will be seen from the above, that the variation of the compass is decidedly on the decrease in this country, although fluctuating very considerably, indeed, more we understand in one day sometimes than in a whole month. But this, and several other interesting facts relating to the phenomenon of magnetism as observation discovers them, as well as the mean magnetic variation for each month, we hope hereafter to be enabled to communicate to our readers.
HOUTMANS ABROLHOS.-By W. J. C. Wickham, Commander, H.M.S.
The extensive cluster of small islands and rocks forming Houtmans Abrolhos, have rarely or never been visited, excepting by those who have been unfortunate enough to suffer shipwreck upon the dangerous reefs that form their sea, or western barrier; and that they have been the cause of such disasters, the western shores of those islands, composing the southern or Pelsarts Group most plainly shew, being literally strewed with wreck, some of which is of a large scantling, and apparently very old, probably the remains of the Dutch ship Batavia, in which Commodore Pelsart was wrecked in 1629, and of the Zeewyk wrecked in 1729.
Whether these shoals were discovered by Frederic Houtman, (whose name they bear,) or by the unfortunate circumstance of Pelsart being thrown upon them in a gale of wind, appears doubtful; at all events, little has ever been known of them, beyond their existence, since that period. They are of great extent, occupying a space of forty-eight miles in a N.N.W. and S.S.E. direction, and form three separate groups, the southernmost of which has been named Pelsart Group, the next Easter Group, and the third the Northern Group.
Wreck Point, which is the south end of the southern island of Pelsarts Group, is in latitude 28° 591 S., and longitude 113° 56' 30" E., or 1° 47' 7" west of Swan River; from this the island extends N, 26° E., nearly six miles, curving slightly to the westward : it is a narrow ridge, chiefly of dead coral, rarely more than an eighth of a mile wide
in any part, but mostly less. The highest and most conspicuous part is a clump of mangroves fifteen feet high, about half way between the north and south extremes. The base of this island, as of all the islands, rocks, and reefs of the Abrolhos, is a calcareous limestone, of which the principal ingredients appear to be coral and shells; and it is remarkable that all the islands on the east side of the two southern groups, are merely ridges of dead coral and shells, nearly void of vegetation, whereas those on the west sides are flat blocks of limestone about five feet above water, with a covering of light sandy soil, that gives growth to a stunted and scrubby vegetation, of which, several kinds of samphire form the principal part.
From Wreck Point a narrow reef extends about a mile to the S.S.W., gradually curving to the north-west, in which direction it extends twelve miles, and forms the sea barrier of a large lagoon, of which, South Island and the reefs and islets north of it are the eastern boundary ; this lagoon is thickly strewed with shoal patches of coral, and has seve. ral small islets in it. For two miles and a half from the north end of South Island is a continuous cluster of coral reefs and small islets, be, tween which and the low coral banks, a little more than a mile to the northward is a clear passage a mile wide, with from thirteen to fifteen fathoms in mid-channel. Three miles N.N.E. of these coral banks is a small Hummock Island, having a small mound thirteen feet high at its south-eastern end; it is surrounded by a reef, which in no part extends more than a quarter of a mile off, but there is discoloured water a long half mile off the north-west end. This islet is in latitude 28° 481 S., and is the north-easternmost of Pelsarts Group; between it and the patch of coral banks to the S.S.W., is a clear passage with twenty to twenty. five fathoms water.
Under the northern part of Pelsarts Group there is anchorage in fifteen to twenty fathoms, and shelter from all southerly winds, and probably a well sheltered anchorage may be found in the lagoon, between Square island and the mangrove islets, where there is from ten to seventeen fathoms water. There are several spaces of deep water in the southern part of the lagoon, where there are no doubt good anchorages, but this part of it is so strewed with shoal coral patches, that it would be a tedious business to get a ship in.
Gun islet is the north-western islet of this group ; observations were taken on its south end, which place it in latitude 28° 53' S., and longitude 1° 53' 30'' W. of Swan River. A brass 4-pounder swivel was found upon it, and several other articles that shewed it had once been the lemporary retreat of some shipwrecked crew; and from the Dutch sketch of a part of these islands, shewing the place where the Zeewyk was wrecked, it is no doubt the same island to which her crew esca ped.
The north-west end of the reef forming the sea barrier of Pelsarts Group, is in latitude 28° 51' S., between which and the southern ex. treme of the reefs off the next cluster, or Easter Group, is a clear passage of four miles in width, through which the crew of the Zeewyk got to sea, in a boat built from the wreck; it has been called Zeewyk Passage in consequence.
It was sailed through by the Beagle, and appears quite free from