Of the Construction and Extent of the Milky Way. By Sir W. HERSCHEL. [1818.]

Of all the celestial objects, consisting of stars not visible to the eye, the milky way is the most striking; its general appearance, without applying a telescope to it, is that of a zone, surrounding our situation in the solar system, in the shape of a succession of differently-condensed patches of brightness, intermixed with others of a fainter tinge.

The breadth of the milky way appears to be very unequal. In a few places it does not exceed five degrees; but, in several constellations, it is extended from 10 to 16. In its course it runs nearly 120 degrees in a divided clustering stream, of which the two branches between Serpentarius and Antinous are expanded over more than 22 degrees.

That the sun is within its plane, may be seen by an observer, in the latitude of about 60 degrees; for, when at 100 degrees of right ascension, the milky way is in the east, it will, at the same time, be in the west at 280; while, in its meridional situation it will pass through Cassiopea in the Zenith, and through the constellation of the cross in the Nadir.

From this survey of the milky way by the eye, I shall now proceed to show what appears to be its construction, by applying to it the extent of telescopic vision.

From the formula which has been given, I calculated a set of apertures, which, by limiting the light of the finder of my seven-feet reflector, would reduce its space-penetrating power to the low gauging powers two, three, and four. I then limited, in the same manner, the space-penetrating power of my night-glass, by using calculated apertures, such as would give the gauging powers five, six, seven, and eight. From the space-penetrating power of the seven-feet reflector, I obtained, by limitation, the successive gauging powers nine, ten, and upwards, to seventeen. And lastly, by limiting the space-penetrating power of my ten-feet reflector, I carried the gauging powers from seventeen to twenty-eight.

With a ten-feet reflector, reduced to a gauging power of 18, I saw a great number of stars: they were of very different magnitudes, and many whitish appearances were so faint, that their consisting of stars remained doubtful. The power 19, which next I used, verified the reality of several suspected stars, and increased the lustre of the former ones. With 20, 22, and 25, the same progressive verifications of suspected stars took place; and those which had been verified

by the preceding powers, received subsequent additional illumination. With the whole space-penetrating power of the instrument, which is 28.67, the extremely faint stars in the field of view acquired more light, and many still fainter suspected whitish points could not be verified for want of a still higher gauging power. The stars which filled the field of view were of every various order of telescopic magnitudes; and were probably scattered over a space extending from the 204th to the 344th order of distances.

From the great diameter of the mirror of the 40-feet telescope, we have reason to believe, that a review of the milky way with this instrument would carry the extent of this brilliant arrangement of stars as far into space as its penetrating power can reach, which would be to the 2300dth order of distances; and that it would then probably leave us again in the same uncertainty as the 20 feet telescope.

What has been said of the extent and condition of the milky way, in several of my papers on the construction of the heavens, with the addition of the observations contained in this attempt, to give a more correct idea of its profundity in space, will nearly contain all the general knowledge we can ever have of this magnificent collection of stars. To enter upon the subject of the contents of the heavens, in the two comparatively vacant spaces on each side adjoining the milky way, the situation of globular clusters of planetary nebulæ, and of far extended nebulosities, would greatly exceed the compass of this paper; I shall therefore only add one remarkable conclusion, that may be drawn from the experiments which have been made with the gauging powers,

Let a circle, drawn with the radius of the twelfth order of distances, represent a sphere containing every star that can be seen by the naked eye; then, if the breadth of the milky way were only five degrees, and if its profundity did not exceed the 900dth order of distances, the two parallel lines in the figure, representing the breadth of the milky way, will, on each side of the centre of the inclosed circle, extend to more than the 39th order of distances.

From this it follows, that not only our sun, but all the stars we can see with the eye, are deeply immersed in the milky way, and form a component part of it.

Astronomical Observations and Experiments, selected for the Purpose of ascertaining the relative Distances of Clusters of Stars, and of investigating how far the Power of our Telescopes may be expected to reach into Space, when directed to ambiguous celestial Objects. By Sir WM. HERSCHEL. IN my last paper on the local arrangement of the celestial bodies in space, I have shown how, by an equalisation of the light of stars of different brightness, we may ascertain their relative distances from the observer, in the direction of the line in which they are seen; and from this equalisation, a method of turning the space-penetrating power of a telescope into a gradually increasing series of gauging powers has been deduced, by which means the profundity in space, of every object consisting of stars, can be ascertained, as far as the light of the instrument which is used upon this occasion will reach.

When the nature or construction of a celestial object is called ambiguous, this expression may be looked upon as referring either to the eye of the observer, or to the telescope by which it has been examined.

If a cluster of stars in a very small telescope will appear like a star with rather a larger diameter than stars of the same size generally have, we shall certainly be authorised to conclude, that an object seen in a larger and more perfect telescope as a star with rather a larger diameter, is also an ambiguous object, and might possibly be proved to be a cluster of stars, had we a superior instrument by which we could examine its nature and construction.

This seems to throw some light upon a species of objects called stellar nebula, 140 of which have been inserted in my catalogues. For, as it has just been mentioned, that a 10-feet telescope may become a finder to a 20-feet one, the 20-feet telescope itself will be but a finder to objects that are so far out of its reach as not to appear otherwise than ambiguous; nay, the 40-feet telescope, when it is but just powerful enough to show the existence of an object which decidedly differs from the appearance of a star, may then truly be called a finder.

Celestial objects can only be said to remain ambiguous, when the telescopes that have been directed to them leave it undetermined whether they are composed of stars or of ne

bulous matter.

In 10 observations, the gauges applied to the milky way were found to be arrested in their progress by the extreme

smallness and faintness of the stars; this can, however, leave no doubt of the progressive extent of the starry regions; for when, in one of the observations, a faint nebulosity was suspected, the application of a higher magnifying power evinced that the doubtful appearance was owing to an intermixture of many stars that were too minute to be distinctly perceived with the lower power; hence we may conclude, that when our gauges will no longer resolve the milky way into stars, it is not because its nature is ambiguous, but because it is fathomless.

In the depth of the celestial regions, we have hitherto only been acquainted with two different principles, the nebulous and the sidereal. The light of the nebulous matter is comparatively very faint, and, except in a few instances, invisible to the eye. It is also in general widely diffused over a great expanse of space, in which, by an increase of faintness, it generally escapes the sight: the light of stars, on the contrary, is comparatively very brilliant, and confined to a small point, except when many of them are collected together in clusters, when their united lustre sometimes takes up a considerable number of minutes of space; but in this case the stars of them may be seen in our telescopes; and by the observations that have been given, it appears that when they are viewed with instruments gradually inferior to those which prove them to be clusters of stars, their diameters, seen with less light and a smaller magnifying power, are generally contracted; a globular cluster is reduced to a cometic appearance, to an ill-defined star surrounded by nebulosity, and to a mere small star with rather a larger diameter than stars of the same size generally have. In consequence of these considerations, it seems to be highly probable that some of the cometic, many of the planetary, and a considerable number of the stellar nebulæ, are clusters of stars in disguise, on account of their being so deeply immersed in space, that none of the gauging powers of our telescopes have hitherto been able to reach them. The distance of objects of the same appearances, but which are of a nebulous origin, on the contrary, must be so much less than that of the former, that their profundity in space may probably not exceed the 900dth order.

The method of equalising the light of stars on which the gauging power of telescopes has been established, may also be applied to give us an estimate of the extent of their power to reach ambiguous celestial objects.

When the united light of a cluster of stars is visible to the eye, there will then be a certain maximum of distance to which

the same cluster might be removed, so as still to remain visible in a telescope of a given space-penetrating power; and if the distance of this cluster can be ascertained by the gauging power of any instrument that will just show the stars of it, the order of the profundity, at which the cluster could still be seen as an ambiguous object, may be ascertained by the space-penetrating power of the telescope through which it is observed. But as the aggregate brightness of the stars depends entirely on their number and arrangement, this method can only be used with clusters of stars that have been actually observed.

On the Anomaly in the Variation of the Needle. By Captain SCORESBY.-[1819.]

THE following are the results of his observations:

All the iron on board a ship has a tendency to become magnetical, the upper ends of the opposite bars being south, and the lower north poles in the northern hemisphere, and vice versa.

The combined influence of all the iron is concentrated into a focus, the principal south pole of which being upward in the northern hemisphere, is situated in general near the middle of the upper deck.

This focus of attraction, which appears to be a south pole in north dip, attracts the north point of the compass, and produces the deviation in the needle.

This deviation varies with the dip of the needle, the position of the compass, and the direction of the ship's head. It increases and diminishes with the dip, and vanishes at the magnetic equator. It is a maximum when the ship's course is west or east, and it is proportional to the sines of the angles between the ship's head and the magnetic meridian.

A compass placed in either side of the ship's deck, directly opposite to the focus, gives a correct indication on an east or west course, but is subject to the greatest deviation when the ship's head is north or south.

Theory of Mists. By Sir H. DAVY.-[1819.]

LAND and water are cooled after sunset in a very different manner. The impression of cooling on the land is limited to the surface, and is very slowly transmitted into the interior; whereas in water the upper stratum, when cooled, descends, and has its place supplied by warmer water from below. The