body over a very bright one, and then viewed through a telescope, were not understood at the time when the observations were made, and it was these which produced, not the astronomical error, but the then inextricable difficulties of the case. The error arose from the observers of the transit seeing without perceiving, and it has been most successfully removed by Mr. Stone, who perceived the meaning of the phenomena without seeing them." The mean, or average, of all the trustworthy recent determinations is 8"-90. This value of the solar parallax, which is the angular measure of the Earth's equatorial semidiameter viewed at the mean distance of the Sun, gives for that mean distance, with only a small probable error, 91,841,000 miles.

If we view the surface of the Sun with an ordinary astronomical telescope, numerous small black irregular patches or spots are generally visible. Sometimes as many as eighty have been counted at one time, while at periods near the minimum frequency, the solar disk is perfectly free from them. Occasionally these spots are of sufficient magnitude to be observed with the unassisted eye, protected only by a coloured glass. Around the principal spots there is generally a fringe of less density called the penumbra. After a spot has been under examination for a short time, it may frequently be observed to undergo considerable alterations of form, while the central part, or nucleus, which is generally much darker than the rest, has been seen on many occasions to revolve. A remarkable instance of rotatory motion was noticed by the Rev. W. R. Dawes, in January, 1852, when the whole spot was observed to have rotated in six days completely around the small black nucleus. The variations in the appearance of the spots do not follow any rule, for occasionally a spot will keep its general contour from the time it enters on the solar disk to the moment of its disappearing on the opposite side, while others have been known to break up within a day or two into twenty or thirty distinct portions. Some extraordinary variations of this kind can be seen in Mr. Carrington's records of solar spots; and the Rev. F. Howlett has laid before the members of the Royal Astronomical Society special accounts of two remarkable spots, the first visible from July 25th to August 4th, 1862, the second in October, 1865. The dimensions of the latter extended to 110" in length, and 60" in breadth; and making allowance for its irregularity of form and density, it spread over a superficial area of not less than 972 millions of square miles. Adding to this, the displacements caused by numerous small spots in other directions, the solar photosphere was disturbed to the enormous extent of 1,137,000,000 square miles, or six times that of the whole surface of the Earth! Solar spots have, however, been observed far exceeding in magnitude those recorded by Mr. Howlett. In June, 1843, a very remarkable one was visible without the aid of a telescope during a whole week; its diameter, according to M. Schwabe, being about 77,000 miles, or nearly ten times that of the Earth. On May 25th, 1837, Sir John Herschel, while located at Feldhausen, Cape of Good Hope, noticed a spot of enormous size, the black centre, or nucleus, being large enough to have allowed a globe of similar magnitude to the Earth to drop through it, leaving a thousand miles clear of contact on all sides of the brink of the tremendous gulf,

THE SUN.

Owing to the rotation of the Sun on its axis, which occupies about 25d 8h, solar spots can be seen to apparently traverse the Sun's disk from east to west. Some which have disappeared at the western edge have been again recognised on their reappearance at the eastern edge, although their form may have undergone considerable modification during the interval. The spots are entirely confined within an equatorial zone, extending between thirty and forty degrees each side of the solar equator, or rather between two zones, one north and the other south of the equator, the intermediate band on the equator itself being comparatively free from them. They are never seen near the poles. Many speculative theories on the nature of solar spots have been put forth by several leading astronomers, but their probable origin is still open to discussion.

The continuous mapping down of the Sun-spots from day to day, persistently carried on for years by M. Schwabe, of Dessau, by Mr. Carrington, and at the Kew Observatory, has produced an accumulation of most valuable facts relating to the distribution of these interruptions to the uniform brilliancy of the solar photosphere. It has been discovered, principally from these records, that the frequency or non-frequency of solar spots is not altogether an accidental circumstance, but that there is a considerable regularity in the times of maximum and minimum. From the observations of M. Schwabe, it seemed tolerably clear that the interval between maximum and maximum generally consisted of about ten years. At one time this periodical variation was supposed to be connected with a similar periodical change in terrestrial magnetic declination, or the variation of the compass. Later observations have, however, thrown some doubt on the apparent coincidence. The positions of Venus and Jupiter in certain parts of their orbits have also been noticed to coincide with the maximum frequency of Sun-spots; this connection, however, is at present far from proved, and a much more extended series of observations must be made before any results deduced from the possible effect of planetary influence on the solar photosphere will be considered as satisfactorily established.

Dark spots of irregular form are not the only objects seen on the solar surface when viewed through an astronomical telescope. It is not very difficult to detect near the limbs, or edges, of the Sun, and also near ordinary Sun-spots, some very luminous streaks of much greater relative lustre than the surrounding photosphere. These streaks are known by the name of faculæ. They are generally many thousands. of miles in length, and frequently connected one with the other over a space equal to thirty or forty thousand miles in length, and from one to four thousand miles in breadth. Other singular appearances on the solar disk are distinctly visible when seen through large telescopes. These phenomena have been distinguished principally by the names of granules, willow-leaves, rice-grains, etc. Observations of the general granular aspect of the solar surface have been frequently made, but the first detection of the small interlacing particles, termed willow-leaves, which are quite distinct things form the ordinary granules, was made by Mr. James Nasmyth, the inventor of the steam-hammer. A considerable controversy was carried on for some years as to the real existence of these objects, but they have now been seen by so many leading

astronomical observers that little doubt remains that these apparently minute particles form an important part of the photosphere. The first announcement by Mr. Nasmyth was made in a private letter to a friend, and afterwards communicated to the public. Mr. Nasmyth remarks that "the filaments in question are seen, and appear welldefined, at the edges of the luminous surface where it overhangs the penumbra, as also in the details of the penumbra itself, and most especially are they seen clearly defined in the details of the bridges,' as I term those bright streaks which are so frequently seen stretching across from side to side over the dark part of the spot. So far as I have as yet had an opportunity of estimating their actual magnitude, their average length appears to be about 1,000 miles, the width about 100. There appears no definite or symmetrical arrangement in the manner in which they are scattered over the surface of the Sun, for they appear across each other in all possible variety of directions."

Among the numerous observers who have seen these well-defined solar particles, we may mention M. Secchi, whose observations made at Rome on April 11-13, 1869, are peculiarly interesting. He noticed that the penumbra of a solar spot under examination was covered with the small luminous particles, termed "willow-leaves," all of which were nearly identical in form. They were arranged apparently in bundles, converging towards the centre of the spot, across which they were thrown in the form of a bridge, arranged in a double line one after the other. M. Secchi considers that the "willow-leaves" form a kind of net-work foundation of the photosphere. The observations of M. Secchi represent very closely what has been seen by the aid of the great thirteen-inch object-glass equatorial at the Royal Observatory, Greenwich.

By his spectroscopic researches on the solar spots, M. Secchi has proved satisfactorily the evident hollow structure of these phenomena. Moreover, he has observed that the cavities are filled with dense metallic vapours, such as may be supposed to exist at the bottom of the solar photosphere. This is only one of the numerous solar discoveries made since the application of spectrum analysis to the comparison of solar light with that emitted by terrestrial substances heated to a state of incandescence, by which our knowledge of the principal elements contained in the solar photosphere has been so considerably increased.

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Since the discovery in August, 1868, of the gaseous nature of the rose-coloured prominences seen in all total eclipses of the Sun, several astronomers, including M. Janssen, Mr. Lockyer, Mr. Huggins, and M. Secchi, have clearly demonstrated that, in addition to the ordinary photosphere, a narrow belt of coloured gaseous matterprincipally hydrogen in a state of combustion-extends over the whole surface of the Sun. Hitherto, portions only of this continuous bed of self-luminous matter have been observed during total eclipses of the Sun, the irregularities or detached portions being known as the rose-coloured protuberances. A large part of the Moon's edge was seen fringed with this rose-coloured matter, during the eclipse of 1851, by the Astronomer Royal, Mr. Hind, Mr. Dawes, and others. In the eclipse of 1860, M. Le Verrier

noticed it also.

THE SUN.

"The Sun," he remarks, "is simply a luminous body on account of its high temperature, covered by a continuous bed of rose-coloured matter, the existence of which is undoubtedly proved." M. Janssen's and Mr. Lockyer's independent

spectroscopic experiments since the solar eclipse of August 18th, 1868, have definitely confirmed what was before considered mixed up with speculation. The first observation in sunlight of the rose-coloured matter composing the protuberances, is very remarkable.

M. Janssen was so struck with the brilliancy of the bright lines in the spectrum of the protuberances during the eclipse, that he resolved to continue his observations after the totality was over, if the weather remained favourable. Unfortunately, the Sun disappeared directly after the eclipse, and continued obscured during the remainder of the day. However, soon after sunrise on the next morning, M. Janssen succeeded far beyond his expectations, for he was not only enabled to observe the bright lines of a protuberance, but he could easily infer from them the rapid changes constantly going on in the form of the prominence. Mr. Lockyer first saw the bright lines in sunlight on October 20th, 1868, before he was aware of the success of M. Janssen in India, which was not announced in Europe before October 26th. Each observer may therefore justly claim a share in this important discovery; but to M. Janssen undoubtedly belongs the honour of being the first person who observed these curious appendages of the Sun without the intervention of the Moon in a total eclipse.

The observation of the rose-coloured matter around the edge of the solar disk has been very successful, not only by M. Janssen and Mr. Lockyer, but by Mr. Huggins, M. Zöllner, and Padre Secchi. Professor Zöllner, of Leipsic, has published some remarkable delineations of the rose-coloured protuberances as observed by him on July 1st and 4th, 1869. The rapidity of motion of the composing matter is very clearly shown in the different sketches of the same prominence, some of them having been made at intervals of only a few minutes of time. a few minutes of time. Mr. Lockyer has on frequent occasions noticed these rapid movements of the gaseous solar matter, sometimes drifting with a velocity of forty miles a second in a vertical direction, and one hundred and twenty miles in a horizontal direction.

It is useless to speculate at present on what insight we may ultimately obtain into the chemical composition of our great central luminary, but the systematic examination of the solar disk, by means of the powerful spectroscopes now in the hands of some of our most eminent amateur astronomers, cannot fail to increase our still imperfect knowledge of solar physics to an extent of which at present we can have only a vague conception.