latitude at different seasons of the year; but much depends upon the atmosphere being free from clouds and fogs, or surcharged with them, as to its perceptible length in all places and at all times. Astronomically speaking, in the latitude of Greenwich, there is no night from the 22d of May to the 21st of July, but twilight from sunset to sunrise. It reaches its minimum three weeks after the autumnal equinox, and three weeks before the vernal equinox, having a duration of one hour and fifty minutes. At midwinter it is longer by rather more than a quarter of an hour than at the former periods, though this is not often perceptible, but rather the contrary, owing to the greater prevalence of clouds and mists at the winter solstice, intercepting, absorbing, and reflecting away from the earth the rays of light that otherwise would visit it. The duration is least at the equator and greatest at the poles; but at the former there are two twilights every twenty-four hours, and at the latter only two in the year. At the north pole the sun is below the horizon for six months. But from the autumnal equinox to the 12th of November, and from the 29th of January to the vernal equinox, the solar depression is within eighteen degrees. Consequently through the whole of these intervals there is twilight, which reduces the extent of the absolute night to about two months and a half. If this region is thus for a long interval deprived of the solar presence, it is compensated by a lengthened possession of it at an opposite season of the year, for from the 20th of March to the 23d of September, the sun is constantly above the horizon. The annexed view exhibits the sun as seen at midnight at the North Cape of Europe, which is within

eighteen degrees of the pole-an appearance novel in the extreme to the dwellers in more southern latitudes. In the locality upon the earth's surface in which we are

situated, the frequent interchange of day and night, with the gradual advance and recession of both, is a benign and beautiful arrangement, the gentle, silent, yet emphatic signal of nature, for man to go forth to his work and to his labour until the evening, when the "ploughman homeward plods his weary way," to give sleep to his eyes and slumber to his eyelids.

Owing to the refractive property of the atmosphere, the disk of the sun when near the horizon loses its circular form, and assumes an oval appearance. This is particularly observable when sunrise or sunset is viewed from the summit of a mountain, or from an eminence by the sea. The refringent power of the atmosphere being the greatest when nearest to the horizon, it follows that the rays of light proceeding from the lower limb of the sun are raised more than those which proceed from the upper point. This diminishes the apparent vertical diameter, while the apparent horizontal diameter is scarcely at all affected, as refraction acts only in a vertical direction. Measured by the micrometer, the vertical height of the solar surface, in the circumstances named, is sometimes found to be four, five, or even six minutes of a degree less than the horizontal width, and hence the term given to the appearance, that of the "horizontal sun." No such effect is perceived in other situations of the solar body, because refraction operates more feebly away from the horizon, and the difference between the refraction of the rays of light issuing from the upper and lower extremities of the vertical diameter is too small to be observed. We are accustomed to speak of the meridian glory of the sun; and independent of the greater purity of the atmosphere at noon, through the dissipation of mists and vapours, a greater quantity of rays reach the eye from the sun when high in heaven than when near the horizon. The air is an absorbent as well as a refractive and reflective medium and however transparent the medium may be, the quantity of light absorbed will increase or diminish according to the extent of atmospheric space it has to traverse. This extent is much smaller with reference to an object in the zenith than one near the horizon. By a reference to the diagram, it will at once be seen that a ray of light passing from z the zenith

Ꮓ

H

will embrace a much less portion of the atmosphere included between the two arcs than one from I the horizon; consequently a less quantity will be absorbed; and hence a celestial object will appear the brighter as its distance from the horizon increases. The comparatively dim and hazy appearance of objects seen in the direction of the horizon, is not only occasioned by the rays of light having to traverse a larger space of the atmosphere, but of its lower strata, where it is the most dense and most absorbent. It is estimated that the solar light is diminished thirteen hundred times in passing through it, and we are thereby enabled to gaze upon the sun when setting without being dazzled by his beams. The apparent diameter of the grand orb varies slightly at different seasons of the year. This is owing to an actual variation of distance, for the ellipticity of the earth's orbit alternately increases and diminishes our proximity to the luminary. The solar diameter appears the least about the summer solstice, because the sun is then in apogee, or most remote from the earth. It is the greatest about the winter solstice, when the sun is in perigee, or at the nearest point to us. It seems extraordinary at first sight, that at mid-winter, when the streams are ice-bound, the snow lies upon the fields, and the traveller shivers in the blast, we should be nearer to the sun than when, at an opposite season of the year, the greensward is burnt up, the cattle pant in the shade of the trees, and men seek a covert from the solar heat. But the effect of the sun's rays is increased or modified by two circumstances, more than sufficient to counterbalance that of the varying distance ;-the length of time during which they act continuously, and

their direction being more or less oblique. When he is farthest from us, as in summer, he is daily above the horizon twice as long as when he is nearest, at the winter solstice. This continued action causes a powerful accumulation of heat; and the nights being short, but little of it is radiated, or given off, during his absence. But temperature is affected by the direction of the sun's rays, whether vertical or oblique, their greatest force being experienced when they are perpendicular to the surface; while in proportion as they are oblique, they glance off, and having to pass through a larger portion of the atmosphere, a larger number are absorbed and dispersed by it. Out of ten thousand rays falling upon the earth's atmosphere, 8123 arrive at a given point if they come perpendicularly, 7024 if the angle of direction is fifty degrees, 2831 if it is seven degrees, and only 5 if the direction is horizontal. Now, in summer, the sun, being north of the equator, rises to a greater elevation in the heavens; the rays reach us in a more vertical direction; and the days being longer than the nights, more heat is absorbed than what is radiated. But in winter he traverses those signs of the zodiac that are south of the equator; and, ascending to a less elevation in the heavens, the rays reach us more obliquely, and the days being short, the solar action is less continuous. Hence in summer we have the greatest heat though the earth is then farthest from the sun; and in winter the greatest cold when it is at the nearest point.

The mean distance of the sun from the earth, as determined by observation of the transits of Venus, is ninety-five millions of miles. This may be confidently regarded as withinth of the true distance, so that no error is involved either way greater than about three hundred thousand miles. The immense magnitude of the solar body appears from the fact, that it occupies so much space in the heavens, and presents such a stately aspect, with so vast an interval between us. If a locomotive had been started five centuries and a half ago, at the termination of the Crusades, and had been travelling incessantly at the rate of twenty miles an hour, it would only now just have accomplished a space equal to that which lies between the terrestrial and the solar surface. Though light comes to the former from the latter in about eight minutes, a cannon ball would not perform the same feat, retaining its full force, under some twelve years. That an object therefore should be so splendidly visible as the sun, so far removed, and should so powerfully influence us with light and heat, argues grand dimensions and wonderful energy. The direct light is supposed to be equal to that of 5570 wax candles placed at the distance of one foot from an object; and so great is the power of the rays, that some of the men employed in constructing the Plymouth Breakwater had their caps burnt in a diving-bell thirty feet under water, owing to their sitting under the focal point of the convex glasses in the upper part of the machine. The sun's diameter of 882,000 miles is equal to 111 times that of the earth; and his circumference of 2,764,600 miles describes a bulk nearly a million and a quarter times larger than our own globe, and above five hundred times greater than the united volume of all the planetary bodies that revolve around him. If his mass occupied the place of the earth, it would fill up the entire orbit of the moon, and extend into space as far again as the path of that satellite. The density of the solar substance is, however, far less than that of the matter of our globe. If the two bodies could be weighed in a balance, the weight of the sun would not preponderate in the same proportion as the bulk, but be only 354,936 times heavier. This proportion is about a fourth less than that of the magnitude; so that the same extent of solar substance would be found four times lighter than the same extent of terrestrial substance.

To the naked eye the disk of the sun ordinarily presents a surface incomparably brilliant and uniformly luminous. There is no spot, or wrinkle, or blemish. The perfect purity of its aspect was an article of faith universally received by the ancient world. It

was not till telescopic views had been obtained, that the apparently smooth, unchequered, uniformly luminous face of the sun was found to be an illusion. The discovery of the vast and mysterious peculiarities, called the solar spots, is due to Galileo, though it has been claimed by and for other observers. They were first discerned by him in April, 1611. In a letter published in the following year, he announced their irregular and variable figures, the unequal term of their continuance, their motions across the disk in parallel lines, and their confinement to a narrow zone north and south of the sun's equator. Scheiner, a German Jesuit; and Fabricius, the friend of Keppler, observed them about the same period, but not with the accuracy of Galileo. Being unacquainted with any method of intercepting a portion of the solar rays to save the eye, Fabricius could only observe the sun at the horizon, when his brilliancy was impaired by the density of the atmosphere, and even then he suffered much from the impression of the solar light. Our own countryman also, Harriot, the companion of Raleigh in his voyage to the New World, must be ranked among the first observers of these phenomena, the discovery and study of which have contributed to correct former opinions respecting the physical constitution of the great orb of day. Though ordinarily to the unassisted sight the sun exhibits a face of uniform and dazzling splendour, yet instances are recorded of his tarnished surface being perceptible to the naked eye. Herschel, who intently studied solar phenomena, and lost one of his eyes through the intense glare, mentions a spot appearing in the year 1779 large enough to be thus discerned. This may explain and justify some statements which have been regarded with incredulity. Thus, we have it upon the authority of Plutarch, that, in the first year of the reign of Augustus, the sun's light was so greatly diminished, that the unprotected eye might steadily contemplate his orb. Abulferagius also relates, that, in the ninth year of Justinian, the sun suffered a diminution of his light, which lasted above a year and two months; and that, in the seventeenth year of the emperor Heraclius, half of his body was obscured, which continued from the first Tisrin till Haziran; that is, from October to June. Keppler likewise states, that once in his time the solar aspect was strangely altered, as though a thick haze enveloped his body; and the stars shone out at mid-day. Hakluyt gives the following entry from the log of a ship on the coast of Africa in December, 1590: "The 7th, at sunset, we saw a great black spot on the sun; and on the 8th, both at rising and setting, we saw the like, the spot appearing about the size of a shilling."

The spots are all evanescent, but some are sufficiently permanent to be recognised as the same after the lapse of a considerable period. They appear upon the eastern edge of the sun, and move towards the western, vanishing when its edge has been gained, reappearing at the eastern extremity in about thirteen days and a half, to pursue the same route. When first seen upon the eastern limb they scarcely seem in motion, afterwards they appear to travel slowly, their velocity increasing till the central regions have been passed, when they apparently relax, and gradually disappear at the western extremity. This is obviously an optical illusion occasioned by the oblique direction in which we view the marginal parts of the sun's body. The fact sensibly demonstrates the solar rotation; but the period included between the appearance of a spot at the eastern edge and its return thither, is greater than the real time of one rotation, owing to the earth advancing all the while in its orbit. The apparent interval of revolution is rather more than twentyseven days; the true time of rotation is somewhat less than twenty-five days and a half, a much longer period than that taken by our own globe, harmonising with the mightier dimensions of the solar machine. The discovery of the chequered physiognomy and rotation of the sun was so repugnant to ancient ideas respecting the glorious orb, that upon Scheiner reporting the evidence of his senses to his provincial superior, the latter treated it as an illusion. "I have read," said he, "Aristotle's writings from end to end many

times, and I can assure you that I have nowhere found anything in them similar to what you mention. Go, my son, and tranquillise yourself; be assured that what you take for spots in the sun are the faults of your glasses, or of your eyes." Scheiner was not allowed to publish his opinions under his own name; and they appeared anonymously.

Some general results of observation may now be succinctly stated. The spots are not uniformly obscure. They consist of a central portion, characterised by intense blackness,

termed the nucleus, surrounded by a very distinct belt of a lighter shade, called the penumbra. The two portions do not gradually blend, but are separated by a well-defined boundary; and this is true of the exterior part of the penumbra, which is in general sharply distinguished from the luminous region around it. By photometrical experiments, Herschel determined that representing the wholly luminous part of the solar surface by 1000, the relative brightness of the penumbra will be represented by 469, and that of the nucleus by 7. On some occasions, though rarely, a large nucleus has appeared without any penumbra; and on the other hand, a penumbra without a nucleus has been seen.

The spots are of very irregular shape, and varying magnitude. Sometimes they are numerous, but small, while individual spots appear of enormous dimensions. The latter are usually formed by simultaneous enlargement on all sides of the nucleus and penumbra from a comparatively minute speck. On the last day of June 1830, a spot of vast extent was observed. Its diameter was estimated at 23,000 miles; and being nearly circular, its area included about 443,000,000 of square miles. But Mayer, in 1754, perceived a spot equal to th of the sun's apparent diameter, which gives it an absolute diameter of more than 45,000 miles. Still greater magnitudes are reported.

Both in form and dimensions, the spots are subject to great changes, which transpire with astonishing rapidity. Two or more, situated very close together, will frequently expand towards each other, and form one large spot. On the contrary, one of great extent has been seen suddenly to crumble into several smaller. Dr Wollaston observed a spot which seemed to burst in pieces, like a lump of ice thrown upon a hard surface. Herschel states, that on the 19th of February 1800, he fixed his attention on several spots; but on looking off, even for a moment, they could not be found again. Sir John Lubbock also remarks, that he has observed spots visible to the naked eye, of which, on the following day, not a trace could be distinguished, even with the aid of a good telescope. These rapid and extraordinary changes indicate that the material subject to them cannot be solid, or liquid, but gaseous.

It is a remarkable circumstance connected with the solar spots, that they invariably appear near the equator, but apart from it. By Galileo they were seen as far as 29° of latitude, north and south. But Scheiner found them extending to 30°, and called the intermediate region, on this account, the "royal zone." They have, however, been occasionally observed at a greater distance on both sides of the sun's equator, but never at