form of a gigantic crab, with its full number of legs and claws. A very irregularly shaped nebula in the southern constellation, Dorado, included within the larger Magellanic cloud, is one of the finest objects of its class in the southern sky. The central portion of this beautiful nebula is composed of patches of light and dark alternately, surrounded by a paler nebulosity, over which are scattered a number of very minute stars. The two most remarkable of their regular nebulæ are those near Theta Orionis and Eta Argûs. The preceding examples of the nebulæ are all taken from the irresolvable class. The clusters which, in low-magnifying telescopes, are seen as nebulous patches of light, are most interesting objects. One in the constellation Hercules is visible to the naked eye on fine nights. It is seen as a luminous round spot, but it may be easily resolved into hundreds of stars, still, however, retaining its globular appearance. Two in the southern sky, one in Centaurus, the other in Toucana, have been sketched by Sir John Herschel. That near Omega Centauri is also visible to the naked eye as a dim round cometic object, in lustre similar to a star of the fourth or fifth magnitude. When seen through large telescopes it is resolved into an immense number of stars from the thirteenth to the fifteenth magnitude, strongly condensed towards the centre. The cluster in Toucana is in the neighbourhood of the lesser Magellanic cloud, in a spot almost free from stars. The condensation in the centre is very marked, and is of a ruddy orange colour. It contrasts very vividly with the white light of the stars immediately surrounding the central part of the cluster. Clusters, like nebulæ, are to be found scattered over the greater portion of the heavens; they are, however, very scarce in some parts. They are the most numerous in or near the Via Lactea and the Magellanic clouds. The richest region where the globular clusters may be found is certainly in the southern hemisphere, especially in that part of the Milky Way between the constellations Sagittarius, Corona Australis, Scorpio, Ara, and Lupus. The Milky Way itself may be considered as the largest and nearest of the clusters, especially if we include our own Sun as one of its members, as has been speculatively asserted by Sir John Herschel and other astronomers. "Fairest of beings! first created light! Prime cause of beauty! for from thee alone The sparkling gem, the vegetable race, The nobler worlds that live and breathe their charms, The lovely hues peculiar to each tribe, From thy unfailing source of splendour draw! In thy pure shine with transport I survey This firmament, and these her rolling worlds, THE SUN. N the solar system the Sun is the great central source of light and heat. Following the expression of Copernicus, the founder of the modern interpretation of the solar system, it may be termed the light of the world, or according to Theon, of Smyrna, the heart of the universe. Practically, however, it is the centre of a number of independent worlds, non-luminous of themselves, forming, as it were, the focus around which they all revolve. In fact, without its attractive power, all the planets, from Mercury to Neptune, could not be preserved for a moment in their respective orbits, but they would disappear into space, one knows not whither. As the different planets are separated from each other by a considerable distance, the apparent magnitude of the Sun as seen from each varies in proportion. As viewed from the Earth, the angular diameter of the Sun is rather more than half a degree, so that it would take about 700 Suns, placed side by side, to make a complete circuit of the celestial sphere. If it were possible to transport ourselves to the surface of Mercury, the nearest known planet to the Sun, we should find that the solar diameter would be apparently greatly increased, and that the intensity of the solar light could not probably be borne by inhabitants constituted like ourselves, even for an instant. On Neptune, on the contrary, the Sun would appear only as a brilliant star, the light and heat received at that immense distance being upwards of six thousand times less than that experienced on the surface of Mercury. The actual diameter of the Sun, as determined from the latest astronomical data, is about 856,500 miles, or 108 times greater than that of the Earth. Its superficial extent is 11,679 times greater, while it would require 1,262,153 globes, each of equal magnitude to the Earth, to form body corresponding in bulk to that of the Sun. But it has been found that the density of the material forming the Sun is about one quarter of the terrestrial density; consequently, if weighed in opposite scales, only 320,000 Earths would be required to equal the total weight of the Sun. These numbers appear large, but they are probably known at the present time within all reasonable accuracy. Whenever any improved determination of the Sun's absolute distance is made, these quantities which represent its bulk must always be corrected in proportion to the alteration of the amount in miles of the distance of the Sun from us, because the radius of the Earth's orbit is the fundamental unit used in the calculation of all the numbers representing the dimensions and bulk of not only the Sun, but of the different members, both primary and secondary, of the solar system. The determination of the value of this fundamental unit in astronomy is considered of so much practical importance, that we do not hesitate to give a brief, but slightly detailed, explanation of the principal methods employed by astronomers to obtain the best possible result. The Astronomer Royal has stated that the measurement of the Sun's distance has always been considered the noblest problem in astronomy. He says, "It is easy to measure a base-line a few miles long upon this Earth, and easy to make a few |