388 SPANISH, LESSONS IN: Measurement of Sides and Mode of Connecting the stone's Bridge Modes of Generating Elec- Decomposition of Water- POPULAR EDUCATOR. LESSONS IN ASTRONOMY.-IX. THE SOLAR SYSTEM-COMPARATIVE SIZES AND DISTANCESTHE SUN-VULCAN. THE student will by this time have become acquainted with many of the more important phenomena of the heavens: we will, therefore, proceed now to notice in detail the principal facts relating to those of the heavenly bodies which are our nearest neighbours in space, and which belong to the same system, or family group, as does the earth. The following is a list of the principal bodies in this group :-The Sun, which is the common centre round which they all revolve; Vulcan, Mercury, and Venus, which are distinguished as the inferior planets, their orbits being included within that of the earth; the Earth; and the superior planets, Mars, the minor planets, or asteroids, Jupiter, Saturn, Uranus, a mile, will stand for Saturn; a full-sized cherry, three-quarters of a mile distant, for Uranus; and a plum, a mile and a quarter off, for Neptune. On this scale the distance of the nearest fixed star would be about 7,500 miles. As the sun is by far the largest of these bodies, we will treat of it first, and the question that immediately occurs to us is, What is the distance of this body? The accurate solution of this question is one of the most important problems in Fig. 15. and Neptune. Several of these have satellites, or secondary planets, revolving around them; and there are also several comets which are included as regular members of our system. These will be enumerated hereafter. As we inquire more particularly into the movements of these bodies, we see many striking points of similarity. They all move round the sun in the same direction, and in elliptical paths of no great eccentricity. They are all likewise opaque bodies like the earth, shining only by reflected light; and all rotate on their axes, so as to produce the changes of day and night. Their orbits, too, are all inclined to the plane of the ecliptic. Orreries are frequently constructed, in which the different planets are represented by different-sized balls moving at various distances round a central one. It is, however, quite impossible to make these on a scale at all true to nature. Fig. 15 illustrates roughly their comparative sizes. The following, however, is a plan for obtaining a tolerably correct idea of their comparative distances and magnitudes, and the relative dimensions of their orbits: A B B Fig, 16. Select a large clear space, and place at one side a ball about two feet in diameter to represent the sun; Vulcan will then be represented by a small pin's head 27 feet from the globe; Mercury by a mustard-seed 82 feet distant; Venus by a pea at a distance of 142 feet; the Earth by a slightly larger pea at a distance of 215 feet; Mars by a large pin's head at a distance of 327 feet; the minor planets by grains of sand between 500 and 600 feet distant. An orange, about 2 inches in diameter, and 1,120 feet distant, will then represent Jupiter; one about two inches in diameter, distant two-fifths of VOL. V. astronomy, as this distance is taken as a measure for determining the distances and magnitudes of most other heavenly bodies. The principle of the problem can easily be understood, though, of course, there are many difficulties in the carrying of it out. Suppose an observer, situated' on the line BC (Fig. 16),. wishes to ascertain the distance of an inaccessible object A; let AC be the visual ray by which it is seen at c; at right angles to this lay off another line, BC, and from B measure accurately the angle CBA. We know then the distance B C, and the measure of the angles at в and c; it is easy, therefore, to calculate the angle BAC and the length of A C. As will at once be seen, the longer BC is, the larger will the angle BA c be, and therefore the less the risk of error in measuring it. When this angle is very small, an Fig. 18. K Fig. 17. exceedingly minute error produces a great difference in the calculated length of c A. Now, in the practical application of this principle, the utmost base-line that can be obtained is the earth's diameter; and this is so small in comparison with the distance of the sun that the angle B AC becomes too minute to be measured directly with a sufficient degree of accuracy. We are enabled, however, in an indirect way, to measure it, and thus solve the problem. The planet Venus travels round the sun in an orbit within that of the earth, and hence, at certain intervals, passes between the earth and the sun, and produces what is called a transit of the planet. On these occasions it is seen as a black spot on the bright disc of the sun, and by means of observations taken at that moment the required angle may be measured. D M Fig. 17 will render the mode of proceeding more clear. AB represents a base-line on the earth's surface, and CD the sun, E being the planet Venus when passing between the two. To an observer at в it will appear to travel across the sun's disc along the line HK, while to one situated at A it will pass along F G. Now if Venus were midway between the earth and the sun, no advantage would be gained, as the angle MB L would then be equal at A L B. The planet's distance from the sun is, 105 however, about two and a-half times as great as its distance from the earth, and hence the angle to be measured is so much greater. Now by accurately observing the times of the planet first coming into contact with the sun's disc at H, and of again emerging from it at к, we shall know exactly the length of HK ; in a similar way, another observer can ascertain the length of the chord F G, and thus we can learn the exact length of the line ML, and the measure of the angle M B L. Of course, many of the details are omitted here, but the student should make himself master of the principle of the calculation, as being the key to all celestial measurements. So great is the importance attached to this problem that at the last transit, in 1769, several expeditions were sent out by Government to take observations at different stations. when the number is at a minimum; the surface being then free from them on more than half the days of observation. They then increase again in number for the next five and a-half years; and thus their period appears to be about eleven years. A most remarkable fact has been noted in connection with this, and that is, that the daily variation in the magnetic needle is found to have a precisely similar period, and to increase or diminish with the increase or diminution in the numbers of spots. Other phenomena seem further to show that there is an intimate relationship between the movements of the magnetic needle and the sun. Whether or not future observations may reveal to us more of the nature of this bond, we cannot say, but fresh discoveries on the subject are frequently being made. These spots are usually accounted for by supposing the sun As a result of the most accurate observations, the sun's to be a dark opaque body surrounded by two atmospheres, the horizontal parallax-that is, the angle that would be subtended outer one highly luminous, and the inner one more dense. It is to an observer in the sun by the semi-diameter of the earth-supposed then that some powerful internal convulsion breaks is found to be about 8.93", and the mean distance of the sun is through these layers, and thus reveals the dark surface of the therefore about 91,430,000 miles. Until quite recently the sun beneath. parallax was taken at 86", and the sun's distance set down at 95,000,000 miles, but subsequent investigations have shown an error in these measurements. The numbers given above must, however, be taken as approximations only, subject to future correction. The next transit of Venus, which will occur in December, 1874, is anxiously awaited to settle the question decisively. It must be remembered that the distance given above is the mean, the difference between the minimum and the maximum being about 3,000,000 miles. Having ascertained the distance of the sun, and knowing its apparent diameter to be about 32', it becomes a simple problem to ascertain its real magnitude; and in this way we find 'that its diameter is about 853,000 miles, or more than 108 times as great as that of the earth. The best idea we can give of this immense size, is to state that if the sun were hollow, and the earth were placed in its centre, there would be room enough for the moon to continue to revolve round it without touching the sun's surface, even if the moon's distance were increased to nearly double what it is now. The sun's volume is so great that it would require 1,300,000 globes of the size of the earth to be rolled into one to equal it, and it is 450 times as large as all the planets that revolve around taken together. Some idea can be formed of its light and heat when we remember the enormous distance we are from its surface, and the degree to which, notwithstanding this, we feel its power. Its light is computed to be equal to that of 5,500 standard candles, placed at a distance of a foot from the surface to be illuminated. We naturally want to know something more of the physical properties of this wonderful and stupendous orb, but we are to a great extent baffled in this inquiry, though many great and important discoveries have been recently effected by means of spectrum analysis. In this way it has been ascertained that many of the metals present in the earth are also present in the sun. When pieces of very dark glass are placed in front of the eye-piece of a telescope, so as to screen the eye from the intense glare of the sun, its surface may be carefully examined, and is found to present an appearance by no means uniform. Many dark spots (Fig. 18), termed macula, are found at times to exist upon its surface. The centre of these is usually of a very dark colour, and is surrounded by a ring much lighter in appearance, which is known as the penumbra. These spots are very irregular in shape, and frequently change in size or disappear altogether. At times, however, they remain permanent sufficiently long to be traced disappearing at one edge of the disc, and after an interval appearing again at the other. We thus learn that the sun, like the other members of our system, is in constant rotation on its axis, and the period of this rotation is found to be 25 days, 7 hours, and 48 minutes. These spots are occasionally so large as to be distinctly observed by the naked eye. Some of them have been measured, and their breadth found to be more than 100,000 miles. Most of them, however, are only visible with the aid of the telescope. Careful records have been kept of the appearance of these spots, and it is found that during some years there is scarcely a day elapses without some being visible. They then diminish in frequency for about five or five and a-half years, In addition to these dark spots, others of unusual brilliancy are frequently observed. These are termed facula. Telescopic investigations show that, besides these markings, the whole surface has somewhat of a mottled appearance. According to Nasmyth, it presents an appearance as if it were covered over with scattered filaments shaped like willow-leaves. The whole question of the physical constitution of the sun is, however, engaging the attention of many astronomers. A total eclipse of the sun presents good opportunities for the observation of many points, and among the most remarkable features in connection with these phenomena is the appearance of red flames or protuberances surrounding the dark body of the moon at the moment of total obscuration. These have recently been seen at other times also, and are believed to be connected with the solar atmosphere. The symbol is frequently employed in almanacks to denote the sun. We now pass on to notice the planets which revolve in ceaseless courses around this grand central luminary. Till recently, Mercury was supposed to be the nearest planet to the sun. About ten years ago, however, the celebrated French astronomer, Le Verrier, having very carefully examined the movements of this planet, found in it a slight variation, which he could only account for by supposing that the mass of the planet Venus was incorrectly ascertained, or else that there was a planet revolving round the sun in an orbit within that of Mercury. These statements of his were published in the hope that some further light might be thrown on the matter. It must be remembered, however, that Mercury itself can only be seen at occasional intervals, and with difficulty, owing to its proximity to the sun; and that therefore a planet much nearer to the sun would never be far enough removed from that body to be clearly Almost the only opportunity, then, of observing it would be when it was in transit. seen. As soon as Le Verrier had made his statement, a French physician named Lescarbault stated that on the 26th of March, 1859, he had seen a small body pass.across the sun, but had not liked to announce the fact, no other observer having called attention to it. Le Verrier at once saw him, and carefully inquired into the matter. At first he thought the whole affair was a delusion: but after questioning the physician, and inquiring about the apparatus he used, he became convinced that he had indeed discovered a new planet, which was then called Vulcan. From this one observation no very decisive details could be drawn, so as to calculate its orbit accurately; its distance from the sun was, however, set down at about 14,000,000 miles, and its time of revolution in its orbit at a little under twenty days. It was conjectured that a second transit might be observed in March, 1860; but though a careful watch was kept, it was not seen, nor has it been seen again up to the present time. Many, on this account, treat the whole affair as a mistake. Instances have, however, been previously recorded of spots resembling planets being seen on the sun, and it may be that future observations will show that the planet really exists, and that these have been transits. We cannot, however, give a definite statement either way, so we merely thus briefly state the fact of the alleged discovery, and shall commence our next lesson with some account of Mercury, the next planet in order of distance. HISTORIC SKETCHES.-XLIV. THE JEWS.-I. So intimately is the history of the Jews bound up with the Holy Scripture narrative, that few persons accustom themselves to regard the two as distinct. In one sense, of course they are not distinct. The Jewish history, like the rest of the Old Testament, was written for our learning, and is profitable for "instruction in righteousness." It is the very groundwork, so to speak, of the Bible. Yet is it well sometimes to consider the remarkable history of this remarkable people apart from its surroundings, to learn from it the meaning of its intense individuality, and to see that had no more been patent to the world than the marvellous series of facts from the delivery out of Egypt to the establishment of Saul upon the throne, men must have been led to the conclusion that some special providence watched over the national life of the Jews, and that the Jews were a chosen people, specially favoured of the Divine Ruler of the universe. Most of the earlier Jewish history is derived from the Bible, but the later portions are drawn from many sources-from the histories of people who made a great figure in the world till they bruised themselves against the rock of Jewish nationality and were overthrown by it—from the histories of peoples who finally dispossessed the chosen people, and cast them forth as wanderers upon the face of the earth till the time of their redemption shall draw nigh. It is proposed in this and one or two succeeding sketches, to portray the Jews as they appeared at distinct epochs in their history, with a view to directing attention to the special features of their case, and to induce our readers to pursue more closely for themselves the study of the most remarkable history known to the experience of the world. "And all the people shouted and said, God save the king." It was a new cry in Israel. Up to that time the Jews had been content to live under the political guidance of spiritual chiefs, acting for and in behalf of that Divine Ruler who had brought them out of Egypt with a mighty hand and a stretched-out arm. Now they wearied of the unseen King who never held courts, nor entertained, nor showed himself, save in a figurative way, but who yet kept awful state in the midst of the people, being made manifest sometimes in the storm, sometimes in the whirlwind, and to those few who could understand Him in that guise, in the still, small voice. So "Samuel took a vial of oil, and poured it upon Saul's head, and kissed him, and said. Is it not because the Lord hath anointed thee to be captain over his inheritance ?" and at Mizpeh Samuel collected the people and announced what he had done, reproaching them at the same time for having rejected the God "who himself saved you out of all your adversities and your tribulations," and for having said, "Nay, but set a king over us." Saul was anointed, the multitude shouted "God save the king," and the first step was taken towards divorcing the State, not from the Church merely, but from the Head of the Church, from God himself. Those who may have noticed it as a curious thing, when reading the historical books of the Old Testament, that the functions of the prophet should have been allowed to clash with the functions of the king, and that what must often have looked like insolence was tolerated, in appearance at least, when it came from the mouth of a man of God, would do well to call to mind the peculiar relationship in which prophet and king stood to each other. Historically considered, the prophet was the creator of the king, the ruler who had governed before ever the idea of monarchical government had entered the mind of the Jewish leaders; the man who, having given, might be supposed to have some power also to take away. The prophet was the avowed oracle of God; the king was a concession to the desire of the people-a desire which was expressed in direct contravention of the will of the Almighty The Israelitish people could not remain satisfied with a system of government which differed in so remarkable a manner from that of the nations by whom they were surrounded, and, in te of Samuel's urgent advice, they persisted in asking for sang. Still it must be borne in mind that, in spite of this change, the principles of government which prevailed among the elites from the time they came out of Egypt to the period when they ceased to be a collective nation, were those of a pure theocracy-that is, of a form of government in which God is the central figure, the head from whom all orders emanate, and to whom all accounts of orders executed are rendered. As the representative of God, and the declarator of His word, a prophet was to be obeyed implicitly whenever he spoke professionally, his authority superseding even that of the king where the two conflicted. It was natural enough that the statesman on the throne should dislike, and vehemently dislike, this sort of imperium in imperio. So long as king and prophet agreed, which they seldom did, upon the course of government, all went smoothly, and the spiritual power came in with might to the aid of the temporal; but whenever there was a conflict, it was war to the knife. Unfortunately for the people, they were seldom on the prophet's side, inclining more frequently to take the part of the prince of this world-who, so long as they paid taxes and gave recruits for the army, allowed them to do pretty much as they pleased-rather than the part of the servant of Jehovah, who, for all that he had brought them out of Egypt, and blessed them in many things beside, was too highly exalted out of their reach for them to have sympathy. with Him, and who was also of purer eyes than to behold iniquity. Instances of conflicts of this sort are many and flagrant in the course of the Old Testament Scriptures, from which it will also be seen that it was a natural tendency in the people to "start aside like a broken bow" whenever the yoke of the Divine King was laid upon them for their good. It was in consequence of this tendency that a temporal king became necessary. Let us, before considering the constitution of the kingdom of Israel, sketch briefly the principal features of Jewish history up to the time when a king was demanded. Certain Arabs, known to us in Biblical writings as the sons of Jacob, fed their flocks and herds in the country westward of Lower Egypt, and led the nomadic life of shepherds in a land barely fruitful enough to support them. This difficulty naturally increased with the increase of population, and at times the chiefs were straitened to know what to do for food. These chiefs were non-elective heads of families, exercising despotic power over children and children's children, their authority being checked only by fear of physical resistance in their subjects. They were what Tartar or Arab chiefs are in the present daypatriarchial rulers, governing not according to any fixed law, but giving judgment according to discretion upon each case as it arose. It so happened that about the year B.C. 1706 a drought of unusual severity forced these Arabs to look beyond their own immediate compounds for sustenance for themselves and their little ones. Many of the cattle and sheep died, and it was becoming a question of human lives also. .To the neighbouring land of Egypt the eyes of the Arabs were turned; the dread of famine overcame their repugnance to mix with people alien to themselves, and some of the great chief's sons were sent down by their father to buy the necessary supplies in the fruitful land of Egypt. Egypt was the market in which they had becn accustomed to sell the surplus of their own stocks. It was already known to them commercially and by repute as one of the most flourishing and richest countries in. the world. Periodically it had been their practice to send down thither, and we have mention made in the Bible of some of these visits. Generally, however, they did their business through agents, merchants who came up expressly from Egypt to deal with the wanderers, and returned with their purchases in a caravan. such dealers, a short while before the famine alluded to, the sons of Jacob sold their younger brother Joseph, against whom they had a jealousy; and from such dealers they would gladly have bought all the supplies of food they needed. But the famine was so sore in the land that the merchants would not come up into it, and those who sought produce from Egypt were therefore compelled to go down into that land and seek it there. The sons of Jacob went down, under the circumstances so familiar to readers of the Old Testament history, and found "corn in Egypt." This was sold to them by the governor of the province, who proved to be their own brother, and whose excellent behaviour and able administration hal won for him the rulership over many cities, and the right of entry into the joy of his lord. To Political circumstances, which were stated at length in the Historic Sketch of Ancient Egypt (Vol. IV., page 222), in |