of His children, who, even in the vegetable world, | [Abridged from MACCULLOCH's Proofs and Illustrations THE CAMERA LUCIDA. THE Camera Lucida, an invention of Dr. Wollaston, like the Camera Obscura, is an instrument employed in making copies of drawings, and in portraying distant objects; but it is of greater service than the Camera Obscura, being much more portable, and, if properly used, reflecting the image of the object without the least distortion. If a piece of thin glass is held at an angle of fortyive degrees with the horizon, at a small distance from the table, and a sheet of white paper is placed immediately beneath it, the reflected image of an object before it will be visible, by looking downwards upon it; and as the glass is transparent, the hand and pencil can also be seen, and an outline of the image can be made upon the paper. In this case the image is inverted. Such an instrument as this can be made off-hand very easily. Let в be a piece of thin plate-glass, about an inch and a quarter long, and three-quarters of an inch wide; A a piece of wood in which the glass is fixed, and c a piece of pasteboard with a small hole in it, forming an eye-piece to keep the eye directed to one point. simplest construction, but possessing the disadvantage of reflecting an inverted image. Α Fig. 2. But the inversion can be corrected by taking a little more pains. Let A be a piece of looking-glass fixed in a wooden or brass frame, and connected with a piece of clear glass, B, so that the angle C BA shall be an angle of one hundred and thirty-five degrees, the image of an object placed at F will be reflected from the looking-glass at A, and proceed to the clear glass at B: from this it will be reflected upwards to the eye at G, and the glass being transparent, the image and the hand will be seen at the same time; in this case the image is erect. But, in general, neither of these plans are resorted to, for in both cases, as there are two reflecting surfaces from the glass, there will necessarily be two reflected images, one of them certainly much less vivid than the other, but still sufficiently visible to distract the eye D Fig. 3. B A The optical portion of the Camera Lucida which is usually sold consists of a prism. Fig. 3 is a section of the prism employed; the angle A is equal to 22 degrees, c to 135 degrees, D 22 degrees, and в is a right angle of 90 degrees. The solid nature of the prism will not allow the hand to be seen through its thickness, and the instrument is used in a different manner to the last contrivance. A is the prism, в a moveable piece of brass, having a small eye-hole in it at в; the reflected ray from c is received near to this corner of the prism, and reflected upwards to the eye; the eye-hole is so adjusted that one-half only is over the prism, B A Fig. 4. the other half leaves a free space through which the hand and pencil can be seen. In using the instrument a vast deal depends on the proper adjustment of this eye-hole. If the light is very powerful on the object, and much less so on the paper, the part of the prism exposed through the hole should be small, and the opening through which the paper is seen large in proportion. On the other hand, if the light on the drawing is weak, a larger part of the prism must be uncovered. In copying a print, great care must be taken that the print itself is perfectly flat and perpendicular to the horizon, and that the side of the prism at B A, fig. 3, which is opposite the print, shall be parallel to it. If this is not attended to, the print will be thrown into perspective and the copy be distorted. If the object is to be copied of its natural size, its distance from the prism in front must be equal to the distance from the eye to the paper; if it is to be reduced it must be placed at a greater distance; if to be enlarged, it must be brought nearer. The Camera Lucida has been fixed to the eye-hole of a telescope or a microscope, in such a manner as to allow the objects within the field of vision to be copied on paper. LONDON: JOHN WILLIAM PARKER, WEST STRAND. PUBLISHED IN WEEKLY NUMBERS, PRICE ONE PENNY, AND IN MONTHLY PARTS, PRICE SIXPENCE. Sold by all Booksellers and Newsvenders in the Kingdom. INTRODUCTION. "WHEN I' consider thy heavens, the work of thy fingers; the moon and the stars, which thou hast ordained;-what is man, that thou art mindful of him? and the son of man, that thou visitest him?"-Psalm viii., 3, 4. When the inspired Psalmist gave utterance to these words, he was evidently under the influence of those feelings of awe, wonder, and admiration, which are sure to be excited in every intelligent mind, by the splendid and sublime phenomena presented to us by the heavenly bodies. When we consider the magnitude and the number of those bodies, the immense distances which separate them one from another, the almost inconceivable velocity with which they move, and that those which we can see form, probably, but a very small part of the whole number;-when we revolve these things in our minds, we are naturally brought to reflect on our own insignificance in the grand scheme of creation. If a man, after having applied to himself the vain and self-satisfying appellation of "lord of the creation," were to remember that the glorious sun, and the planets which revolve around it, form but one particular division, class, or system in the universe,-that the earth is but an humble member of that system, and that he, man himself, is but a moving particle on the surface of this earth,-he may well be expected to give utterance to the sentiments of David, and to wonder how the Great Deity can regard with parental care so humble a member of so magnificent a whole. But this feeling, as Addison has beautifully shown, arises from the narrow powers of our own minds. We know that our perceptive faculties soon reach a boundary beyond which we cannot pass: we study the laws of Optics, but we know not the nature of Light:-we feel that we live and think, but we know not what constitutes life and thought. When, therefore, we judge of the Great Creator, by our own standard, we are lost in wonder at the vastness and at the minuteness, as also at the countless number of the objects which are under the Divine protection. But when we consider God as an Omnipresent and Omniscient Being, we then admit, indeed, that nothing is too vast, VOL. XII. nothing too minute, nothing too numerous, for his notice; that He who could create and arrange the whole, can also watch over and preserve the minutest parts of that whole. Our notions of great and small are derived from our own imperfect experience, and strikingly show the limited scope of our minds. The distance of the sun from the earth is a quantity so immense, as almost to perplex the mind which reflects on it; and yet that distance is small, compared with the distance of the fixed stars :-again, the minuteness of the nerves and smaller blood-vessels of the human body, is such as to require the microscope to aid us in an examination of their structure, and yet there are other entire animals, endowed with life and powers of motion, which are so minute that the eye cannot perceive them. The words great and small, then, are for man's use; to the Almighty nothing is great, nothing is small; the revolving planet, and the animalcule whose world is a drop of water, being equally objects of his ever-active care. This divinelysustaining power of Him, in whom "we live, and move, and have our being," is so obvious, that we may exclaim with the poet Thomson Were every falt'ring tongue of man, Thy Works themselves would raise a general voice, By human foot untrod;-proclaim thy power, Th' eternal cause, support, and end, of all! Nothing is more calculated to elevate the mind, and to display to us the wonders of Creation, than the study of Astronomy. We propose, therefore, to place before our readers a popular view of the elements, which serve for the basis of the astronomer's study. In doing this, we need not have recourse to the mathematical reasonings on which the various statements of the astronomer are founded; but we shall confine ourselves to such a simple explanation of the Mechanism of the Heavens as may pave the way for the study of a more systematic treatise. We trust, therefore, that both those who have, and those who have not, an opportunity of referring to more elaborate works, will not find the following pages devoid of instruction. 369 GENERAL APPEARANCE OF THE HEAVENS. LET us suppose a man to be totally ignorant of Astronomy, From these two facts, it would be by no means unreasonable to arrive at the conclusion, that the sun moves in a circle; and that, while he is invisible to us, he is passing round the earth, in a direction from west to east, at which latter point he arrives at the moment that the observer sees him rise in the morning. Here a difficulty arises, which it may well be supposed, What can be would perplex the uninitiated observer. meant by going round the earth, if the earth be, as it seems to our eyes, a flat and extended surface of ground, or water, which seems to touch the sky at the farthest points which the eye can reach :-how then can the sun pass round this? There can scarcely be a more reasonable question, so long as we judge merely by what meets the eye; but, if we extend our observations to certain appearances on the ocean, we begin to see proof that the surface of the water is not quite flat. On land, we have not the means of making a similar observation, because of the intervention of mountains and valleys. The proof that the surface of water is not quite flat, is derived from the following circumstance. If we are at sea, on a clear day, and watch the approach of a vessel from a distance, we find that the masts are visible sooner than the hull: we first see the top of the masts; then the whole height of the masts gradually comes into view; and finally, we see the hull. These different appearances of the ship, as it approaches towards the observer, are represented in the following figure. This could not occur if the earth were perfectly flat, because then the hull and masts would come into view at the same time. It matters not what part of the world be chosen as the place of experiment; the result is almost exactly the same every where, and the amount of the bending of the surface of the ocean may be illustrated thus:-If we had a piece of string four miles long, and were to apply both ends to the surface of still water, and could possibly draw the string into a perfectly straight line, the middle of the string would be about sixteen inches below the surface of the water; thus showing that the water is not quite flat. Thus might be supposed to arise the first conjecture that the earth is round, like a ball; a fact which was clearly proved by Captain Cook, who was the first to perform a voyage completely round the earth, about the year 1769. He left a given spot, and arrived again at that spot by an opposite course. There is an abundance of proof, derived from other sources, that the earth is a globe; but those will open upon us more plainly, as we advance. We have hitherto endeavoured to trace what would probably be the feelings and opinions of one who, without previous instruction in Astronomy, should note the appearances presented by the sun. But that golden orb is not seen to be a solitary inhabitant of the sky: another luminous body about as large in apparent size as the sun, would soon attract the notice of the gazer. He would see it rise in the east, soar aloft, and then sink in the west. After the lapse of a few hours it would again appear in the east, attain a height from which ..... With a boundless tide Of silver radiance, trembling round the world, it would again descend, to sink as before in the west. The We should in this way find the means of accounting for the change in the shape of the moon. We shall, by and bye, show that these suppositions are really true. But another point would attract the notice of the observer, independently of the change in apparent form. If the two brilliant bodies, the sun and moon, were observed to be near each other on a certain day when the latter appeared as a thin crescent to the left of the sun, they would be seen the next day further removed, and on the following day, at a still greater distance from each other: if, therefore, the motion of the moon round the earth be admitted, it is necessary to suppose that motion to be slower than the sun's motion round the earth. But in addition to the glowing light of the sun and moon, a glittering assemblage of smaller bodies meets the eye of the observer: a crowd of little spangles adorns the sky when the sun has left it, and softens the dreary darkness which results from his absence. These stars are seen to resemble the sun and moon in the circumstance of rising in or near the east, attaining a certain altitude, and setting in the west; from which circumstance the observer infers that the stars, like the sun and moon, revolve round the earth. But, by a careful attention to different stars, he would find that the same remark does not apply to all. Those which rise exactly in the east, set exactly in the west; those which rise to the south of east, set south of west; and many which rise somewhat north of east, set somewhat north of west; but in looking northward he sees stars which appear neither to rise nor to set, but which perform a complete circle round a particular point of the heavens. For example, there are seven stars, which, to most persons who pay any attention to the appearance of the heavens, are known under the name of the Great Bear. These stars never rise or set to an inhabitant of London. If they become invisible, it is either because clouds obscure them, or the superior brilliancy of the sun drowns their comparatively feeble light. The same may be said of five conspicuous stars known by the name of Cassiopeia's Chair. If we watch the progress of these stars, and others in their vicinity, we shall find that they describe a circle round a star called the Pole-star. The position of this star we shall find to be nearly this: if we suppose the distance from the north point of our horizon (which is the line in which the surface of the earth appears to touch the sky) to the zenith (which is the point immediately over our heads) to be divided into five equal parts, then at about the height of three of those parts from the earth, will be seen the Pole star. All this would seem to show to an observer, that the Pole-star is the end of an axis round which all the stars revolve, and that the reason why we cannot see the whole circular path of any stars except of those in the vicinity of the Pole-star, is, that they pass round under the earth during a part of their journey, and are therefore concealed from our view. Those stars which are further removed from the Pole-star describe a larger circle than those which are nearer, while the Pole-star is almost stationary. This is exactly what takes place when we see a wheel turn round; the axle remains in one spot, but any particular point on the outer edge of the wheel describes a larger circle than that which is described by any point between the axle and the circumference. If we consider the Pole-star to form one end of the axle or axis round which the stars revolve, then it is obvious that there must be an opposite end of the same axis, in the contrary direction. If then we turn towards the south, with the expectation of seeing such a point, we shall find that it will not be realized; for none of the stars in that quarter are seen to describe circles; for they all rise and set. Still, however, the semicircles or portions of semicircles, commonly called arcs, which they describe, appear to have a common centre, which is some distance below the horizon; and this centre may be considered as the southern end of the axis before spoken of. If we suppose the observer to have arrived at this amount of information respecting the stars, he will be prepared to notice the uniformity of the positions of the stars with respect to one another. The Great Bear, for instance, whether it be under or over the Pole-star, or at the right or the left of it, will always have its seven principal stars at the same relative distances from each other. The changes which take place in the distance of the sun from the moon, would lead to the opinion that these revolve round the earth in unequal times; and by similar reasoning, the constant maintenance of the same distance between any two stars, would seem to imply that the stars revolve in equal times. The seeming myriads of stars which present themselves to the notice of the observer all appear to follow this rule, of remaining at the same relative distances from one another, with very few exceptions. These exceptions, are, indeed, so few, that a constant watching of the same part of the heavens for a considerable period, would be necessary to determine that a star had actually changed its place relatively to other stars. There have, however, been discovered at different times ten stars, more or less brilliant, which change their relative distances from one another, and from other stars. These we know by the name of Planets; and by a careful attention to their movements, it is seen that each one travels in a curved path among the other stars, and returns again nearly to the point from whence it set out. At intervals, again, star-like bodies of another order present themselves, whose progress among the other stars is more rapid than that of the planets; and which at a period more or less brief, vanish altogether from the view of the observer, and do not return till after a long absence. His longitude through Heaven's high road; the gray His mirror, with full face borrowing her light THEORIES TO EXPLAIN THE MOTIONS OF THE HEAVENLY BODIES. IN very early ages, before Europe occupied a page in the history of nations, the phenomena of the heavens were studied with great attention by several nations of the East. The Chaldeans, the Indians, the Chinese, and the Egyptians, have all left evidences of the industry and ingenuity with which their observations were conducted. They constructed observatories,-invented instruments for observing and measuring with accuracy,-separated the stars into different groups, called Constellations, for the facility of finding any particular star,-gave particular names to most of the moving stars or planets, and noted the period which each took to move through its apparent path in the heavens; and, in many other ways, the ancients helped to lay the foundation of that mass of astronomical knowledge which the men of later ages have brought to more maturity. Various opinions were formed respecting the motions of the sun, moon, and stars of all kinds, both with reference to one another, and also to the earth; but the first theory which had attained a name and an importance in the carly ages of the world, was that of Ptolemy, a distinguished Egyptian astronomer, who lived about one hundred and thirty years after the birth of Christ. He conceived that the various bodies which had been distinguished by the appellation of "the heavenly host," were disposed in the order represented in the annexed diagram. He supposed, according to the popular opinion, that the Earth was fixed as the centre of the universe, and that the Sun, Moon, Planets, and Stars, revolved round it in the following order; namely-the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn; the Moon being the nearest, and so on; exterior to all of which, he supposed that a great concave sphere in which all the stars were fixed, kept on revolving round the earth. From the early history of Astronomy, we learn that before the time of Ptolemy, it had been conjectured by some, that the earth passed round the sun, and not the sun round the earth; but the difficulty of believing a statement so contrary to appearances and to the evidence of one's physical senses, led to the rejection of this opinion; and although it was afterwards found to be Seven small stars clustered together in the constellation Taurus. These stars rise with the sun about the time of Spring, and our poet, in this passage, intimates the old and common opinion that the Creation took place in the Spring. correct, yet nearly two thousand years elapsed before such a theory was generally admitted by philosophers. After Ptolemy had promulgated the theory which bears his name, he found that there were certain difficulties which followed from the adoption of it. He conceived that the seven bodies mentioned before which revolved round the earth, moved in the same general direction from West to East. But on watching the progress of some of the planets, he found that they did not appear to travel uniformly round the earth, but seemed to have, at certain times, a retrograde or opposite motion, with reference to the other planets; while at other times they seemed to be stationary. To account for this, he was obliged to suppose that those planets did not revolve in a perfect circle round the earth, but that they described a peculiar path called an epicycloid, of the nature of which some idea may be formed from the following illustration:-if we had a large coach wheel, and by any contrivance could make a smaller wheel roll round the outside of it, at its circumference, then, any particular point on the small wheel would describe that curve which is called an epicycloid: the axle of the small wheel would describe a perfect circle, but it is easy to see that any point on the edge would not describe a circle. Ptolemy was obliged to multiply these epicycloids to a most perplexing extent, in order to account for the various appearances of the planets. It is to this circumstance that Milton alluded, when he spoke of the shifts and difficulties which beset the progress of those who build their opinions on a wrong foundation : When they come to model Heaven Cycle and epicycle, orb in orb.- -Par. Lost, b. viii. There is nothing which more beautifully shows the power and force of truth, than the embarrassments which retard the progress of those who do not take truth for their guide. In religion, in morals, in science, he whose steps are guided by the light of truth, can arrive, by a short and pleasant path, at results which others can scarcely obtain by a complex and wearying track. Thus Ptolemy was forced to assume the existence of much unwieldly machinery in the scheme of the heavens, in order to account for those motions which are most simple and beautiful; this was because he placed (in his own imagination,) the earth in the midst of the heavenly bodies, all of which were made to revolve round it. Great as were the difficulties which attended the adoption of the Ptolemaic system, it yet retained its ground, with some slight modifications, until the time of Nicholas Copernicus, an eminent astronomer, who was born at Thorn, in Polish Prussia, in 1473. This distinguished individual perceived the unreasonable results which follow from the theory of Ptolemy. That all the planets, the sun, the moon, and all the stars and comets, should revolve round the earth, seemed to him much more unnatural and complex than that the earth should revolve on its own axis, and move in an orbit round the sun. All the appearances of the heavenly bodies can be explained with much greater ease and simplicity by the latter supposition than by the former; and Copernicus was thus led, after the study of forty years, to the adoption of a theory which had been advocated by Pythagoras and Thales of Greece, five or six hundred years before the time of Ptolemy. The frontispiece to this number is a representation of the solar system, according to the theory of Copernicus. In this system, the sun, the glorious source of light and heat to us, is placed in the centre. Round him the planets revolve in the following order :-Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Since the time of Copernicus, five more planets have been discovered, namely, Uranus, Pallas, Vesta, Juno, and Ceres. In order to account for the phenomena presented by the moon, Copernicus assumed, (what has since been confirmed,) that the moon has a twofold motion,-round the earth and round the sun: a small circle, therefore, surrounds the earth, which circle represents the path of the moon round the earth, while the earth and the accompanying moon together revolve round the sun. The frontispiece of course represents the Copernican system with the addition of the five planets since discovered. The circles represent the orbits of the several planets; that is, the paths in which they travel in their progress round the sun. The distances of these circles from the centre at which the sun is placed, could not conveniently be in the same proportion one to another, as the real distance of the planets is respectively from the sun, because the innermost orbits would be too small to be conveniently seen. The real proportions, however, which exist between the distances of the planets from the sun, admit of being easily understood. For instance, the earth, (as we shall hereafter explain more fully) is about ninety-five millions of miles from the sun. If now we call that distance 1, the distances of the other ten planets from the sun, will be represented by the following numbers, with sufficient nearness for our present purpose. Mercury Venus Earth ૐ Ceres Pallas} Jupiter. 2 |