Pleasure 's the portion of th' inferior kind, BARBAULD. QUESTIONS.-1. What has the microscope done for us? 2. What is the appearance of grains of sand when examined by the eye, and by the microscope? 3. Mouldiness? 4. What is said of the green surface of standing water? 5. What is the appearance of animalcules in the infusions of pepper?-new hay? 6. What appearance has the edge of a lancet? 7. Sting of a bee? 8. Fine lawn? 9. Silk worm's web? LESSON 39. The Telescope and Telegraph. Sat'ellite, a small planet revolving round a larger, a moon. O'ral, delivered verbally, not written. No invention in the mechanic arts has ever proved more useful and entertaining than the production of the telescope; its utility both by sea and land is too well known to need observation; and without such assistance the science of astronomy must have been far short of its present state. A telescope is useful, not only for discovering those distant objects that are invisible to the naked eye, but for rendering more clear and distinct those that are discernible; it is constructed to act either by refraction or reflection. It is the sole business of all telescopes to enable the eye to see the object under a larger angle. For this purpose a new image of an object is produced by the object-glass of the telescope, and then this image is viewed by means of the eye-glasses. The first impression, conveyed to the mind by a telescope, is that of bringing the object nearer, which is only another mode of declaring that it is enlarged, or seen under a larger angle. To show objects in their natural posture, a telescope must have three eye-glasses. The two additional lenses simply give an erect position to objects. If you remove one of the eye-glasses from a common telescope, every thing will appear in an inverted position. The three eye-glasses have all their focal distances equal, and the magnifying power is found by dividing the focal distance of the object-glass by the focal distance of one of the eye-glasses. The two additional lenses are not necessary for astronomical telescopes; for no inconvenience arises from seeing the celestial bodies inverted. When very great magnifying power is required, telescopes are constructed with concave mirrors, and called reflecting telescopes. Mirrors are used in order to bring the image nearer the eye; and a lens or eye-glass is for the same purpose as in the refracting telescope, that is, to magnify the image. The Newtonian reflecting telescope consists of a tube, towards the end of which a concave mirror is placed. The reflected converging rays, before they reach the focus, are made to fall upon a plane mirror placed at an angle of forty-five degrees, and thus are thrown upwards to the focus of a convex lens fixed in the upper side of the telescope, through which the eye looks down on the image. In the telescopes made by Dr. Herschel there is but one mirror, which is placed at the lower end of the tube, with such an inclination, that the rays are brought to a focus and the image formed near the edge of the upper end of the tube. The image, therefore, is formed by only one reflection, and its brightness, when viewed through the lens is, on this account, greater than that in the Newtonian telescope. The head of the observer, when a large aperture is wanted, may be placed entirely at one edge of the tube, so as not to intercept any of the rays at the time of making an observation; but as the eye looks down the tube, the back must be turned to the object. Dr. Herschel's grand telescope is nearly forty feet long, and four feet ten inches in diameter. The concave polished surface of the great mirror is forty-eight inches in diameter, and it magnifies six thousand times. This noble instrument was, in all its parts, constructed under the sole direction of Dr. Herschel it was begun in the year 1785, and completed August 28th, 1789, on which day was discovered the sixth satellite of Saturn. The telegraph is a machine for communicating intelligence at a considerable distance, by making various signals, which have been previously agreed upon between two parties, to represent letters, words, or ideas. No machine for making signals can with propriety be called a telegraph, unless it is adapted to express a sufficient number of letters or words to form a complete language, and which can be made, therefore, to communicate any information which can be expressed by oral or written language. Less perfect sys tems of signals which extend only so far as to communicate intelligence of events which have been foreseen, and the appropriate signals, previously arranged, are called signal flags, signal lanterns, and signal guns or fires. Telegraphs have been constructed in various ways: What is called the English telegraph consists of six octagonal boards, each of which is poised upon an axis in a frame, and worked by means of ropes in the manner of bell-ropes, so that it can either be placed vertically, and appear with its full size to the observer at the nearest station, or it becomes invisible to him by being placed horizontally, so that the narrow edge alone is exposed, which from a distance cannot be seen. Six boards make thirty-six changes, by the most plain and simple mode of working; and they will make many more, if more were necessary; but as the real superiority of the telegraph, over all other modes of making signals, consists in its making letters, it is not necessary that the changes should be more than the letters of the alphabet, and the arithmetical figures. Telegraphs of this description are set up on eminences at the distance of eight, ten, or twelve miles; and a line of them, by repeating each other's signals, conveys a message at the rate of a hundred miles in about five minutes. A telescope for the use of the observer is fixed in the watch-tower of each station. QUESTIONS.-1. Of what advantage is the telescope? 2. Why does it seem to bring an object nearer? 3. What is said of the eye-glasses and the magnifying power of telescopes? 4. Why are mirrors used in reflecting telescopes? 5. Describe the Newtonian telescope. 6. Describe the telescope as made by Dr. Herschel. 7. His grand telescope. 8. What is a telegraph? 9. How is a proper telegraph distinguished from other machines for making signals? 10. Describe the English telegraph. 11. What is said of its number of changes? 12. At what rate will such telegraphs convey a message? 13. How may an idea of the Newtonian telescope be obtained by looking at fig. 27. LESSON 40. Astronomy. Locomotive, having the power of removing, or changing place. ASTRONOMY is the science which teaches the magnitudes and motions, distances, periods, and order of the celestial bodies. It is the boldest and most comprehensive of all our speculations. It is the science of the material universe considered as a whole. The wide-spreading firmament, while it lifts itself above all mortal things, exhibits to us that luminary, which is the light, and life, and glory of our world, and when this retires from our view, is lighted up with a thousand lesser fires, that never cease to burn, that never fail to take their accustomed places, and never rest from their slow, solemn, and noiseless march. Among the objects more immediately about us, all is vicissitude and change. Plants arise out of the earth, flourish awhile and decay, and their place is filled by others. Animals also have their periods of growth and decline. Even man is not exempt from the general law. Nations are like individuals, privileged only with a more protracted existence. The firm earth itself, the theatre of all this change, partakes in a degree of the common lot of its inhabitants, and the sea once heaved its waves where now rolls a tide of wealth and population. Situated as we are, in this fleeting, fluctuating state, it is consoling to be able to dwell upon an enduring scene, to contemplate laws that are immutable, an order that has never been interrupted, to fix, not the thoughts only, but the eye, upon objects that after the lapse of so many ages, and the fall of so many states, cities, human institutions, and monuments of art, continue to occupy the same places, to move with the same regularity, and to shine with the same pure, fresh, undiminished lustre. Astronomy is the most improved of all the branches of knowledge, and that which does the greatest credit to the human understanding. We have in this obtained the object of our researches. We have solved the great problem proposed to us in the celestial motions; and our solution is as simple and as grand as the spectacle itself, and is in every respect worthy of so exalted a subject. It is not the astronomer only, who is thus satisfied, but the proof is of a nature to carry conviction to the most illiterate and skeptical. Our knowledge, extending to the principles and laws which the author of nature has chosen to impress upon his works, comprehends the future; it resembles that which has been regarded as the exclusive attribute of supreme intelligence. We are thus enabled, not only to explain those unusual appearances in the heavens, which were formerly the occasion of such unworthy fears, but to forewarn men of their occurrence; and by predicting the time, place, and circumstances of the phenomenon, to disarm it of its terror. There is, however, nothing perhaps so surprising in this science, as that it makes us acquainted with methods, by which we can survey those bright fields on which it is employed, and apply our own familiar measures to the paths which are there traced, and to the bodies that trace them; that we can estimate the form, and dimensions, and inequa lities of objects so immense, and so far removed from the little scene of our labours. What would be the astonishment of an inhabitant of one of those bodies, of Jupiter for instance, to find that, by means of instruments of a few feet in length, and certain figures and characters still smaller, all of our own invention, we had succeeded in determining the magnitude and weight of this great planet, the length of its days and nights, and the variety of its seasons, that we had watched the motions of its moons, calculated their eclipses, and applied them to important domestic purposes? What would be our astonishment to learn, that an insect, one of those for instance which serve sometimes to illuminate the waters of the ocean, though confined by the exercise of its proper organs and locomotive powers, to the sphere of a few inches, had, by artificial aids of its own contriving, been able to extend its sphere of observation to the huge monsters that move about it; that it had even attempted, not altogether without success, to fathom the depth of the abyss, in which it occupies so insignificant a place, and to number the beings it contains? The first use of the telescope, about the commencement of the seventeenth century, opened a new and most brilliant era in the science of astronomy. The defect of the natural organ with respect to the objects of this science had never been recognised. We had gazed upon them without comprehending what we saw. We had cast a vacant eye over the splendid pages of this volume, as children amuse themselves with a book which they are unable to read. We had caught here and there a capital letter, or a picture, but we had failed to distinguish those smaller characters on which the sense of the whole depended. It is not the least of the advantages of this wonderful instrument, that it has taught us the importance of those means of improvement and enjoy |