EQUATION OF TIME. 123 Civil months are those which are framed to serve the uses of life, and approach nearly to the quantity of astronomical months either lunar or solar; being made, with the exception of February, to consist of thirty and thirty-one days. To the days of a week, the Pagans gave the names of the sun, moon, and planets; and for the first two days and last day of our weeks, those names are still retained. A natural or solar day is the time which the sun takes in passing from the meridian of any place till it comes round to the same meridian again; or it is the time from noon to noon. A sidereal day is the time in which the earth revolves once about its axis. The rotation of the earth is the most equable and uniform motion in nature, and is completed in twenty-three hours, fifty-six minutes, and four seconds, for any meridian on the earth will revolve from a fixed star, to that star again in this time. Sidereal days, therefore, are all of the same length; but solar or natural days are not. The mean length of a solar day is twenty-four hours, but it is sometimes a little more, and sometimes less. The reason of the difference between the solar and sidereal day is, that as the earth advances almost a degree eastward in its orbit, in the same time that it turns eastward round its axis, it must make more than a complete rotation before it can come into the same position with the sun that it had the day before; in the same way, as when both the hands of a watch or clock set off together, as at twelve o'clock, for instance, the minute hand must travel more than a whole circle before it will overtake the hour hand, that is, before they will be in the same relative position again. It is on this account that the sidereal days are found to be, on an average, shorter than the solar ones by three minutes and fifty-six seconds. As a clock is intended to measure exactly twenty-four hours, it is evident that, when a solar day consists of more than twenty-four hours, it will not be noon by the sun till it is past noon by the clock; in which case the sun is said to be slow of the clock. But when a solar day consists of less than twenty-four hours, it will be noon by the sun before it is noon by the clock; and the sun is then said to be fast of the clock. Time measured by a clock is called equal or mean time, and that measured by the apparent motion of the sun in the heavens, or by a sun-dial, is called apparent time. The adjustment of the difference of time, as shown by a 124 EQUATION OF TIME. well-regulated clock and a true sun-dial is called the equation of time. Since the stars are found to gain three minutes and fifty-six seconds upon the sun every day, amounting in a year to one diurnal revolution, it follows that, in three hundred and sixty-five days as measured by the sun, there are three hundred and sixty-six days as measured by the stars. This regular return of the fixed stars to the meridian affords an easy method of determining whether our clocks and watches keep true time. For if through a small hole in a window-shutter, or in a thin plate of metal fixed for that purpose, it be observed at what time any star disappears behind a chimney or the corner of a building at a small distance; then if the star disappears the next night three minutes and fifty-six seconds sooner by the clock or watch than it did the night before, on the second night seven minutes fifty-two seconds sooner, and so on, it is a certain sign that the machine goes right; but if it does not observe this rule, it is evidently not accurate, and as the disappearing of a star is instantaneous, we may depend upon this information to half a second at most. QUESTIONS.-1. What is said of the form and commencement of the year among different nations? 2. What is an astronomical month? 3. Lunar month? 4. Solar month? 5. Civil month? 6. Solar day? 7. Sidereal day? 8. How does it appear that sidereal days are all of the same length? 9. Why is there a difference between the lengths of a solar and sidereal day? 10. When is the sun said to be slow of the clock? 11. Fast of the clock? 12. What is mean time? 13. Apparent time? 14. Equation of time? 15. What follows in consequence of the stars gaining upon the sun? 16. What is an easy method of determining whether clocks and watches keep true time? [NOTE. The inequality of solar days, as caused by the eccentricity of the earth's orbit, and the obliquity of the ecliptic, is clearly illustrated in Wilkins' Elements of Astronomy: the work has been recommended as containing a judicious selection and concise statement of the leading facts and principles of the science.] THE PLANETARY SYSTEM. 125 LESSON 57. The Planetary System. FAIR star of eve, thy lucid ray Directs my thoughts to realms on high; Mercury and Venus first appear, Then Mars is seen of fiery hue; Mars soon his revolution makes, In twice twelve months the sun surrounds; Jupiter, greater limit takes, And twelve long years declare his bounds. With ring of light, see Saturn slow, 126 THE PLANETARY SYSTEM The Georgium Sidus next appears, Six moons are his, by Herschel shown, And lo! by astronomic scan, Three stranger planets track the skies, Whence those successive worlds arise. Next Mars, Piazzi's orb is seen, Four years six months complete his round; And gild Palermo's favoured ground. Daughters of telescopic ray, Pallas and Juno, smaller spheres, Are seen near Jove's imperial way, Tracing the heavens in destined years. Comets and fixed stars I see, With native lustre ever shine; How great! how good! how dreadful! He, Oh! may I better know his will, And more implicitly obey; Be God my friend, my father still, From finite-to eternal day. MANGNALL. NOTE. The foregoing rhymes were made, probably, before Vesta was discovered, and some of the facts, relating to the other new planets, not so well ascertained as at present. Ceres is sometimes called Piazzi, after the discoverer. CHEMISTRY is an instructive, interesting, and valuable science. Within the last sixty years its empire has been wonderfully extended. There is scarcely an art of human life which it is not fitted to subserve; scarcely a department of human inquiry or labour, either for health, pleasure, ornament, or profit, which it may not be made in its present improved state, eminently to promote. To the husbandman this science furnishes principles and agents of inestimable value. It teaches him the food of plants, the choice and use of manures, and the best means of promoting the vigour, growth, productiveness, and preservation of the various vegetable tribes. To the manufacturer chemistry has lately become equally fruitful of instruction and assistance. In the arts of brewing, tanning, dyeing, and bleaching, its doctrines are important guides. In making soap, glass, pottery, and all metallic wares, its principles are daily applied, and are capable of a still more useful application, as they become better understood. Indeed, every mechanic art, in the different processes of which heat, moisture, solution, mixture, or fermentation is necessary, must ever keep pace in improvement with this branch of philosophy. To the physician this science is of still greater value, and is daily growing in importance. He learns from it to compound his medicines, to disarm poisons of their force, to adjust remedies to diseases, and to adopt general means of preserving health. To the student of natural history chemistry furnishes instruction at every step of his course. To the public economist it presents a treasure of useful information. By means of this science alone can he expect to attack with success the destroying pestilence, and to guard against other evils to which the state of the elements gives rise. And to the successful prosecution of numberless plans of the philanthropist, some acquaintance with the subject in question seems indispensably necessary. Finally, to the domestic economist this science abounds with pleasing and wholesome lessons. It enables him to make a proper choice of meats and drinks; it directs him to those measures with respect to food, clothing, and |