LESSON XLV. ATMOSPHERE. stion. WHAT is air? ILLUSTRATED swer. It is an elastic, invisible fluid, which surIs the earth. Of what, besides air, is the atmosphere composed? Is the atmosphere of the same density as we ascend from the It grows thinner or less dense. What is the estimated height of the atmosphere? What is the pressure of the atmosphere upon the earth? It is 816 times lighter than water. ASTRONOMY. Q. What is the apparent place of a planet? A. It is the place where it appears to be when s from the surface of the earth. Q. What is the true place of a planet? A. It is the place where it would appear to be if s from the centre of the earth, or centre of motion. Q. Where is the parallax of a heavenly body the greatest? A. They are divided into two kinds, diurnal a annual parallax. The pressure of the atmosphere is equal to a column of water, and Moon. t height? Thirty-three feet. Of what is air composed? Of oxygen and nitrogen gases. In what proportions? Twenty parts of oxygen to eighty parts of nitrogen. LESSON XLVI. REFRACTION. stion. WHAT is refraction? swer. It is the deviation of the rays of light from a ht line. What is astronomical refraction? It is the deviation of the rays of light in their ge through the atmosphere. What is the cause of this refraction? It is caused by the increase of the density of the sphere towards the earth. In what part of the heavens is the light of a body most refracted? In the horizon. What effect does this refraction have upon the sun at its rising ting? It makes the sun appear above the horizon when actually below it. (See Diagram.) Does this affect the length of the day? Q. What is annual parallax? A. It is the apparent difference in the situation of star as seen from the earth in opposite points of orbit. Q. Have the stars been observed to have any sensible parallax A. A few have been observed to have a small par lax of a part of a second. (NOTE-No parallax h been discovered in more than 30 or 40 of them.) Q. What is the cause of their having no appreciable parallax? A. Because they are at such an immense distan from us. Q. If the earth's orbit were a solid ring, how large would it appe when viewed from the nearest fixed star?. A. Not as large as a lady's finger ring. LESSON XLVIII. LIGHT AND HEAT. Question. WHAT bodies produce light? Q. Is light a substance thrown off from a luminous body, or is caused by a vibratory motion? A. It is probably caused by the undulations of a extremely subtle fluid. Q. In what direction are the rays of light thrown off from a lum It makes the day from six to ten minutes longer, nous body? sun rise to sun set. Is the light of a body refracted when it is in the zenith? What is twilight? A. In straight lines, and in all directions. Q. With what velocity does light move? A. By observing the eclipses of Jupiter's moons. Q. In what proportion do the light and heat of the planets increas or decrease? A. In inverse proportion to the squares of their dis tances from the sun.. Q. Which planet has the most light and heat, and which the least A. Mercury has the most, and Leverrier the least. Q. If a board a foot square be placed one foot from a lighted candle how many feet square would the shadow be upon the wall, nine fee from the candle? A. Nine feet square, or eighty-one square feet. Q. What amount of light and heat would fall upon the one foot and upon the 81 feet? A. The same amount of light and heat would fall upon each. EFRACTION, PARALLAX LIGHT & HEA THE REFRACTION DECREASES FROM THE HORIZON TO THE HORIZON I SOL DOJILD WOULD APPEAR CARRASER THAN A LADY'S FINGER RING, WHEN SEEN FROM LESSON ILLUSTRATED ASTRONOMY. XLIX.. STRIAL AND CELESTIAL GLOBES. AT is a globe? globe is a round body or sphere. kinds of globes are there used in astronomy?. rrestrial and celestial globes. the terrestrial globe represent? ents the earth. drawn upon the surface of the terrestrial globe? ats, islands, mountains, oceans, seas, rivers, gdoms, empires, &c. the celestial globe represent? ents the heavens as seen from the earth. Q. What angle expresses the right ascension? A. The angle between the meridian passing through the body, and the one passing through the first point of Aries. Q. How far is right ascension reckoned? A. 360 degrees, or quite round the heavens. Q. What are circles of latitude on the celestial globe? A. It is its distance north or south of the ecliptic, measured on a circle of celestial latitude. Q. What is the longitude of a heavenly body? A. It is its distance east of the first point of Aries, measured on the ecliptic. Q. What angle expresses the longitude? A. The angle between the circle of latitude passing through the body, and the one passing through the first oup of stars, to which is applied the name point of Aries. l or object. e number of constellations? hree. the terrestrial globe, where is the observer supposed arface. the celestial globe, where must the observer suppose ed? entre, looking towards the heavens. (Inut.) e galaxy or milky way? Q. Where is this angle formed? A. At the poles of the ecliptic, where the circles of latitude intersect each other. Q. How far is celestial longitude reckoned? LESSON LI. Question. WHAT is a vertical circle? Answer. It is a great circle in the heavens, passing minous belt forming a complete circle in through the zenith and padir, and cutting the horizon at right angles. Q. What vertical circle is the meridian? A. It is that vertical circle, which passes through the north and south points of the horizon. Q. Which is the prime vertical? A. The vertical circle which passes through the east and west points of the horizon. Q. What are measured on the vertical circles? A. Altitude and zenith distance. Q. What is the zenith distance of a heavenly body? A. It is its distance from the zenith. Q. To what are the altitude and zenith distance always equal? A. They are equal to 90 degrees. Q. What is the azimuth of a heavenly body? A. It is its distance east or west of the meridian. Q. What angle expresses the azimuth? A. The angle between the meridian and the vertical circle passing through the body. Q. What is the amplitude of a heavenly body? A. It is its distance north or south of the prime vertical. A. The angle between the prime vertical, and the vertical circle passing through the body. Q. Where are the angles expressing azimuth and amplitude formed? A. At the zenith where the vertical circles intersect each other. Q. On what circle are these angles measured? Q. To what is the sum of the azimuth and amplitude always equal? [The diagram can be used to illustrate azimuth, amplitude, altitude, and zenith distance, by supposing the ecliptic to represent the celestial horizon, and the circles of celestial latitude to represent vertical circles.] |