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persection of that instrument, the greater, in a very high proportion, is the number of the stars, which may be observed. Galileo found 80 stars in the belt of Orion's sword. De Rheita counted 188 in the Pleiades, and more than 2000 in the constellation of Orion, of which only 78 are visible to the naked eye. The bxed stars, as seen through a telescope, are found to be collected in clusters. When a small magnifying power is used, these clusters appear like small light clouds, and hence have been called nebulæ. Dr. Herschel has given a catalogue of more than 2000 nebulæ, which he has discovered. When these nebulæ are examined with a telescope of great magnifying power, they are foond to consist of immense multitudes of stars. Dr. Herschel is of opinion, that the starry heaven is replete with these nebulæ : that each nebula is a distinct and separate system of stars; and that each star is the sun or centre of its own system of planets. That bright, irregular zone, which we call the Milky Way, he has very carefully examined, and concludes that it is the particular nebula to which our sun belongs. In examining it, in the space of a quarter of an hour, he has seen the astonishing number of 116,000 stars pass through the field of view of a telescope of only 15' aperture ; and, in 41 minutes, he saw 258,000 stars pass through the field of his telescope. It is probable that each nebula in the heavens is as extensive, and as well furnished with stars, as the milky way; that many nebulæ, within the reach of the telescope, have not yet been discovered ; and that very many more lie beyond its reach, in the remote regions of the universe. If this be true, the number of 75,000,000, which La Lande assigned, as the whole number of the fixed stars, will be seen to fall far short of the truth.
The distance of the fixed stars, however, is so great, that their bumber will, probably, never be calculated with certainty. The diameter of the earth's orbit is 190,000,000 miles. Of course, when the eye is placed at one end of this diameter, it is so much nearer given stars, than when at the opposite end. Yet this immense distance makes no apparent difference in the size of any of them,nor any difference in their relative situations. The distance of the nearest fixed star is estimated to be more than 5,000,000,000,000 miles from us, a distance which a cannon ball, moving at the rate of 480 miles an hour, would not pass over in less than 1,180,000 years. Astronomers generally, however, have calculated the distance of the nearest fixed star, at 400,000 times the diameter of the earth's orbit.
The real magnitudes of the fixed stars are not known. In astronomical calculations they are generally supposed to be equal to that of the sun.
With regard to their nature we can make nearer approaches to certainty. We know that they shine by their own light, because if they borrowed their light from any large luminous body which was near them, that body would itself be visible. They resemble the sun in several other particulars. Many of them are observed to revolve on an axis ; to have spots on their surface, and changeable spots, too, like those of the sun. Hence they are very
fairly concluded to be suns, each one a centre of light, and warmth, and motion for its own system of planets.
THE GLOBES, AND THEIR USE.
A globe is a round body, whose surface is every where equally remote from the centre. But by the globes, sometimes called artificial globes, is here meant two spherical bodies, whose convex surfaces are supposed to give a true representation of the earth and the apparent heavens. One of these is called the terrestrial, the other the celestial globe. On the convex surface of the terrestrial globe, all the parts of the earth and sea are delineated in their relative size, form, and situation.
On the surface of the celestial globe, the images of the several constellations and the unformed stars are delineated ; and the relative magnitude and position, which the stars are observed to have in the heavens, are carefully preserved.
In order to render these globes more useful, they are fitted up with certain appurtenances, whereby a great variety of useful problems are solved in a very easy and expeditious manner.
The brazen meridian is that ring in which the globe hangs on its axis, represented by two wires passing through its poles. The circle is divided into four quarters of 90 degrees each ; in one semicircle the divisions begin at each pole, and end at 90 degrees on the equator, where they meet. In the other semicircle, the divisions begin at the equator, and proceed thence toward each pole, where they end at 90 degrees. The graduated side of this brazen circle serves as a meridian for any point on the surface of the earth, the globe being turned about till that point come under it.
The hour circle is a small circle of brass, divided into 24 hours, the quarters and half
quarters. It is fixed on the brazen meridian, with its centre over the north pole ; to the axis is fixed an index, that points out the divisions of the hour circles as the globe is turned round its axis. Sometimes the hour circle, with its divisions, is described or marked about the north pole on the surface of the globe, and is made to pass under the index. In some of Adams's globes, the equator is used as an hour circle, over which is placed a semicircular wire, carrying two indices, one on the east side of the brazen meridian, and the other on the west.
The horizon is represented by the upper surface of the wooden circular frame encompassing the globe about its middle. On this wooden frame there is a kind of perpetual calendar, contained in several concentric circles. The inner one is divided into four quarters of 90 degrees each ; the next circle is divided into the 12 months, with the days in each according to the new style ; the next contains the 12 equal signs of the zodiac or ecliptic, each being divided into 30 degrees; the next the 12 months and days according to the old style ; and there is another circle, containing the 32 points of the compass, with their halves and quarters. Although these circles are on most horizons, yet they are not always placed in the same order.
The quadrant of altitude is a thin slip of brass, one edge of which is graduated into ninety degrees and their quarters, equal to those of the meridian. To one end of this is fixed a brass nut and screw, by which it is put on and fastened to the meridian; if it be fixed in the zenith, or pole of the horizon, then the graduated edge represents a vertical circle passing through any point of the horizon, to which it is directed.
Beside these, there are several circles, described on the surfaces of both globes; as the equator, ecliptic, circles of longitude and right ascension, the tropics, polar circles, parallels of latitude and declination, on the celestial globe; and on the terrestrial, the equator, ecliptic, tropics, polar circles, parallels of latitude, hour circles or meridians, to every 15 degrees; and on some globes, the spirat Thumbs flowing from several centres, called flies.
In using the globes keep the graduated side of the meridian towards you, unless the problem require a different position. With respect to the terrestrial, we are to suppose ourselves situated at a point on its surface; with respect to the celestial, at its centre. The motion of the former represents the real diurnal motion of the earth; that of the latter, the apparent diurnal motion of the heavens.
The following PROBLEMS, as being most useful and entertaining, are selected from a great variety of others, which are easily solved with a terrestrial globe, fitted up with the aforesaid appurtenaaces.
1. The latitude of a place being given, to rectify the globe for that
Let it be required to rectify the globe for the latitude of Boston, 42 degrees 23 minutes north.
Elevate the north pole, till the horizon cut the brazen meridian in 42 23, and the globe is then rectified for the latitude of Boston, Bring Boston to the meridian, and you will find it in the zenith, or directly on the top of the globe. And so of any other place.
II. To find the latitude and longitude of a place on the globe.
Bring the given place under that half of the graduated brazen meridian, where the degrees begin at the equator, and under the graduated side of it; then the degree of the meridian over it shows the latitude ; and the degree of the equator, under the meridian, shows the longitude.
Thus Boston will be found to lie in about 42 23 north latitude, and 71 west longitude from Greenwich,
III. To find the sun's place in the ecliptic,
Look the day of the month on the horizon, and opposite to it, you will find the sign and degree the sun is in that day. Thus on
the 25th of March, the sun's place is 4} degrees in Aries. Then look for that sign and degree in the ecliptic line marked on the globe, and you will find the sun's place; there fix on a small black patch, so is it prepared for the solution of the following problems.
Note. The earth's place is always in the sign and degree opposite to the sun ; thus, when the sun is 4 degrees in Aries, the earth is 41 degrees in Libra; and so of any other.
IV. To find the sun's declination, that is, its distance from the equi
noctial line, either northward or southward.
Bring its place to the meridian ; observe what degree of the meridian lies over it, and that is the declination. If the sun lie on the north side of the line, the declination is north, but if on the south side the declination is south.
Thus on the 20th of April the sun has 11 degrees of north declination, but on the 26th of October, it has 124 of south declination,
Note. The greatest declination can never be more, either north or south, than the distance of a tropic from the equator.
V. To find where the sun is vertical on any day.
Bring the sun's place to the meridian, observe its declination, or hold a pen or wire over it; then turn the globe round, and all those countries which pass under the wire, will have the sun vertical, or nearly so, that day at noon. Thus on the 16th day of April, the inhabitants of the north part of Terra Firma, Porto-Bello, Philippine Isles, southern parts of India, Abyssinia, Ethiopia, and Guinea, have the sun over their heads that day at 12 o'clock.
Note. This appearance can only happen to those who live in the torrid zone.
VI. To find at any hour of the day, what o'clock it is at any place.
Bring the place where you are, to the brass meridian ; set the index to the hour, turn the globe till the place you are looking for come under the meridian, and the index will point out the time required.
Thus when it is 10 o'clock in the morning, at Boston, it is 24 minutes past 12 at Olinda in Brazil, and 8 at Mexico in NewSpain; the former being at 35 degrees west longitude, and the latter at 100 degrees west longitude.
Note. By this problem you may likewise see at one view, in distant countries, where the inhabitants are rising, where breakfasting, dining, drinking tea, where going to assemblies, and where to bed.
VII. To find at what hour the sun rises and sets any day in the year
at a place, the latitude of which does not exceed 661 degrees ; and also on what point of the compass it rises and sets.
Rectify the globe for the latitude of the place ; bring the sun's place to the meridian, and set the index to 12; then turn the sun's place to the eastern edge of the horizon, and the index will point out the hour of rising ; if you bring it to the western edge of the horizon, the index will shew the hour of setting.
Thus on the 10th day of April, the sun rises at half an hour after five o'clock at Boston, and sets half an hour before seven.
Note. In summer the sun rises and sets a little to the northward of the east and west points; and in winter a little to the southward of them. If, therefore, when the sun's place is brought to the eastern and western edges of the horizon, you look on the horizon directly against the little patch, you will see the point of the compass on which the sun rises and sets that day.
VIII. To find the length of the longest and shortest day at a given
Rectify the globe for that place; if its latitude be north, bring the beginning of Cancer to the meridian; set the index to 12, then bring the same degree of Cancer to the east part of the horizon, and the index will show the time of the sun's rising, which doubled, gives the length of the shortest night.
If the same degree be brought to the western side, the index will show the time of the sun's setting, which doubled will give the length of the longest day.
If we bring the beginning of Capricorn to the meridian, and proceed in all respects as before, we shall have the length of the longest night and shortest day.
Thus in Egypt and Florida the longest day is 14 hours, and the shortest night 10 hours. The shortest day is 10 hours, and the longest night 14 hours.
At Petersburg, the capital of Russia, the longest day is about 194 hours, and the shortest night 4+ hours. The shortest day 4} hours, and the longest night 19 hours.
Note. In all places near the equator, the sun rises and sets at 6 o'clock, through the year. Thence to the polar circles, the days increase as the latitude increases ; so that at those circles the longest day is 24 hours, and the longest night the same. From the polar circles, to the poles, the days continue to lengthen into weeks and months ; so that at the pole, the sun shines for six months together in summer, and is below the horizon six months in winter. Note also, that when it is summer with the northern inhabitants, it is winter with the southern, and the contrary; and every part of the world partakes of an equal share of light and darkness.