from the equator, being always at the same distance from the poles that the tropics are from the equator. The northern is called the Arctic circle, and the southern the Antarctic circle.

16. Meridians are imaginary great circles drawn through the poles of the worid, cutting the equator and the equinoctial at right angles.

Every place on the Earth, and every corresponding point in the heavens, is considered as having a meridian passing through it; although astronomers apply but 24 to the heavens, thus dividing the whole concave surface into 24 sections, each 15° in width, These meridians mark the space which the heavenly bodies appear to describe, every hour, for the 24 hours of the day. They are thence sometimes denominated Hour Circles. In measuring distances and determining positions on the Earth, the equator and some fixed meridian, as that of Greenwich, contain the primary starting points; in the heavens these points are in the ecliptic, the equinoctial, and that great meridian which passes through the first point of Aries, called the equinoctial colure.

17. Latitude on the Earth, is distance north or south of the equator, and is measured on the meridian.

Latitude in the Heavens, is distance north or south of the ecliptic, and at right angles with it.

Longitude on the Earth, is distance either east or west from some fixed meridian, measured on the equator.

Longitude in the Heavens, is distance east from the first point. of Aries, measured on the ecliptic.

18. Declination is the distance of a heavenly body either north or south of the equinoctial, measured on a meridian.

Right Ascension is the distance of a heavenly body east from the first point of Aries, measured on the equinoctial.

It is more convenient to describe the situation of the heavenly bodies by their declination and right ascension, than by their latitude and longitude, since the former oorresponds to terrestrial latitude and longitude.

Latitude and declination may extend 90° and no more. Terrestrial longitude may extend 180° either east or west; but celestial longitude and right ascension, being reckoned in only one direction, extend entirely round the circle, or 360°.

It is easy to convert right ascension into time, or time into right ascension, for if a heavenly body is one hou: in passing over 15°, it will be one fifteenth of an hour, or four minutes, in passing over 1'.

If the first point of Aries be on the meridian at 12 o'clock, the next hour line, which is 15° E. of it, will come to the meridian at 1 o'clock; the second hour line at 2 o'clock; the third at 3, &c. Of any two bodies whose right ascensions are given, that one will pass the meridian first which has the least right ascension.

19. In consequence of the Earth's motion eastward in its orbit, the stars seem to have a motion westward, besides their apparent diurnal motion caused by the Earth's revolution on its axis; so that they rise and set sooner every succeeding day by about four minutes, than they did on the preceding. This is

16. Meridians? How many? What other name? How measure distances on the earth? In the heavens? 17. What is latitude on the earth? In the heavens? Longitude on the earth? In the heavens? 18. Declination? Right ascension Why describe by D. and R. A.? Extent of latitude? Declination? Longitude and R. A? How Convert R. A. into time? Which of two bodies given will first pass the meri. Jian? 19. What a parent motion of stars? Cause? Results?

called their daily acceleration. It amounts to just two hours a month. On this account we have not always the same constellations visible to us throughout the year. While some, that were not visible before, are successively rising to view in the east, and ascending to the meridian, others sink beneath the westerr. horizon, and are seen no more, until, having passed through the lower hemisphere, they again reappear in the east.

DESCRIPTION AND USE OF THE MAPS.

20 THE first map of the atlas represents, upon a large scale, a general view of the solar system. This will be more fully described in the second part of the work.

The next six maps represent different sections of the concave surface of the heavens. The first of these exhibits the principal constellations visible to us in October, November, and December; the second, those visible in January, February, and March; the third, those visible in April, May, and June; and the fourth, those visible in July, August, and September; with the exception, however, of the constellations which lie beyond the 50th degree of north and south declination, of which, indeed, those around the North Pole are always, and those around the South Pole, never visible to us.

21. These constellations are represented on the sixth and seventh maps, called circumpolar maps, which are an exact continuation of the others, and if joined to them at their corresponding degrees of right ascension and declination, they might be considered as constituting one map. The scale on which all the above-mentioned maps are drawn is that of a 16-inch globe. The lines drawn on the maps have been already defined; and their use, being nearly the same with those in geography, will be readily understood. Those which are drawn from right to left, on each side of the equinoctial and parallel to it, are called Parallels of Declination. Those which are drawn up and down through the maps, at intervals of 15°, are called Meridians of Right Ascension, or Hour Circles.

The scale at the top and bottom of the first four maps, and in the circumference of the circumpolar maps, indicates the daily progress of the stars in right ascension, and shows on what day of the month any star will be on the meridian at 9 o'clock in the rvening.

20. What said of maps? First? Next six? Describe lines? Scale indicates what?

21. Sixth and seventh? Scale?

22. The first four maps of the heavens are so constructed that the pupil in using them must suppose himself to face the south, and to hold them directly overhead in such manner that the top of the map shall be towards the north, and the bottom towards the south; the right hand side of the map will then be west, and the left-hand east. In using the circumpolar maps he must suppose himself to face the pole, and to hold them in such a manner that the day of the given month shall be uppermost.

The constellation called the Great Bear is an exception to this rule; in this constel. lation the principal stars are marked in the order of their right ascension.

That point of projection for the maps which would exhibit each successive portion of the heavens directly overhead at 9 o'clock in the evening, was chosen, because in sum. mer at an earlier hour the twilight would bedim our observation of the stars, and at other seasons of the year it is easier to look up to stars that want an hour of their meridian altitude than to those which are directly overhead.

CLASSIFICATION OF STARS, NEBULÆ, &c.

23. FOR purposes of convenience in finding or referring to particular stars, recourse is had to a variety of artificial methods of classification. First, the whole concave of the heavens is divided into sections or groups of stars, of greater or less extent, called Constellations.-(Of the origin of these figures see page 143). Next, they are classified according to their magnitudes, (as already stated art. 4), and designated on the maps accordingly. Thirdly, the stars of each constellation are classified according to their magnitudes in relation to each other, and without reference to other constellations. Thus, for instance, the largest star in Taurus is marked a, Alpha; the next largest 6, Beta; the next, y, Gamma, &c., till the Greek alphabet is exhausted. Then the Roman (or English) is taken up, and finally, if necessary, recourse is had to figures.

This useful method of designating particular stars by the use of the Greek and Roman alphabet, was invented by John Bayer, of Augsburg, in Germany, in 1603. It has been adopted by all succeeding astronomers, and extended by the addition of the Arabic notation 1, 2, 3, &c., wherever the stars in a constellation outnumber both alphabets.

As Greek letters so frequently occur in catalogues and maps of the stars and on the celestial globes, the Greek alphabet is here introduced for the use of those who are unacquainted with it. The capitals are seldom used for designating the stars, but are here given for the sake of regularity.

22. How use the first four maps of the heavens? Circumpolar? What exception? What point of projection chosen, and why? 28. Classification or designation of stars? By whom invented, and when?

24 As a further aid in finding particular stars, and especially in determining their number, and detecting changes, should any occur, catalogues of the stars have been constructed, one of which is over two thousand years old. Several of the principal stars have specific names, like the planets, as Sirius, Aldebaran, Regulus, &c.

25. The stars are still further distinguished, as single, double, triple, multiple, binary, variable, new, and nebulous.

A single star is one that appears as a unit under the most powerful telescopes. Double, triple, and multiple stars, are those that appear single to the naked eye, but by the aid of telescopes are found to consist of two or more stars. Binary stars are double stars revolving around each other, often called Binary Systems. Variable stars are those that are found to undergo certain fluctuations in their brightness, sometimes becoming quite invisible. In most cases these changes are periodical and regular, on which account they are called Periodical stars. New stars are those that suddenly blaze forth in some portion of the heavens previously void. Nebulous stars are those which are surrounded by a faint nebula, or halo of light or mist.

26. A cluster of stars is an assemblage or group, thrown promiscuously together, like the Pleiades and Hyades in Taurus, and the Bee Hive in Cancer. A Nebula is a cluster so remote as to appear only like a faint cloud or haze of light. Resolvable Nebula, are those that can be resolved into distinct stars by the aid of a telescope. Irresolvable Nebula are those that have not

24. What further aid? Age? Names of stars? 25. Stars, how further distia guished? Single stars? Double, &c.? Binary? What other name? Variable stars. What other name and why? New stars? Nebulous? 26. What are clusters? Nebu beat? Resolvable Nebula? Irresolvable? Annular? Planetary?

as yet been thus resolved. Annular Nebula are those that have the form of an annulus or ring. Planetary Nebula are those that resemble planets in form, and in the sharpness of their outline. Stellar Nebula are those with a star in the centre, the same as nebulous stars, already described (25).

A more detailed account of the double stars, clusters and nebulæ, will be given after the student has become somewhat familiar with the constellations.

27. We may now imagine the pupil ready to begin the study of the visible heavens. The first thing of importance is to fix upon the proper starting point. This, on many accounts, would seem to be the North Polar Star. Its position is apparently the same every hour of the night throughout the year, while the other stars are continually moving. Many of the stars also in that region of the skies never set, so that when the sky is clear, they may be seen at any hour of the night. They revolve about the pole in small circles, and never disappear below the horizon.

On this account they are said to be within the circle of perpetual apparition. On the other hand, the identity of the North Polar Star, strange as it may appear, is not so casily determined by those who are just entering upon this study, as that of some others. For this reason, the point directly overhead, called the zenith, is preferable, since upon this point every one can fix with certainty in whatever latitude he may be. It will be alike to all the central point of the visible heavens, and to it the pupil will learn imperceptibly to refer the bearing, motion, and distances of the heavenly bodies.

That meridional point in each map, whose declination corresponds with the latitude of the place of observation, represents the zenith of the heavens at that place; and those constellations of stars which occupy this position cn the maps, will be seen directly overhead at 9 o'clock in the evening of the day through which the meridian passes. Thus in Georgia, for instance, the starting point should be those stars which are situated in this meridian near the 33d degree of north declination, while in New England it should be those which are situated in it near the 42d degree.

28. We might, however, begin with the stars near either of the meridians represented on the maps, the only rule of selection being to commence at that which approaches nearest to being overhead at the time required. We have chosen for our starting point in this work that meridian which passes through the vernal equinox at the first point of Aries, not only because it is the meridian from which the distances of all the heavenly bodies are measured; but especially because the student will thus be enabled to observe and compare the progressive motion of the constellations according to the order in which they are always arranged in catalogues, and also to mark the constellations of the Zodiac passing overhead as they rise one after another in their order, and to trace among them the orbits of the Earth and of the other planets.

27. What first important in commencing study of the heavens? What star would seem best starting point? Why? Why not the best? What point preferable, and why? Illustration from map. 28. With what stars might we begin? What meridian

hosen by the author? Why?