were at rest, and all its motion communicated to the other in an opposite direction.*

Hence, if two bodies move alike, they will, when seen from each other (without reference to other near bodies, but only to the starry sphere), appear at rest. Hence, also, if the absolute motions of two bodies be uniform and rectilinear, their relative motion is so also.

(80.) The stars are so distant, that as we have seen it is absolutely indifferent from what point of the earth's surface we view them. Their configurations inter se are identically the same. It is otherwise with the sun, moon, and planets, which are near enough (especially the moon) to be parallactically displaced by change of station from place to place on one globe. In order that astronomers residing on different points of the earth's surface should be able to compare their observations with effect, it is necessary that they should clearly understand and take account of this effect of the difference of their stations on the appearance of the outward universe as seen from each. As an exterior object seen from one would appear to have shifted its place were the spectator suddenly transported to the other, so two spectators, viewing it from the two stations at the same instant, do not see it in the same direction. Hence arises a necessity for the adoption of a conventional centre of reference, or imaginary station of observation common to all the world, to which each observer, whereever situated, may refer (or, as it is called, reduce) his observations, by calculating and allowing for the effect of his local position with respect to that common centre (supposing him to possess the necessary data). If there were only two observers, in fixed stations, one might agree to refer his observations to the other station; but, as every locality on the globe may be a station of observation, it is far more convenient and natural to fix upon a point equally related to all, as the common point of reference; and

This proposition is equivalent to the following, which precisely meets the case proposed, but requires somewhat more thought for its clear apprehension than can perhaps be expected from a beginner :

PROP.-If two bodies, A and B, be in motion independently of each other, the motion which B seen from A would appear to have if A were at rest is the same with that which it would appear to have, A being in motion, if, in addition to its own motion, a motion equal to A's and in the same direction were communicated to it.

this can be no other than the centre of the globe itself. The parallactic change of apparent place which would arise in an object, could any observer suddenly transport himself to the centre of the earth, is evidently the angle C S P, subtended at the object S by that radius C P of the earth which joins its centre and the place P of observation.

Fig. 10.

P

S

CHAPTER II.

TERMINOLOGY AND ELEMENTARY GEOMETRICAL CONCEPTIONS AND RELATIONS.-TERMINOLOGY RELATING TO THE GLOBE OF THE EARTH-TO THE CELESTIAL SPHERE.-CELESTIAL PERSPECTIVE.

(81.) SEVERAL of the terms in use among astronomers have been explained in the preceding chapter, and others used anticipatively. But the technical language of every subject requires to be formally stated, both for consistency of usage and definiteness of conception. We shall therefore proceed, in the first place, to define a number of terms in perpetual use, having relation to the globe of the earth and the celestial sphere.

(82.) DEFINITION 1. The axis of the earth is that diameter about which it revolves, with a uniform motion, from west to east; performing one revolution in the interval which elapses between any star leaving a certain point in the heavens, and returning to the same point again.

(83.) DEF. 2. The poles of the earth are the points where its axis meets its surface. The North Pole is that nearest to Europe; the South Pole that most remote from it.

(84.) DEF. 3. The earth's equator is a great circle on its surface, equidistant from its poles, dividing it into two hemispheresa northern and a southern; in the midst of which are situated the respective poles of the earth of those names. The plane of the equator is, therefore, a plane perpendicular to the earth's axis, and passing through its centre.

(85.) DEF. 4. The terrestrial meridian of a station on the earth's surface, is a great circle of the globe passing through both poles and through the plane. The plane of the meridian is the plane in which that circle lies.

(86.) DEF. 5. The sensible and the rational horizon of any station have been already defined in art. 74.

(87.) DEF. 6. A meridian line is the line of intersection of the

plane of the meridian of any station with the plane of the sensible horizon, and therefore marks the north and south points of the horizon, or the directions in which a spectator must set out if he would travel directly towards the north or south pole.

(88.) DEF. 7. The latitude of a place on the earth's surface is its angular distance from the equator, measured on its own terrestrial meridian: it is reckoned in degrees, minutes, and seconds, from 0 up to 90°, and northwards or southwards according to the hemisphere the place lies in. Thus, the observatory at Greenwich is situated in 51° 28′ 40′′ north latitude. This definition of latitude, it will be observed, is to be considered as only temporary. A more exact knowledge of the physical structure and figure of the earth, and a better acquaintance with the niceties of astronomy, will render some modification of its terms, or a different manner of considering it, necessary.

(89.) DEF. 8. Parallels of latitude are small circles on the earth's surface parallel to the equator. Every point in such a circle has the same latitude. Thus, Greenwich is said to be situated in the parallel of 51° 28′ 40′′.

(90.) DEF. 9. The longitude of a place on the earth's surface is the inclination of its meridian to that of some fixed station. referred to as a point to reckon from. English astronomers and geographers use the observatory at Greenwich for this station; foreigners the principal observatories of their respective nations. Some geographers have adopted the island of Ferro. Hereafter, when we speak of longitude, we reckon from Greenwich. The longitude of a place is, therefore, measured by the arc of the equator intercepted between the meridian of the place and that of Greenwich; or, which is the same thing, by the spherical angle at the pole included between these meridians.

(91.) As latitude is reckoned north or south, so longitude is usually said to be reckoned west or east. It would add greatly, however, to systematic regularity, and tend much to avoid confusion and ambiguity in computations, were this mode of expres sion abandoned, and longitudes reckoned invariably westward from their origin round the whole circle from 0 to 360°. Thus, the longitude of Paris is, in common parlance, either 2° 20′ 22′′ east, or 357° 39′ 38" west of Greenwich. But, in the sense in which we shall henceforth use and recommend others to use the

term, the latter is its proper designation. Longitude is also reckoned in time at the rate of 24 h. for 360°, or 15° per hour. In this system the longitude of Paris is 23h. 50m. 384s.*

(92.) Knowing the longitude and latitude of a place, it may be laid down on an artificial globe; and thus a map of the earth may be constructed. Maps of particular countries are detached portions of this general map, extended into planes; or rather, they are representations on planes of such portions, executed according to certain conventional systems of rules, called projections, the object of which is either to distort as little as possible the outlines of countries from what they are on the globe-or to establish easy means of ascertaining, by inspection or graphical measurement, the latitudes and longitudes of places which occur in them, without referring to the globe or to books-or for other peculiar uses. See Chap. IV.

(93.) DEF. 10. The Tropics are two parallels of latitude, one on the north and the other on the south side of the equator, over every point of which respectively, the sun in its diurnal course passes vertically on the 21st of March and the 21st of September in every year. Their latitudes are about 23° 28' respectively, north

and south.

(94.) DEF. 11. The Arctic and Antarctic circles are two small circles or parallels of latitude as distant from the north and south poles as the tropics are from the equator, that is to say, about 23° 28'; their latitudes, therefore, are about 66° 32'. We say about, for the places of these circles and of the tropics are continually shifting on the earth's surface, though with extreme slowness, as will be explained in its proper place.

(95.) DEF. 12. The sphere of the heavens or of the stars is an imaginary spherical surface of infinite radius, having the eye of any spectator for its centre, and which may be conceived as a ground on which the stars, planets, &c., the visible contents of the universe, are seen projected as in a vast picture.†

* To distinguish minutes and seconds of time from those of angular measure we shall invariably adhere to the distinct system of notation here adopted (°'", and h. m. s.). Great confusion sometimes arises from the practice of using the same marks for both.

The ideal sphere without us, to which we refer the places of objects, and which we carry along with us wherever we go, is no doubt intimately connected by association, if not entirely dependent on that obscure perception of sensation in the retina of our eyes, of which, even when closed and unexcited, we cannot en