Sir John Herschel, for instance, did not go to the Cape of Good Hope for the purpose of reporting on the inhabitants of the moon, although a certain hoax, that may be remembered, would imply this was expected of him on making so distant an expedition; but he went to ascertain the position of stars with greater accuracy than could be done here, the neighbourhood of the Equator being also more suitable as commanding both hemispheres. Great power is indeed necessary for many remote sidereal observations that form subjects of interest now, but the distinction must be remembered between such instruments as enable you to look into the crevices of the moon, or admire the rings of Saturn and divide stars into two or three separate parts, and such as mark down in the chronicles of time the minutest change of position, and thus afford work to those who would find out the cause of that change. All the perfections of the law of gravitation have been thus discovered often when phenomena have been for years thought inexplicable.

The mural circle is a nobly constructed instrument for this purpose; the distances of stars from each other are not measured by it as they are seen in the heavens at any given time; sufficient accuracy could never be obtained by such kind of measurement, for it is obvious that the plane of the measuring circle must in that case be constantly moved in order to bring it into coincidence with the plane of two particular stars. The mural circle, from its name, signifies that it is immovably fixed in a wall of stone, like a transit instrument, turning freely in the plane of the meridian. A star, therefore, can only be observed during its passage across the meridian. If the meridian altitude of a star is thus observed, and the moment of its crossing the meridian noted down, you have the exact position of that star ascertained. Each star is thus independently mapped down in the heavens, and any relative distances can thus be easily known. The curious method of ascertaining the exact meridian altitude may be described as follows, the principles being applicable of course to other instruments as well:-A telescope is fixed in the plane of this circle, enabling the observer to see the star clearly, as a point, without the confusion of light that surrounds a star when seen by the naked eye; smaller stars, also, than could otherwise be visible, may thus be dealt with. The field of vision, therefore, in this telescope includes the meridian, but some means are necessary to know precisely which is that line. For this purpose wires are inserted within the foci of the glasses, two or three horizontally, and one, to mark the centre or the meridian, perpendicularly. This wire gives the finest perceptible idea of the division of space. The observer then fixes the circle with the telescope as near as possible in the direction of the

star's meridian height. When the time approaches at which he knows the star will pass that line, he places his eye to the glass, his whole body being in perfect repose on couches screwed up to suit his position. He then sees the star enter his field of vision, running along with that steady and silent motion that pervades the universe, and is our truest idea of the inevitable nature of time. He then moves the delicately suspended, though weighty circle, till the centre horizontal wire coincides with the course of the star; in a few seconds the little traveller will reach the perpendicular wire, and then will have passed the meridian. The instant of this passage is noted by the chronometer close by, and its altitude fixed by the immediate clamping of the whole machine to its then position. The altitude is thus read off on the division of degrees at the edge of the circle. But an elaborate operation is necessary to do this, owing to the imperfection of all circles. The angular movement of the circle is measured, commonly in six different places, none of which will exactly correspond; a mean is, therefore, taken of these errors, and that is put down as the observation. These measurements are effected by means of the micrometer microscope at each of the six points alluded to. By this instrument a small portion of the circle is magnified very highly, and illuminated by a small lamp, and then divided within the microscope in its magnified state; no actual divisions marked on the circle could give any but very general measurements. The exactness of the observation depends on thus magnifying space, and then dealing with that ocular deception as if a real thing.

Amidst the many branches, however, of science treated in this book, we cannot dwell long on one. We therefore pass over a great variety of wonderful instruments that have to do with the general principles of optics, and proceed on our way to other subjects.

With regard to our own earth, if we deal with it as an astronomical body, we find many curious illustrations of the great laws of nature. It is well known that the earth is not quite a sphere, but is flattened at the poles into a spheroid. The reason of this is obvious, though some difference of opinion exists as to the most likely process by which that cause acted. Sir John Herschel is content with showing that, whether the earth has ever been in a hardened state as now, or once was molten, in either case its rotatory motion must have produced the present form. If the earth once was of soft materials, it is plain that the tendency of rotatory motion would be to spread out those parts more remote from the axis, and therefore possessing greater centrifugal force, proportionably contracting the poles. But he also takes the other case. Commence with the

earth hard and spherical, and surrounded with water, then set it whirling; the water following the principle just described will gather round the equator and leave the poles, which consequently will be the only dry land. Another process would then commence of a similar kind, but slower in operation. The action of the water would dissolve and wear off the portions of solid matter, against which it was driven, or on which it rested, and enable that solid matter to follow the water, and form continents at the equator, like sand-banks at the mouth of a river. Which of these actions is most consistent with geological facts, is not our present subject to consider; moreover, it has once before been discussed in the pages of this review. Be this as it may, the actual difference in the diameter of the earth from pole to pole, and from equator to equator, is about twentysix miles and a half, which, compared with the whole diameter, (7,899 miles in the lesser instance,) is so small that it would not be a perceptible departure from the sphere in the terrestrial globe.

Another great geographical phenomenon, which owes its existence to the earth's rotation, is that of the trade winds. The description of this, as given by Sir John Herschel, is most clear and interesting; but as its length forbids our extracting the whole, we will endeavour to give its substance in as few words as possible. The atmosphere on the surface of the earth, if in repose, must have the same velocity as that surface on which it rests, therefore its velocity depends on its distance from the earth's axis, which of course is greatest at the equator or lessens to nothing at the pole. Now the effect of the sun's heat at the equator is to rarify the air immediately under it and make it ascend; its place is supplied by colder air from the north and south, sucked in by the vacuum. This air, however, comes from localities which have not so great velocity from the earth's rotation as the ground of the equator; and as all matter retains its existing motion till something alters it, it following that until the surface at the equator gains entire hold of that air, it will be left behind, and therefore cause a wind contrary to the earth's motion. Again, the heated air will descend to the regions vacated by the process just described, and, still retaining some portion of its equatorial velocity, will move faster than the ground, and therefore cause a wind in the same direction as the earth's motion. In these several cases, the actual motion of the wind will be a compound between the eastern or western, and the northern or southern directions, arising from the causes mentioned. Particular parts of the earth will be in these various currents, as the position of the sun in the ecliptic makes it more or less vertical to them.

Such then is the principle, and if our readers will be

at the trouble to work this out, they will find that the trade winds are accounted for.

The construction of maps is an interesting subject, and more complicated than at first may appear. To make a globe is indeed simple enough if you have the power of obtaining correct observations, but in a map the spherical surface has to be flattened, which is done in various ways. 'In the orthographic 'projection every point on the hemisphere is referred to its 'diametral plane or base by a perpendicular let fall on it, so that 'the representation thus mapped on its base, is such as would actually appear to an eye placed at an infinite distance from it.' In this kind of projection the extremities will be crowded together and distorted, therefore it can only be used in representing small portions of the earth's surface. The stereographic projection is the perspective view of the concave surface of a hemisphere as seen from the antipodes of that hemisphere. Mercator's projection is altogether artificial, representing the sphere as it cannot be seen from any one point. In it the degrees of longitude and those of latitude bear always to each the same proportion; the equator is conceived to be extended into a straight line, and the meridians are straight lines at right angles to it. Instead therefore of the meridians converging at the pole, they are parallel, and the scale of the map increases in size as the pole is approached, the whole is thus out of all proportion, and is only correct for tracing portions of the earth at a time, as is required in navigation.

Having considered the earth's surface, let us now look to its motions in the expanse of heaven. The mind may almost be bewildered at the very thought of the multitude of these motions. From one astronomical correction to another we seem to be referred till a definite result would appear to be ever receding. In one sense this is true, but in another the more numerous these little varieties of motion are, the more do they resemble the effects of gravitation as witnessed among things close at hand. The real state of the case is most simple; every cause produces a result, and every variety in one motion produces irregularities in others ad infinitum, for all things depend one upon another.

There are two important motions that affect the position of the earth's axis, which as a general rule is considered stationary, and always pointing nearly to the polar star. The first is the precession of the equinoxes, the second the nutation of the axis. The precession of the equinoxes, or the moving of those points in the heavens, where the sun crosses the equatorial line, arises from a shifting in the earth's axis. The axis is inclined 23° 28′ to a line perpendicular to the orbit round the sun called the ecliptic. This inclination, however, is not always in the same

direction, for it performs a cycle round the above supposed perpendicular ever at the same angle. The cycle is not quickly accomplished, for it would occupy 25,868 years to go all round; but still, within the records of astronomy, there is a material difference in the point of the heavens to which our axis points.

'The visible effect of precession on the aspect of the heavens consists in the apparent approach of some stars and constellations to the pole and recess of others. The bright star of the Lesser Bear, which we call the pole star, has not always been, nor will always continue to be, our cynosure: at the time of the construction of the earliest catalogues it was 12° from the pole-it is now only 1° 24', and will approach yet nearer, to within half a degree, after which it will again recede, and slowly give place to others, which will succeed in its companionship to the pole. After a lapse of about 12,000 years, the star a Lyræ, the brightest in the northern hemisphere, will occupy the remarkable situation of a pole star approaching within about 5o of the pole.

'At the date of the erection of the Great Pyramid of Gizeh, which precedes by 3970 years (say 4000) the present epoch, the longitudes of all the stars were less by 55° 45' than at present. Calculating from this datum the place of the pole of the heavens among the stars, it will be found to fall near a Draconis; its distance from that star being 3° 44′ 25′′. This being the most conspicuous star in the immediate neighbourhood was therefore the pole star at that epoch. And the latitude of Gizeh being just 30° north, and consequently the altitude of the north pole there also 30°, it follows that the star in question must have had at its lower culmination, at Gizeh, an altitude of 26° 15′ 35′′. Now it is a remarkable fact, ascertained by the late researches of Col. Vyse, that of the nine pyramids still existing at Gizeh, six (including all the largest) have the narrow passages by which alone they can be entered, (all which open out on the northern faces of their respective pyramids,) inclined to the horizon downwards at angles as follows.

'Of the two pyramids at Abousseir also, which alone exist in a state of sufficient preservation to admit of the inclinations of their entrance passages being determined, one has the angle 27° 5', the other 26°.

At the bottom of every one of these passages therefore, the then pole star must have been visible at its lower culmination, a circumstance which can hardly be supposed to have been unintentional, and was doubtless connected (perhaps superstitiously) with the astronomical observation of that star, of whose proximity to the pole at the epoch of the erection of these wonderful structures, we are thus furnished with a monumental record of the most imperishable nature.'-Pp. 191–193.

The nutation of the earth's axis is another small perturbation which affects the whole mass of our globe, and consequently the apparent position of all the heavenly bodies. This motion and its combination with the previous one are thus described :

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