been shewn, that when the ocean is el& vated at the zenith, Z, it is also elevated at the opposite point, or nadir, N, at the same time ; and therefore, in this situa tion of the sun and moon, the tides will be augmented. Again, whilst the full moon, F, raises the waters at N and Z, di rectly under and opposite to her, the sun S, acting in the same right line, will also raise the waters at the same points, Z and N, directly under and opposite to him ; and therefore, in this situation also, the tides will be augmented, their joint ef fect being nearly the same at the change as at the full; and in both cases they oc casion what are called the spring tides. Pursuing the illustration in the same way, let now F and T (fig. 6.) be the moon in her first and third quarters, and the rest as before. Then, since the sun and moon act in the right lines S H and F T, which are nearly perpendicular to each other, their forces will tend to produce contrary effects; because the one raises the waters in that part where the other depresses them. The sun's attraction at It and If will diminish the effect of the moon's attraction at Z and N ; so that the wa ters will rise a little at the points un der and opposite to the sun, and fall as much at the points under and opposite to the moon ; and of course the lunar tides will be diminished in those parts. This respects the moon only in her first quar ter, at F ; but the same reasoning will evidently hold, when applied to the moon in her third quarter, at T ; for as the sun and moon still act in lines which are per pendicular to each other, they must pro duce the same diminution as before ; and in both these cases they occasion what are called the neap tides. But it must be ob served, that neither the spring nor neap tides happen when the sun and moon have the precise situations here mentioned ; because, in this case, as in others of a si milar kind, the actions do not produce the greatest effect when they are the strongest, but some time afterwards. The effects of the disturbing forces of the sun and moon depend, likewise, upon their respective distances from the earth, as well as upon their particular situations. For the less the distances are, the great er will be their effects; and, consequent ly, in winter, when the sun is nearer to the earth, the spring tides will be greater than in summer, when he is farther off; and the neap tides, on that account, will be less. For a like reason, as the moon
moves in an elliptical orbit round • the earth, and is nearer to us at some times 1.
than at others, the tides will at those tunes be greater, and at the opposite points of her orbit less. Some variations likewise take place in consequence of the different declinations of the sun and moon at dif ferent times. For if either of these lumi naries were at the pole, it would occasion a constant elevation both there and at the opposite one, and a constant depression at the equator ; so that as the sun'and moon gradually decline from the equator, they lose their effect, and the tides become less; and when they are both in the equa tor, the tides of course become greater.
Astronomy is sometimes divided, in books, with respect to its different states, into " new" and " old." The former re fers to the art, as it stood under Ptolemy and his followers, with all the apparatus of solid orbs, epicycles, &c. &c. By new astronomy is meant the science, as it has been cultivated since the period in which Copernicus flourished. By that great man the constitution of the heavens was reduced to more simple, natural, and certain principles. The substance of the old astronomy is given by 'racquet, and of the new by Whiston, in his " Prelec tiones Astronomicx," published in 1707. The whole doctrine, both according to the ancients and moderns, is explained by Mercator, in his Institutiones Astron.
Having concluded this brief sketch of' a very important science, we shall refer to other articles, in which many subjects will be discussed, that usually find place in a treatise of astronomy. Under the word Sew will be found some interesting specu lations of Dr. Herschel ; under that of Moox, an account of the methods, of measuring its mountains, an explication of the harvest moon and horizontal moon. For equation of time, see TDIE ; see also EARTH, figure ECLIPTIC ; E au r NOTES, precession of ; GALAXY ; GRAVI TATION ; NEBULA: ; SATURN, ring of ; ASTRONOMY, practical ; and ASTRONO MICAL instruments : See OBSERVATORY; SATELLITES ; TRANSIT; &C. &C. &C.