SIGNS, or CONSTELLATIONS. Collections, or groups, of stars. Those of the zodiac are twelve, corresponding with the twelve months in the year. In the centre of these the ecliptic is situated. The sun appears to pass in succession through these signs; entering the first degree of Aries, which is accounted the first sign, about the 21st of March. SKY. That vast expanse, or space, in which the heavenly bodies are situated. Its blue appearance is supposed to arise from the particles of which the atmosphere is composed, possessing the property of reflecting the blue rays, in greatest abundance. SOLAR. Appertaining to, or governed by, the sun: as the solar system, the solar year, solar eclipses. SOLID. Not fluid. Having its parts connected so as to form a mass. Solid bodies, are not absolutely so, all undoubtedly containing pores, or spaces void of matter. SOLSTICES. The middle of summer and the middle of winter; those two points in the orbit of the earth, in which its poles point most directly towards the sun. SONOROUS BODIES. Those bodies which are capable of being put into a state of vibration, so as to emit sounds. SPECIFIC GRAVITY. The relative weight of bodies of different species, when the same bulk of each is taken. Water has been chosen as the standard for comparison. If we say that the specific gravity of a body is 6, we mean, that its weight is six times as great as that of a portion of water, exactly equal to it in bulk. SPECTRUM. That appearance of differently coloured rays, which is produced by the refraction of the solar ray, by means of a prism, is called the prismatic spectrum; it exhibits most distinctly, and beautifully, all the colours seen in the rainbow. SPHERE. A globe, or ball. SPHEROID. Spherical; a body approaching nearly to a sphere in its figure. The earth, is denominated an _oblate spheroid_; it not being an exact sphere, but flattened at the poles, so as to cause the polar diameter to be upwards of thirty miles less than the equatorial. Oblate, is the reverse of oblong, and means shorter in one direction, than in another. SPRING TIDES. Those tides which occur at the time of new, or of full moon. The tides then rise to a greater height than at any other period. SQUARE. A figure having four sides of equal length, and its angles all right angles. In numbers; the product of a number multiplied into itself; thus, the square of 3 is 9, and the square of 8 is 64. STAR. The _fixed_ stars are so called, because they retain their relative situations; while the planets, by revolving in their orbits, appear to wander amongst the fixed stars. SUBTEND. This term is applied to the measurement of an angle; when the lines by which it is bounded recede but little from each other, they are said to subtend; that is, to be contained under, a small angle. SUPERFICIES. The surface of any figure. Space extended in length and width.