CONJUNCTION. When three of the heavenly bodies are in a straight or right line, if you take either of the extreme bodies, the other two are in conjunction with it; because a straight line drawn from it, might pass through the centres of both, and join them together. At the time of new moon, the moon and sun are in conjunction with the earth; and the moon and earth, are in conjunction with the sun. CONSTELLATION, OR SIGN. A collection of stars. Astronomers have imagined pictures drawn in the heavens, so as to embrace a number of contiguous stars, and have named the group after the animal, or other article supposed to be drawn; an individual star is generally designated by its fancied location; as upon the ear of _Leo_, the Lion, &c. CONVERGENT RAYS, are those which approach each other, so as eventually to meet in the same point. CRYSTALS. Bodies of a regular form, having flat surfaces, and well defined angles. Nitre, and other salts, are familiar examples. Many masses of matter, are composed of crystals too minute to be discerned without glasses. CURVILINEAR, consisting of a line which is not straight, as a portion of a circle, of an oval, or any curved line. CYLINDER. A body in the form of a roller, having flat circular ends, and being of equal diameter throughout. DEGREE. If a circle of any size be divided into 360 equal parts, each of these parts is called a degree. One quarter of a circle contains ninety degrees; one twelfth of a circle, thirty degrees. The actual length of a degree, must depend upon the size of the circle. A degree upon the equator, upon a meridian, or any great circle of the earth, is equal to 69-1/2 miles. Straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bearing no relationship to the degrees upon a circle. DENSITY. Closeness of texture. When two bodies are equal in bulk, that which weighs the most, has the greatest density. DIAGONAL. A line drawn so as to connect two remote angles of a square, or other four-sided figure. DILATATION. The act of increasing in size. Bodies in general, dilate when heated, and contract by cooling. DISCORD. When the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is produced, which is called discord. DIVERGENT RAYS. Those which proceed from the same point, but are continually receding from each other. DIVISIBILITY. Capability of being divided, or of having the parts separated from each other. This is called one of the _essential properties_ of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &c. as the largest mass of matter. ECHO. A sound reflected back, by some substance, so situated as to produce this effect.