afternoon; only the hours must be reversed, as in figure 4, and the hypothenuse a g, of the gnomon agf, must make an angle with the dial-plane equal to the colatitude of the place. As the suu can shine no longer on this dial, than from six in the morning until six in the evening, there is no occasion for having any more than 12 hours upon it. See Ferguson's Mechanics.

DIAL, Equinoctial, is that described on an equinoctial plane, or a plane representing that of the equinoctial. Dials of this kind are usually distinguished into upper, which look towards the zenith, and lower, which respect the nadir. Now as the sun only illumines the upper surface of an equinoctial plane, while he is in our hemisphere, or on the northern side of the equator, an upper equinoctial dial will only shew the hour during the spring and summer season. And again, as the sun only illumines the lower surface of an equinoctial plane, while he is in the southern hemisphere, or on the other side of the equator, a lower equinoctial dial will only shew the hour in autumn and winter. To have an equinoctial dial therefore that shall serve all the year round, the upper and lower must be joined together; that is, it must be drawn on each side of the plane.

DIAMETER, in geometry, a right line passing through the centre of a circle; and terminated at each side by the circumference thereof: the chief properties of the diameter are that it divides the circumference of a circle into two equal parts; hence a method of describing a semi-circle upon any line assuming its middle point for the centre. The diameter is the greatest of chords. See CONIC SECTIONS.

DIAMETER, in astronomy. The diameters of the sun and planets are either apparent or real: the apparent diameters are such as they appear to the eye, and being accurately measured by an instrument, are found different in different parts of their orbits. The apparent diameter of the sun is found to vary from 32′ 38′′ in January when it is nearest to us; to 31' 33" in July when it is farthest from us. The apparent diameter of the moon varies from 29' 28" to 33′ 36": her real diameter is about 2180 miles in length. The apparent diameters of the planets when at their respective mean distances from the earth are as follow:

Mercury 11"

Venus

Jupiter 39" 58" Saturn 18'

Mars

27"

Herschel, 3" 54""

From these apparent diameters and the respective distances from the earth, the real diameters of the sun and planets have been determined in English miles, which are given in the following num

DIAMOND, this is the most valuable and hardest of gems, and though found of different shapes, and sometimes accidentally tinged to several colours ; yet it ever carries the same distinguishing characters, and is very evidently in all those states the same body. It is, when pure, perfectly clear and pellucid as the purest water, and is eminently distinguished from all other substances, by its vivid splendor, and the brightness of its reflections. It

is extremely various in shape and size, being found in the greatest quantity very small, and the larger ones extremely seldom met with; the largest diamond ever certainly known to have been found is that in the possession of the Great Mogul, which weighs 279 carats, and is computed to be worth £.779,244.

The diamond has certainly one proper and deter minate figure, into which it must naturally concrete, when in a state of rest, and impeded by no other accident in its formation: the true figure is an equilateral octahedron; and wherever it has concreted in a perfect manner, and without any interrupting accidents, it has always formed itself into this figure; and often in this its several surfaces are as bright as if polished by art: but, as in common salt, though its figure be pyramidal, yet very trifling accidents can determine it into cubes and parallelopipeds; so the diamond has often, in the state of formation, been thrown into two other figures, both also seeming regular ones; the one a prismatic columnar one of six angles, somewhat emulating the figure of crystal, the other an oblong quadrilateral column with two truncated ends: these seem the only regular figures of this gem; but besides these it is every day found in numberless other mis-shapen forms, often roundish, emulating the shape of pebbles, but full of small flat planes ór faces; frequently oblong, very often flat, and as often tapering, either from one end to the other, or else from the middle to both ends. It is common for diamonds to be too thick or deep for the extent of their surface, and there is a certain proportion of depth, beyond which the gem should not be al

VOL. II.