LIE 



LIG 



balance. The expression is applied by 

 astronomers to an apparent oscillation of 

 the moon, in consequence of which she 

 exhibits sometimes a little more, some- 

 times a little less, of one side or the 

 other. 



1. Lihration in Longitude. The phe- 

 nomenon by which the moon, during the 

 period of acceleration, exhibits on the 

 east some portion of her surface not 

 previously seen, whilst corresponding 

 parts disappear on the west. The converse 

 of this takes place during retardation. 



2. Lihration in Latitude. The pheno- 

 menon by which the moon, in conse- 

 quence of her axis of rotation being in- 

 clined to her orbit, and of its always 

 preserving its parallelism, turns each of 

 her poles to us alternately, and displays 

 to us the spots situated about it. 



3. Diurnal Lihration. The phenome- 

 non by which the moon, when on the 

 horizon, presents some parts the more 

 of one side, and a corresponding portion 

 the less of the other side. This is in 

 consequence of the spectator being re- 

 moved from the centre of the earth 

 (towards which the same hemisphere of 

 the moon is constantly turned) by the 

 length of the earth's radius. 



LICHE'NES. The Lichen tribe of the 

 Aphyllae, or leafless plants. Aerial, leaf- 

 less, perennial plants, spreading over 

 almost all dry surfaces, of trees, stones, 

 &c. ; reproductive organs are sporules 

 lying in thecae in the medullary sub- 

 stance, or separated cellules of the me- 

 dullary layer of the thallus. 



LFCHENIN. Lichen starch ; a pecu- 

 liar vegetable product, obtained from the 

 cetraria islandica, or liverwort ; it is said 

 to combine, like the alkalies, with acids, 

 but it does not form crystallizable salts 

 •with them. 



LICHTENBERG'S FIGURES. The 

 phenomena observed on discharging a 

 positive spark and a negative spark on 

 different parts of the same polished re- 

 sinous surface ; the two electricities will 

 be retained, in a latent state, on their re- 

 spective spots. If the surface be now 

 sprinkled with flowers of sulphur, the 

 powder will be attracted and retained by 

 the electricities; and, if the rest of the 

 powder be blown away, a radiant star- 

 like figure will be found at the positively 

 electrified spot, and a round clouded 

 speck at the spot which was negatively 

 electrified. 



LIE'VRITE. A pyroxenic mineral, 

 also called ilvaite and yenite, occurring in 

 197 



particularly perfect crystals, chiefly from 

 Elba. 



LI'GAMENT {ligo, to bind). An ex- 

 ternal substance, by which the two valves 

 of acephalous testacea are united, and 

 which, in fact, is the true hinge. The 

 internal part is generally composed of 

 another substance, and is called the car- 

 tilage. 



LIGHT. The agent of vision, or the 

 cause of those sensations and colours 

 which we refer to the eye, or sense of 

 seeing, as their source. It is distin- 

 guished into two kinds; viz. natural 

 light, proceeding from the sun and stars ; 

 and artificial light, proceeding from 

 bodies which are strongly heated, and 

 thus become incandescent. 



1. A ray is a single line of light, as it 

 comes from a luminous body. A beam 

 of light is a body of parallel rays. A 

 pencil of light is a body of diverging or 

 converging rays. Divergent rays are 

 such as come from a point, and con- 

 tinually separate wider apart, as they 

 proceed. Convergent rays are those which 

 approach each other, so as to meet at a 

 common point. Luminous bodies emit 

 rays, or pencils of light, in every direc- 

 tion, so that the space through which 

 they are visible is filled with them at 

 every possible point. 



2. Light barometrical. A luminous 

 appearance exhibited in the vacuum of 

 some barometers, when the mercury is 

 shaken in the dark— a phenomenon pr<>- 

 bably depending on electricity. 



LIGHT-EQUATION. Roemer ob- 

 served that the eclipses of the satellites 

 of Jupiter, which occurred at and about 

 the opposition of the planet, or its nearest 

 point to the earth, took place too soon 

 for his calculation ; whereas those which 

 happened when the earth was in the 

 part of its orbit most remote from Jupi- 

 ter, were always too late. The cor- 

 rection required in consequence of the 

 time employed by light in traversing 

 the solar system, is called the light-equch 

 tion. 



LIGHTNING. A phenomenon usually 

 referred to the accumulation of sensible 

 electricity in the atmosphere, and its 

 consequent neutralization : a strong elec- 

 tric spark traverses a stratum of air 

 intervening between oppositely electrified 

 clouds, or between a cloud and the earth's 

 surface. See Conductors Mel^allic. 



LI'GNIN {lignum, wood). The basis 

 of woody fibre— the most durable product 

 of vegetation. When heated in clos^ 

 K3 



