UMB 



UND 



said to be a ratio of equality. The me- 

 thod of prime and ultimate ratios was in- 

 troduced by Newton, and the terms re- 

 late to the ratios of variable quantities 

 considered as receding from, or approach- 

 ing to, the ratios of the limits to which 

 they continually and simultaneously ap- 

 proach. 



ULTRAMARINE. A blue pigment 

 of great permanence, prepared entirely, 

 until lately, from the lapis lazuli or lazu- 

 lite. It is now made of silica, alumina, 

 soda, and sulphur. 



ULVA'CEiE {ulva, laver). A tribe of 

 cryptogamic plants belonging to the in- 

 articulate division of the order AlgacecB. 

 They comprise plants found in fresh and 

 salt water, with a membranous, reticu- 

 lated structure, and the reproductive 

 organs imbedded in the substance of the 

 plant. They are the lavers, the sea- 

 purse, the water-gut, &c. 



UMBEL {umbella, an umbrella). A 

 form of inflorescence, in which all the 

 pedicels of the flowers proceed from a 

 single point, and are of equal length or 

 corymbose. When each pedicel bears a 

 single flower, as in Eryiigium, the umbel 

 is said to be simple ; when each pedicel 

 divides, and bears other umbels, as in 

 Heracleum, the umbel is termed com- 

 pound. In the latter case, the assem- 

 blage of umbels is called the universal 

 umbel, each of the secondary umbels 

 being called the partial umbel. The 

 peduncles which support the partial um- 

 bels are called radii. 



VMBELLVFERJE (umbella, an umbel, 

 fero, to bear). The Umbel-bearing tribe 

 of Dicotyledonous plants. Herbaceous 

 plants, with leaves usually divided ; flowers 

 in umbels ; calyx entire or 5-toothed ; 

 petals 5, alternate with 5 stamens; ova- 

 rium didymous, with 2 styles and solitary 

 pendulous ovula. 



UMBER. An ore of iron and manga- 

 nese, occurring in beds with brown jasper 

 in the isle of Cyprus, and used as a 

 brown pigment. 



UMBILI'CUS (dim. of umbo, the boss 

 of a shield). 1. A term applied in con- 

 chology to the hollow axis of those spiral 

 shells in which the inner sides of the 

 whorls or volutions do not touch one an- 

 other. 2. In Botany, the term is syno- 

 nymous with hiium, and denotes the 

 scar where the seed is united with the 

 placenta. 3. The term has sometimes 

 been applied to the focus of an ellipse ; 

 but, in modern works, it signifies a point 

 of a surface through which all its lines of 

 352 



curvature pass. At such a point the two 

 principal curvatures are equal. 



UMBO. Literally, the boss of a shield ; , 

 hence applied, in concholoa[y, to that 

 point in a conchifer or bivalve shell 

 which constitutes the nucleus or apex 

 of each valve, and which is generally 

 situated above the hinge, and always 

 near it. 



UMBRA. A shadow; the shadow of 

 the earth or of the moon in an eclipse. 

 See Penumbra. 



U'NCIiE. The name given by the old 

 algebraists to the coeflBcients of the letters 

 in the expansion of any power of a bino- 

 mial. 



UNCONFORMABLE. A term applied 

 to a set of geological strata, when their 

 planes are not parallel to those of another 

 set which are in contact. See Conformable. 

 UNDE'CAGON {undecim, eleven, 

 ycovia, an angle). A barbarous term for 

 a plane figure bounded by eleven sides, 

 and, consequently, containing eleven 

 angles. Endecagon is classical. 



UNDER-SHRUB. Suffrutex. The 

 under-shrub differs from the shrub in 

 perishing annually, either wholly or in 

 part; and from the herb, in having 

 branches of a woody texture, which fre- 

 quently exist more than one year. It is 

 exactly intermediate between the shrub 

 and the herb ; such is the mignionette in 

 its native country, or in the state in 

 which it is known as the Tree Mignio- 

 nette. 



UNDETE'RMINED. This word is 

 applied in Mathematics to a number 

 which has not been determined, but is 

 capable of being determined ; whereas an 

 indeterminate number is one which can- 

 not be determined at all in the given 

 case. 



UNDULATED (undula, a little wave). 

 Wavy ; a term nearly synonymous with 

 sinuated, but more particularly applied 

 to the colouring of shells. 



UNDULA'TION {undula, a little 

 wave). A tremulous motion or vibration 

 observable in a fluid, whereby it alter- 

 nately rises and falls like the waves of 

 the sea. The constituent particles of all 

 bodies are usually held, by the operation 

 of certain internal and external forces, 

 in a state of equilibrium ; and, where 

 these particles have been to a certain ex- 

 tent disturbed by any cause, they return 

 to the state of equilibrium, by alternately 

 approaching the position of rest and re- 

 ceding from it, until, by a gradually de- 

 creasing recession, they attain the conr 



