UNG 



UNI 



dition of repose. These movements are 

 called undulations, vibrations, or oscilla- 

 tions. 



1. Progressive undulation. In this 

 kind of movement, the undulation suc- 

 cessively traverses the different parts of 

 the hody; those particles which have 

 been immediately excited by the disturb- 

 ing cause communicating motion to those 

 "Which are next to them, and so on. In 

 this case the movement of the particles 

 is successive, so that the position they 

 assume at any particular moment during 

 the continuance of their motion, appears 

 to advance from one place to another. 



2. Stationary undulation. In this kind 

 of movement all the particles of a body 

 begin their undulations simultaneously, 

 and complete them at the same instant. 

 Even when the body is divided into 

 several vibrating portions, each of them 

 ceases to move independently of the rest, 

 the different parts of the body being 

 separated by points which remain 

 throughout in a state of rest. 



U'NDULATORY THEORY {undula, 

 a little wave). A theory for explaining 

 the nature of light. According to this, 

 light has no material existence, but its 

 phenomena are produced by the vibra- 

 tions, or undulations, of a subtle ethereal 

 fluid, diffused through all nature, and set 

 in motion by the presence of luminous 

 bodies. The phenomena of vision are 

 thus produced by pulsations of ether on 

 the retina of the eye, as those of sound 

 are by pulsations of air on the nerve of 

 hearing. See Emission. 



UNEQUALLY PINNATE. Impari- 

 pinnate. That modification of the pin- 

 nate leaf, in which an odd leaflet termi- 

 nates the petiole. 



UNGUICULA'TA {unguis, a claw). 

 Clawed animals ; a section of the Mam- 

 malia, comprising those which have the 

 digits armed with claws, but free for the 

 exercise of touch upon their under sur- 

 face. The feet of these animals are bifid 

 in the camel, multifid in the elephant, 

 the ape, &c. 



UNGUIS. The Latin term for a nail. 

 In Botany,! it denotes the lower part of a 

 petal which tapers conspicuously towards 

 the base, as in the pink. The upper part 

 is called the limb. The petal itself is 

 termed unguiculate. 



U'NGULA. A hoof; and, from the re- 

 semblance of this part of the animal to 

 that part of a cone which is separated from 

 the portion containing the vertex by an 

 oblique plane, such a solid is called an 

 353 



ungula, and rules for determining its con- 

 tent are given inbooks of mensuration. 



UNGULA'TA (ungula, a hoof). Hoofed 

 animals ; a section of the Mammalia, 

 comprising those species which have the 

 digits enclosed in hoofs, the under sur- 

 face not being left free for the exercise 

 of touch. The hoof is solidipedous in the 

 horse, bisulcate in the ox, the goat, &c. 



U'NIFORM. Though this word means 

 nothing more than "of one form," it has 

 a signification in mathematics which 

 might be better rendered by "of one 

 value," or "of one degree," when we 

 speak to the mathematical proficient. 

 But it is a convenience, though only an 

 accidental one. that the word does not 

 imply the idea of value absolutely. Pen. 

 Cycl. 



UNILO'CULAR {unus loculus, one 

 cell). The designation of shells which 

 are not divided into chambers ; of seed- 

 vessels which are not separated into 

 cells. 



UNIO'NID^. River Mussels or 

 Unios; a family of atrachian bivalves, 

 which unite the atrachian with the ma- 

 crotrachian mollusca. 



UNIPE'LTATES {una pelta, one 

 buckler). A family of stomapodous 

 crustaceans, including those in which 

 the carapace is composed of a single 

 shield-like plate. 



UNIPO'LAR. A term applied by Ehr- 

 man to substances of imperfect conduct- 

 ing power, which are capable of receiving 

 only one kind of electricity, when made 

 to form links in the voltaic chain. 



UNIT JAR. An apparatus contrived 

 by Mr. Harris for charging Leyden jars 

 with known proportions of electricity, the 

 quantity of electricity employed being 

 proportioned to the number of charges. 



UNIT OF MEASURE. A term ap- 

 plied in Geometry to a line by which an- 

 other is measured ; that is, which is ap- 

 plied to another line, in order to deter- 

 mine the number of times that the latter 

 contains it. The primary unit by which 

 angles and their corresponding arcs are 

 numerically expressed^ is the degree or 

 the ninetieth part of a right angle or of a 

 quadrant. 



UNIVALVES {unus, one, valva, fold- 

 ing-doors). A class of mollusks, whose 

 shell is composed of one piece, generally 

 with spiral volutions. 



UNIVERSAL PROPOSITION. A 

 proposition whose predicate is affirmed 

 or denied of the whole of the subject. 

 Thus, in the language of logic, we have 



