sou 



SPE 



thecae developed on the under surface of 

 the fronds of Ferus. 



SO'THIAC PERIOD. A terra em- 

 ployed in the calendar of the Egyptians 

 and the Persians. Their year is supposed 

 to have comprised 365 days, so that every 

 four years they lost a day in the solar year, 

 and, after a period of 1460 years, called 

 the Sothiac Period, or great canicular 

 year, the civil and the solar years recom- 

 menced at the same time; or, in other 

 words, the canicula or dog-star would 

 then a^'ain rise heliacally at the be- 

 ginning of their year. The 365 days of 

 the year composed twelve months, each 

 of thirty days, and the five remaining 

 days were added under the name of 

 epagomence, or supplementary days. 



SOUND. A sensation produced through 

 the ear, by the vibrating or tremulous 

 motion of a sounding body. This mo- 

 tion is communicated to the surrounding 

 air, and is conveyed by this medium to 

 the drum of the ear, which also undergoes 

 a vibrating motion, and this last motion, 

 by throwing the auditory nerves into 

 action, produces the sensation of hear- 

 ing. 



1. Sound, waves of. Every vibration of 

 a sonorous body produces a progressive 

 wave in the air, which moves in a di- 

 rection perpendicular to the surface of 

 such body. These undulations are called 

 waves of sound, and every right line, per- 

 pendicular to their surface, or to th^t of 

 the sonorous body, is called a ray of 

 sound. It has been supposed that a 

 wave of vibration, " the sij^h of a drown- 

 ing slave,"— proceeds in constant undu- 

 lation through space, and a similar idea 

 has been applied to the radiations of 

 light from external objects. 



2. Sound, Musical. When a number 

 of sounds, or vibrations, succeed one 

 another with such regularity as to pro- 

 duce the impression of a single sound, 

 this is called a musical sound. The 

 quantity of a musical sound depends on 

 the extent and velocity of the vibrations, 

 and corresponds to the strength or in- 

 tensity of the sound. The quality of a 

 sound is independent of its quantity, and 

 relates to the different instruments by 

 which it is emitted, as by the human 

 voice, the cord of a violin, &c. Our 

 appreciation of the quality of sounds has 

 not been satisfactorily explained. 



SOUNDINGS. This term denotes, in 



hydrography, the depths of water in rivers, 



harbours, along shore, and even in the 



open seas. The term is also applied to the 



309 



nature of the ground at the bottom of the 

 water. 



SPA WATER. An acidulous chaly- 

 beate, containing more iron and carbonic 

 acid than any other mineral spring. 



SPACE {spatium). A general term for 

 extension in all directions. From the 

 idea of space follows that of form, which 

 is the conception of the manner in which 

 one part of space is separated from the 

 rest ; and from the investigation of forms 

 arises geometry. In this restricted sig- 

 nification, space denotes area, as when 

 it is said that two straight lines cannot 

 enclose a space, that is, an area. 



SPADIX. A form of inflorescence, in 

 which the flowers are arranged close to- 

 gether upon a succulent axis, which is 

 enveloped in a sheath, or spathe. This 

 mode of inflorescence is characteristic of 

 the natural orders Araceae and Acoraceae. 



SPANGLED ROD. A glass tube, on 

 the surface of which are pasted spangles 

 of tinfoil in a spiral form and at small 

 intervals. An electric spark transmitted 

 along such a conductor appears at the 

 same instant at all the intervals. 



SPANISH CHALK. Steatite, or 

 soapstone; a sub-species of rhomboidal 

 mica. 



SPAR (spath, German). A term fre- 

 quently applied to stones, the broken 

 surfaces of which present polished shi- 

 ning plates, placed over one another in 

 horizontal layers. This is the sparry 

 texture, and minerals of this kind are 

 generally called spathose. 



SPA'RID^. Sparoides. A family of 

 acanthopterygious fishes, named from 

 the genus sparus, or Gilt-head. The 

 palate is edentulous, but the jaws are 

 well furnished with teeth, and it is upon 

 the various modifications of the teeth 

 that the genera are formed. 



SPARRY ANHYDRITE Cube spar. 

 A sub-species of prismatic gypsum, found 

 in the salt-mines of Halle, &c. 



SPARRY IRON. Carbonate of iron, 

 found abundantly in limestone. It 

 affords an iron well suited for conver- 

 sion into steel. 



SPATHE' {(nrd0n, any broad plate, of 

 wood or metal ; a fiower-sheath). The 

 botanical term for a large bract, which 

 envelopes the spadix of the arum and 

 other plants. 



SPECIES. A logical species is a pre- 

 dicate which is considered as expressing 

 the whole essence of the individuals of 

 which it is affirmed. A predicable which 

 expresses only a part, is of two kinds : it 



