476 



S1KKNIA 



S1IUUS 



tin- blast of air is broken up into a succession of 

 puffs, whose nuinlier per second U exactly equal t<> 

 tin 1 iiunilirr of holes which pa.-s in front of the 

 nozzle in one second. Knowing the rate of rotation 

 per second of the disc and the number of holes in 

 the circle that is lieiii}. 1 used, we can readily calculate 

 the frequency of the musical note produced (see 

 SOUND). Bv shifting the nozzle to bear upon a 

 different circle of holes we get a different note for 

 the same rate of rotation. 



In Cagniard de Latour's siren the Wast of air 

 whose interruption gives the note also drives the 

 siren. Fig. 1 shows the upper surface of the siren, 

 SS, resting on the cover of the wind-chest, AA. 

 In fig. 2 the instrument is shown in vertical 

 section through the line tin of fig. 1. The blast 

 enters by the pipe, BB. The cover of the wind- 

 chest is pierced with exactly the same number 



Ffc. 1. 



Fig. 2. 



of holes as the disc, the only difference being 

 that the holes pierced obliquely in both disc and 

 wind-chest cover are oppositely directed in their 

 obliqueness, as shown at a in fig. 2. When the 

 corresponding holes are in apposition the blast of 

 air is driven through all the holes, and by its 

 impact on the obliquely cut walls of the holes in 

 the disc forces the disc round. This brings the 

 unpierced part of the disc opposite to the holes in 

 the cover, and the air is cut off until the holes 

 again come into apposition. The stronger the 

 blast the more quickly will the disc be driven arid 

 the higher the note produced. A much more 

 powerful tone can lie obtained from this form of 

 siren than from Seebeck's, inasmuch as all the holes 

 are used simultaneously. Von Helmholtz's Double 

 Siren is a combination of two of these on the same 

 axis, each siren, by a suitable arrangement of con- 

 centric rings of holes which can ! closed or opened 

 at pleasure, being able to sound four distinct notes, 

 singly or simultaneously. It is an invaluable in- 

 strument for demonstrating the laws of beats and 

 combination tones (see SOUND). Other forms of 

 siren used in fog-signalling are descrilted under 

 LlMlTiiorsK. 



Sin-ilia, an order of aquatic mammals now 

 represented by the dugong (Halicore) and tin' 

 manatee (Manatiis). Another form, S teller's sea- 

 cow (Ithytinn stelleri), was exterminated during 

 the 18th century. In the Miocene and early 

 Pliocene seas there seem to have lieen abundant 

 Sirenians, many of which Moused to the genus 

 Halitherium. Ax the characteristics of the dugong, 

 the manatee, and Steller's sea-cow are descril>ed in 

 separate articles, it is enough to state here that 

 the Sirenians form a very distinct, order, that in 

 spite of their Hiiperlirial resemblance they have 

 certainly no near afliiiitie- with Cetaceans, that at 

 present we must he content to regard them as old- 

 fashioned mid, it is to be feared, moribund tynes, 

 occupying a lowly position in the Mammalian aeries. 



The name, which suggest* mermaids, seems to re NT 

 to the appearance of the dugong, when it raises its 

 head above the water or carries it- young one under 

 its arm. 



Siren*, sea-nymplis in Greek Mythology who 

 sat on the shores of an island between Circe's isle 

 and Seylla, near the south-western coast of Italy, 

 and sang with Switching sweetness songs that 

 allured the pa .inn sailor to draw near, but only to 

 meet with death. In Homer there are two, in later 

 writers three, Ligeia, I.enUosia, and Parthenope, or 

 Aglaopheme, Mol|>e, and Thelxiepeia. If any sea- 

 man could resist the enticements of their magic 

 music they themselves were doomed to die, but 

 Ulysses or the Argonaute alone succeeded in doing 

 so. In the Odyssey we read how Ulysses, by the 

 advice of Circe, Mopped the ears of his companions 

 with wax, and lashed himself to a mast, until he 

 had sailed out of hearing of the fatal songs. The 

 Argonaute got safely past because Orpheus pro- 

 tected them by the stronger spell of his own 

 singing, whereupon the sirens threw themselves 

 into the sea and were nan-formed into rocks. The 

 Latin poets give them wings, and in works of art 

 they are often represented as birds with the faces 

 of maidens, and are provided with musical instru- 

 ments. According to J. P. Postgatc (Cambridge 

 Journal of Philology, vol. ix.), the original mean- 

 ing of the word is 'bird.' In later days they are 

 represented on tombs as singers of dirges for the 

 dead, and more generally as syml>olising the magic 

 power of eloquence and song. Parallel conceptions 

 are the Mermaid (q.v. ) of western Europe and the 

 Lorelei (q.v.) of the Rhine. See Miss Harrison'* 

 Mytfis of the Odyssey in Art and Literature ( 1881 ). 



Sirhiml. a tract in the Punjab, l>eing the north- 

 eastern part of the plain between the Jumna and 

 the Sutlej, which is watered by the great Sirhind 

 Canal (main branch finished in 1882) and its 

 branches. Sirhind, which is not an administrative 

 division, contains live British districts (one being 

 Umballa) and nine native states (including Pali 

 ala). It is named from an ancient town in Patiala, 

 now in ruins. 



Sir-i-kol, a great lake of the Pamir (q.v.). 



Sir-i-|ml. a river, city (75 miles NE. of 

 Maimana; pop. 15,000), and district in Afghan 

 Turkestan. 



Siriiis. otherwise called Canicvla, or the Dog- 

 star, the brightest star in the heavens, is situated 

 in the constellation of Cants Major, or the 'Great 

 Dog.' Its distance in light-units is 8-6 (see STARS). 

 It has long lx>en known to possess a ' pror>cr motion ' 

 i.e. an independent progressive motion which 

 was for a time l>elieve< U to In- in a straight line, but 

 was in 1844 shown by Bessel to consist of an undula- 

 toiy progressive motion on each side of a middle line. 

 This motion was investigated by Professor Peters 

 of the Pulkowa Observatory, Russia, on the sup- 

 position that its anomalous character was produced 

 1>y the attraction of some unseen neighbour, and 

 his calculations were completed and verified b\ 

 Mr Safl'ord of Washington. In .January 1862 Mr 

 Alvan Clark of New York, chancing to ol*erve 

 Siritis through a powerful teleseope, detected a 

 minute star (which mid never before l>een olwerved) 

 situated at an angular distance of 7" from Sirius, 

 ami it is generalh l>elie\ed that this is the disturber 

 in question. By photometric measurement it has 

 lieen shown that, adopting tin 1 latest measures of 

 its distance. Sirius gives seventy times as much 

 light as our sun would at the same distance, and 

 has a moss three times that of the sun. The 

 Kgyptians called this star Sot his, and at one time 

 it- ' Heliacal Rising' (q.v.) was regarded as a sure 

 forerunner of the nsing of the Nile ; while among 

 the Romans it WON considered a star of evil omen. 



