THR 



714 



THU 



THORN'BACK. A flsh, the Raia clatata, 

 Cuv. and Yarr. ; the maiden skate of 

 Scotland. It grows to about two feet 

 long, is very voracious, feeding on every 

 kind of small flounder, herrings, sand- 

 eels, and crustaceous animals, as crabs 

 and lobsters. It is common in some of 

 the British rivers and on the coasts. 



THO'ROCGH. Among builders. See PER- 



BEND. 



THO'ROCGH-BASE, ) In music, the basso 



THO'ROUGH-BASS. } continue of the Ita- 

 lians, and the accompaniment of the 

 Trench. Thorough base on the harpsi- 

 chord, organ, &c., consists in the execu- 

 tion of a complete and regular harmony, 

 by seeing only the notes of one part of 

 the harmony ; and this part is called the 

 base, being in reality the basis of the 

 whole composition. This base is played 

 with the left hand, and the harmony with 

 the right. 



THRALI,. The Saxon word for slave: 

 whence thraldom, slavery. 



THRAVE. Threave of corn. Twenty four 

 sheaves or four shocks (ttooks in Scotland;, 

 of six sheaves to the shock. 



THREAD, Sax. thred, thraed. 1. A small 

 line made up of a number of fibres of 

 some vegetable or animal substance, such 

 as flax, cotton, or silk , whence its name 



of linen, cotton, and silk thread. 2. 



The filament of any fibrous substance, as 



of bark ; the filament of a flower. 3. 



The prominent spiral part of a screw. 



THRIPS. The name given by Linnaeus 

 to a genus of homopterous hemiptera, 

 from fl<-4/, a moth, from Tg<, to bore. 

 The species live on flowers, plants, and 

 under the bark of trees. 



THROAT'WORT. In botany, (1.) The 

 throatwort of Britain is a species of the 

 bell-flower, the Campanula trachelium, a 

 perennial of several varieties; (2.) The 

 trachelium of two species. &TRACHE- 



I.IUM. 



THROS'TLE. 1. In cotton-spinning, the ma- 

 chine otherwise called the water-frame, 

 because it requires considerable power to 

 put it in motion, and could only be 

 worked, before the application of the 

 steam-engine as a moving power, in such 

 factories as had water-power. It takes 

 the name throstle from the peculiar noise 

 (like the singing of a throstle or thrush !), 

 which it makes in working. The yarns 

 spun by it are much harder than those 

 spun by the jenny, and on that account it 

 is better adapted to the spinning of 

 warps. It is now, however, in a great 



measure superseded by the mule. 2. In 



ornithology, the song-thrush or mavis 

 (Tutdus wtustcus, Lin.), the finest of our 

 native singing birds. 



Taau* 'TLE- MTE. In tttam-enpims. a 

 vije contrived to regulate the supp/y of 

 M&oi ui the cylinder, it u brought into 



operation by the action of the governor, 



and takes its name from its enlarging or 



diminishing the throat of the engine so as 



to allow a wider or narrower p;>- 



the steam, as a greater or less velocity is 



required. 



THRUM'MINO. A nautical term, signify- 

 ing the inserting in a sail, mat, &c., 

 through small holes made by a bolt-rope, 

 needle, or a marlin-spike, a number ol 

 small pieces of rope or spun-yarn. 



THRUSH. 1. In ornithology, see TUR- 



DUS. 2. In nosology, small ulcerations 



which appear first in the mouth, but 

 often affect the alimentary duct: techni- 

 cally called aphtha;. 



THUGS. A numerous class of professed 

 assassins and robbers among the Hin- 

 doos. They form a society, proceed upon 

 fixed principles, and are with difficulty 

 detected. 



THUM'MIM. A Hebrew word denoting 

 perfections. See URIM and THUMMIM. 



THCN'BEB, Sax. thunder, Pers. thondhor. 

 The sound which follows an explosion or 

 discharge ojf electricity in the atmosphere. 

 The snapping noise which the electric 

 spark makes in passing through a por- 

 tion of the atmosphere, is due to the sud- 

 den compression of the air ; and there 

 can be no doubt that the awful thunder- 

 clap itself is produced by the same action. 

 The report is in this instance modified by 

 a variety of circumstances, such as dis- 

 tance, echo, &c. ; and the sudden dying 

 away and return of the sound may be ac- 

 counted for on well-known principles. 

 Sound travels in air at a velocity of only 

 1130 feet in a second, but light at the 

 rate of 195,000 miles in the same period of 

 time. The time in which the flash of 

 lightning reaches us from the different 

 parts of its course may therefore be taken 

 as instantaneous ; but the time which 

 the explosion occupies will be very ap- 

 preciable, and will vary with the dis- 

 tance of the several parts of the long zig- 

 zag which the discharge traverses. By 

 a calculation, founded upon the interval 

 between the flash and the sound, and the 

 duration of the thunder-clap, it has been 

 found that a flash of lightning frequently 

 traverses a space of nine or ten miles, and 

 when we take into account the irregular 

 course which it follows, its alternate ap- 

 proach and recession will account for the 

 phenomena in question. Such would b 

 the effect produced upon an observer, 

 placed at the end of a long file of soldiers, 

 who were to discharge their muskets at 

 the same instant. He would not hear a 

 single report but a succession of reports, 

 which would produe an irregular rolling 

 strcnd. 



THUN'DER-BOLT. 1. The lightning of a 



el'ctnc discharge in the atmosphere. 



2. lu rtutxralogy, thunder-bolt* are cry*- 



