EXPLANATION OF SCIENTIFIC TERMS. 



19 



the weight of the atmosphere from that 

 part of the well which is covered with 

 the tube, leaving it to press on the other 

 parts of the surface, is also, metaphori- 

 cally, termed suction. 



TABA SHEER. This substance, which 

 has been long famous as a medicine in 

 many parts of the East, is, originally, a 

 transparent fluid in the jointed cavities 

 of the bamboo cane. This fluid thickens, 

 spontaneously, until, by degrees, it is 

 converted into a white, or a bluish white 

 solid, something like a small fragment of 

 a shell. It is almost wholly composed of 

 Silica (the material of common flint), but 

 is easily crumbled between the fingers. 

 By imbibing water, it becomes transpa- 

 rent : the white bits in a low degree, but 

 the bluish nearly as much so as glass. 

 See OPTICS, p. 62. Similarly formed con- 

 cretions are not uncommon. Humboldt 

 discovered Tabasheer in the bamboos of 

 South America ; and Sir Joseph Banks 

 found " a solid pebble, about the size of 

 half a pea, so hard as to cut glass," 

 in a green bamboo that was reared in a 

 hothouse at Islington 1 



TANGENT, TANGENTIAL. A straight 

 line touching, but not cut- Fig 13. 

 ting, a curve is termed a 

 Tangent, from the Latin 

 tangere, to touch. Thus 

 AB, Jig. 13, which 

 touches the circle D E 

 C F at C, is a tan- 

 gent at that point ; and, 

 in mechanics, a force acting upon a 

 wheel in the direction of the line 

 AB, is said to be Tangential. It is 

 in this direction that motion is com- 

 municated between wheels and pinions, 

 or from one wheel to another. A plane 

 which touches a curvilineal solid is, in 

 like manner, termed a tangential plane. 

 It is this tangential plane from which we 

 measure the angles of incidence on the 

 point C, whether the impinging ray be 

 reflected or refracted. See Reflexion, 

 and Refractive Power. 



TANGENT of an Angle. See Angle. 



TELESCOPE, ACHROMATIC. See 

 Achromatic. 



TEMPERATURE. The temperature of 

 a body is the comparative degree of active 

 heat accumulated in that body, as mea- 

 sured by an instrument, or generally, by 

 its effects on other bodies. 



TETRAHEDRON. See Rhombus. 



THERMOMETER (from two Greek 

 words, signifying a measurer of heat) is 

 an instrument which serves to compare 

 the degree of active heat existing in other 

 bodies. This comparison is made by 

 marking the effect of the heat ; and gene- 

 rally by its power in expanding a fluid, 

 confined in a glass tube hermetically 

 sealed. The fluid is chiefly contained in 

 a bulb, and rises, or falls, in a very nar- 



row tube, supplied from the fluid in the 

 bulb, which contained fluid expands or 

 contracts when immersed in, or applied 

 to, the body whose heat is to be measured. 

 The upper part of the tube is freed from 

 air, to allow the fluid to expand. The 

 fluid capable of sustaining the greatest 

 degree of heat without boiling, is mer- 

 cury. Higher degrees are measured by 

 other instruments called Pyrometers 

 (Greek for fire-measurers} ; but these 

 are all very imperfect, so much so, that a 

 good pyrometer is yet a desideratum. 



THERMOMETER, CENTIGRADE, 

 FAHRENHEIT'S AND REAU- 

 MUR'S See those several articles. 



TORRICELLIAN VACUUM is the 

 vacuum at the top of the column of mer- 

 cury in a barometer, and so called from 

 Torricelli, the inventor of that instru- 

 ment. 



TOURMALINE. This stone is hard 

 enough to scratch glass, and becomes elec- 

 tric by heat. It is of various colours and 

 forms ; it is transparent when viewed 

 across the thickness of a crystal, but 

 perfectly opaque when turned in the op- 

 posite direction. 



TRANSVERSE DIAMETERS. See Co- 

 nic Sections. 



VACUUM (Latin vacuus, empty) is lite- 

 rally an empty place, but is "generally 

 used to denote the interior of a close 

 vessel, from which the atmospheric air 

 and every other gas has been extracted. 

 See Torricellian. The vacuum produced 

 by means of an air-pump is always im- 

 perfect ; the vessel is, nevertheless, 

 termed an Exhausted Receiver. 



VALVE, a close lid affixed to a tube, or 

 opening in a vessel, by means of a hinge, 

 or some other sort of moveable joint, and 

 which can be opened only in one direc- 

 tion. 



VAPOUR is any liquid expanded into an 

 elastic, or gaseous fluid, by means of 

 heat. It differs from gas in its want of 

 permanency ; for it returns into the li- 

 quid state, when exposed to a diminished 

 temperature. 



VELOCITY is the comparative celerity of 

 motion in a moving body. 



VERNIER. A Vernier' (so called from 

 the name of its inventor) is a small move- 

 able scale, running- parallel with the fixed 

 scale of a quadrant or other instrument, 

 and having the effect of subdividing the 

 divisions of that instrument into more 

 minute parts. See Scale. 



Let A G, fig. 14, be any proportion of 

 the limb (or circular part) of a qua- 

 drant : for example, half a degree divided 

 into six parts, A B, B C, &c. of five 

 minutes each. I^et H I be another limb 

 of equal extent, divided into five parts. 

 In consequence of the relation of these 

 divisions, we see that the line/ b will be 

 further advanced than N by a fifth part ; 

 C 2 



