THO 



THY 



the temperatures are so very high or so 

 very low that our ordinary pyrometers 

 and thermometers cannot be relied on ; 

 or, lastly, in places in which other kinds 

 of apparatus could not be used. The 

 strength of the current generated by the 

 heat in a thermo-battery measures its 

 temperature ; the intensity of the cur- 

 rent being estimated, as in hydro-electric 

 batteries, by the deflection of a magnetic 

 needle. 



THE'RMO PHONE {eepntr heat, <pu>vr], 

 sound). An apparatus, also called, from 

 its discoverer, Trevelyan's instrument, 

 for producing sounds from heated metals 

 when placed under certain circum- 

 stances. It consists of a metallic body, 

 generally of brass, which is first heated 

 and then laid on a cold block of lead, to 

 which it gradually imparts its heat ; 

 during the process of cooling, the former 

 body contracts in a manner by regular 

 pulsations, and in so doing emits musical 

 sounds, which cease entirely when the 

 temperatures of the two metals are 

 equalized. 



THERMOSCOPE (^fp/urj, heat, oko- 

 weo), to observe). The name of a parti- 

 cular kind of thermometer, which shows 

 or exhibits the changes of heat to the eye. 

 So, pijroscope is the name of a particular 

 kind of pyrometer. 



THERMOSTAT (^epMn, heat, iVrnjui, 

 to fix). Heat-governor. A self-acting 

 apparatus for regulating temperature, 

 constructed on the principle of the un- 

 equal expansion of metals by heat. 



THIN OUT. A term employed when 

 a geological stratum, in the course of its 

 prolongation in any direction, becomes 

 gradually less in thickness; the two 

 surfaces approach nearer and nearer, 

 and when, at last, they meet, the 

 stratum is said to thin out or disap- 

 pear. 



THIRD. An interval in music, classed 

 among the imperfect concords, from its 

 variable nature, as it may be either major 

 or minor. The ratio of the major third 

 is 5 : 4 ; that of the minor, 6 ; 5. The 

 former comprises one major and one 

 minor tone, as C E ; the latter, a major 

 tone and a semitone, as A C. 



THO'MPSONITE. A mineral of the 

 zeolite family, found near Kilpatrick. 



THORITE. A mineral found in 

 syenite, in Norway, containing about 

 58 per cent of thorina. 



THO'RIUM. A metal obtained from 

 a black mineral, called thorite, from the 

 coast of the North Sea, and named from 

 338 



the Scandinavian deity Thor. Thorina 

 is considered to be a protoxide. 



THOROUGH-BASE. A technical 

 term expressive of the art of playing 

 an accompaniment on keyed instru- 

 ments from figures representing chords, 

 and placed over or under the notes of 

 the instrumental base staff. 



THREE, RULE OF. The name given 

 to an arithmetical rule which teaches, 

 from three given quantities, to find a 

 fourth, bearing a certain relation to them. 

 The relation is, that the first quantity 

 shall bear the same proportion to the 

 second as the third to the fourth, or the 

 required quantity. The rule may there- 

 fore be briefly stated to be that by which 

 a fourth proportional is found to three 

 given quantities. 



Double Rule of Three. Questions may 

 arise in which, instead of 3 terms given 

 to find a fourth, we may have 5 given to 

 find a sixth. The rule in such cases 

 might be called the Rule of Five. " Write 

 the given quantities in two lines, keep- 

 ing quantities of the same sort under 

 one another, and those which are con- 

 nected with each other in the same line. 

 Draw a curve through the middle of 

 each line and the extremities of the 

 other. There will be three quantities on 

 one curve and two on the other. Divide 

 the product of the three by the product 

 of the two, and the quotient is the an- 

 swer to the question." De Morgan. 



THULITE. A mineral, usually granu- 

 lar, found in Norway, and consisting 

 principally of silica, alumina, and lime. 



THU'MERSTONE. A mineral found 

 in beds at Thum in Saxony. It is also 

 called axinite, from the axe-like form 

 and edge of its crystals. 



THUNDER. This phenomenon is 

 supposed to be occasioned by the sudden 

 rushing of the air into a partial vacuum 

 produced by the heat of lightning. For 

 its prolongation, various causes have 

 been assigned. 



THYRSUS. A form of inflorescence, 

 consisting of a compact panicle, the 

 middle branches of which are longer 

 than those of the apex or of the base, as 

 in lilac. The thyrsus has been also de- 

 fined as an inflorescence at first centri- 

 petal, afterwards centrifugal. 



THYSANO'URA {Ovaata, obsolete ; 

 from 0va>, to move rapidly, olpa, a tail). 

 The Spring-tails ; an order of insects 

 which jump by means of their tail. They 

 are wingless, and do not undergo meta- 

 morphosis. 



