72 J IK A VEKS. 



Thermo-Dynam'ics (Gr. thermc, licat, and dnnamis, force). 

 — The .science which treats of the mechanical action or re- 

 lations of lieat. 



Ther'mogen (Gr. therme, heat, and gcnao, I bring forth). — 

 Tiic (Icmcntarij matter of heat. 



Thermom'eter (Gr. thermC, lieat, and metron, measure). — 

 An instrument for measuring heat. 



HEAVENS. 



Astron'omy (Gr. aatron, a star, and nomas, a law or rule). — 

 The science which treats of the heavenly bodies, explaining 

 the motions, times and cau.scs of the motions, distances, 

 magnitudes, gravities, light, etc., of the sun, moon, and 

 stars, the nature and causes of the eclipses of the sun and 

 moon, the conjunction and apposition of the planets, and 

 any other of their mutual aspects, with the times when 

 they <lid or will happen. 



Aberration. — A small apparent motion of the fixed stars, 

 occasioned by the progressive motion of light and the 

 earth's annual motion in its orbit. By this they sometimes 

 appear twenty seconds distant from their true situation. 



Am'plitude. — An arc of the horizon intercepted between the 

 true east and west points and the centre of the sun, or a 

 star at its rising or setting. 



Anom'aly. — The angular distance of a planet from its perihe- 

 lion, as seen from the sun ; either true, mean, or eccentric. 



Aphelion. — That point of a planet's orbit whicb is most dis- 

 tant from the sun. 



Ap'ogee. — That point in the orbit of the moon wliicli is at the 

 greatest distance from the earth. 



Appari'tion. — The first appearance of a star or other luminary 

 after having been obscured. 



Ap'pulse. — The approach of a planet towards a conjunction 

 with the sun or any of the fixed stars. 



Ap'sides, or Ap'sis. — The two points of a planet's orbit in 

 which it is at its greatest and least distance from the sun. 



Aqua'rius (Lat., the Water-Bearcr). — The eleventh sign of 

 the zodiac, which the sun enters about the 21st of January. 



A'ries (Lat., the Ram). — The first of the twelve signs of the 

 zodiac, which the sun enters about the 21st of 3Iarch. 



