EQUATOR 



EQUINOCTIAL 



As another example, the re- 

 action between washing soda and 

 vinegar may be used. Washing soda 

 is sodium carbonate (Na 2 COj), 

 with ten molecules of water of 

 crystallisation which need not be 

 shown in the equation. The acidity 

 of vinegar is due to the acetic acid 

 it contains. The formula for acetic 

 acid can be written in several 

 ways, e.g. HC,H 3 0,; C,H 4 0, ; CH 3 . 

 COOH; orC ? H 3 .OH. "Taking the 

 first expression as most suitable, 

 we obtain the following equation : 



,0 2 + H,0 +C0 a 

 118 + 120 = 176 + 18 + 44 



On the right hand side results 

 are shown, sodium acetate (which 

 remains in solution), water, and 

 carbon dioxide. We have thus 

 accounted for the products of the 

 reaction in accordance with the 

 doctrine of the indestructibility 

 of matter. The sums of the mole- 

 cular weights on both sides are 

 equal (rounded figures have been 

 used), showing that all the atoms 

 are accounted for. 



EQUATION OF' TIME. A solar 

 day is the interval which elapses 

 between two successive passages 

 of the sun over the meridian 

 of a given place (e.g. London). 



Equatorial telescope mounted to enable 



the observer to follow the heavenly 



bodies across the sky 



By courtesy of T. Cooke & Son 



Owing to the obliquity of the 

 ecliptic to the equator, and to the 

 varying velocity of the earth in its 

 orbit, this interval is not always 

 the same. Hence solar time differs 

 from the mean solar time, regis- 

 tered by a perfectly even-going 

 clock, constructedso as to record 24 

 hours to a mean solar day. There 

 will be a difference between noon 

 as registered by the sun and as 



Equator. Perspective view of the 



earth, showing where the equator 



cuts Africa and a portion of S. 



America 



registered by the clock, and the 

 difference will vary from day to 

 day. This difference is called the 

 equation of time, and is expressed 

 in the minutes and seconds at 

 which apparent noon takes place 

 before or after mean noon. See 

 Horology. 



Equator (Lat. aequare, to equal- 

 ise). Circle drawn round the 

 globe midway between the N. and 

 S. poles. At the equator the sun 

 is seen directly overhead at noon 

 at the equinoxes. Latitude is 

 measured N. and S. of this circle. 

 It is the longest line, in one plane, 

 that can be drawn round the earth, 

 measuring approximately 24,902 m. 



Strictly speaking, this line is the 

 terrestrial equator. The great 

 circle in which the plane of the 

 terrestrial equator cuts the celestial 

 sphere is called the celestial 

 equator. The magnetic equator is 

 that line drawn round the earth 

 at any point on which the vertical 

 components of the force of the 

 earth's magnetism is zero. See 

 Earth ; Equinox ; Latitude. 



Equatorial. Instrument so 

 mounted that it can follow a 

 heavenly body from its rising to its 

 setting. This continuous obser- 

 vation is regulated by clockwork 

 machinery. In the best arrange- 

 ment, a strong steel pillar supports 

 a headpiece, in <Khich is fixed the 

 polar axis of the instrument, parallel 

 to the axis of the earth. This polar 

 axis is turned round once in 

 twenty-four hours. A telescope 

 fixed to such an axis will always 

 move in a " circle of declination," 

 and thus a clock driving the tele- 

 scope in one direction as fast as the 

 earth is carrying it in the opposite 

 direction will always keep the tele- 

 scope fixed on the same point in the 

 sky. It is not convenient to at- 

 tach the telescope directly to the 

 polar axis, because its range is 

 thereby limited ; it is therefore 

 fixed to a declination axis placed 

 above the polar axis and at right 

 angles to it. Most of the great 



modern refractors are equatorials. 

 See Observatory ; Telescope. 



Equatorville. Alternative name 

 for the administrative and trading 

 centre of the Belgian Congo, better 

 known as Coquilhatville (g.v. ). 



Equerry. Originally an official 

 of the royal stables. In the British 

 royal household the equerries are 

 army officers in the department of 

 the master of the horse. The chief 

 or crown equerry is a permanent 

 official, who looks after the stables 

 and stud. The sovereign always 

 has an equerry in attendance. The 

 form of the word, originally mean- 

 ing stable (Fr. ecurie, late Lat. 

 scuria), has been influenced in 

 English by a supposed connexion 

 with Lat. equus (horse). 



Equidae (Lat. equus, horse). 

 Family of Ungulate mammals, in- 

 cluding the horses, asses and zebras. 

 In geological history, the horse 

 family can be traced back to an- 

 cestors that had five toes instead 

 of the single toe of modern horses. 

 The Phenacodus, a five-toed animal 

 about the size of a bull-dog, lived 

 at the beginning of the tertiary 

 period. The next stage is seen in 

 Hyracotherium, with four equal 

 toes on the fore limbs, found in 

 lower eocene strata. Succeeding 

 stages are represented by Anchi- 

 therium, with three toes and a di- 

 minutive fourth, in the Miocene 

 age in Europe; Hipparion, with one 

 large middle toe and two smaller 

 side toes, in the Pliocene age ; and 

 Equus, two diminutive toes on each 

 side of the large toe, in the Pliocene 

 age. See Horse. 



Equilibrium (Lat. aequus, 

 equal ; libra, balance). In a system 

 of forces a state of equilibrium 

 exists when the forces under con- 

 sideration are so arranged that they 

 balance or have no resultant at 

 any point. A body is in stable equi- 

 librium when it returns to its ori- 

 ginal position after being disturbed; 

 in unstable equilibrium, when it 

 continues to move in the direction 

 given to it by a disturbing force. 



CHEMICAL EQUILIBRIUM. A bal- 

 anced action between chemicals 

 similar to that indicated by equili- 

 brium in mechanics. It mostclosely 

 resembles the mechanical equili- 

 brium established when friction is 

 large or inertia small, because in 

 a chemical system there is nothing 

 corresponding to the oscillations in 

 mechanics. See Dynamics ; Motion. 



Equinoctial Gales (Lat. 

 aequus, equal ; nox, night). Term 

 indicating a belief that gales nor- 

 mally occur about the equinoxes. 

 In N. America and Europe and 

 over the N. Atlantic Ocean, from 

 Nov. to Jan. is the period of most 

 frequent and most intense gales. 

 See Meteorology ; Weather. 



