EPO 



EQU 



17. Epi-phlceum{(})\oi6i, har'k.). A term 

 applied by Link to the second portion of 

 the bark of plants, consisting of several 

 layers of thin-sided tubular cells, rarely 

 coloured green. This is the phloeum or 

 peridermis of Mohl. 



18. Epiphragma {(ppafna, a fence). A 

 term applied, in Bryology, to the mem- 

 brane (peristomium) which sometimes 

 closes up the orifice of the theca. It is 

 sometimes called tympanum, from the 

 drum-like appearance which the theca 

 thus presents. 



19. Epi-phyllospermous {<pv\\ov, a leaf, 

 cnepiJia, seed). An old term for those 

 plants which bear their organs of repro- 

 duction upon the back of their fronds, as 

 the ferns. These are now called dorsi- 

 ferous, from dorsum, the back, and fero, 

 to bear. 



20. Epi-phyllous {(pvWov, a leaf). A 

 terra applied to any thing which is in- 

 serted upon the leaf of a plant. 



21 . Epi-phyie {(pvTov, a plant). A plant 

 which grows upon other plants, adhering 

 to their bark, and rooting among the 

 scanty soil found on their surface. The 

 term is generally restricted to those Or- 

 chidaceous plants which grow upon trees. 

 The old term was epidendrum. 



22. Epi-pterous {mepov, a wing). A 

 term applied to a fruit or seed which is 

 furnished with a broad margin or wing, 

 when it terminates. See Pterous. 



23. Epi-rrheology {knippoi], a flowing 

 on, X670f, description). That branch of 

 science which treats of the effects of ex- 

 ternal agents upon living plants. 



24. Epi-sperm {air^pfxa, seed). The 

 terms episperm and perisperm are ap- 

 plied by Richard to the testa, or integu- 

 ment of a seed — the spermoderm of De 

 Candolle. 



25. Epi-zoa i^utov, an animal). An 

 order of the Entozoa, which are at first 

 free and natant larvae, afterwards fix- 

 ing themselves permanently, by strong 

 organs of attachment, to the soft super- 

 ficial parts of aquatic animals. 



EPI'STOLOGRA'PHIC WRITING 

 (enKTToXi], a letter, ^pa^a, to write). A 

 designation of the demotic or enchorial 

 mode of writing, in common use among 

 the Egyptians, and approaching very 

 nearly to the Chinese m^'thod. 



E'POCH (eTTox*;, a pause in the reck- 

 oning of time). In Chronology, a point 

 at which one period closes and another 

 begins. The epoch of a star is its place 

 in the heavens. In order to predict the 

 longitude of a planet at any epoch, some 

 124 



preceding epoch must be taken, at which 

 the longitude is known. The latter is 

 emphatically called the epoch; and the 

 term longitude at the epoch has been 

 shortened into epoch. 



EQUATION (in Astronomy). The 

 term applied by astronomers to their 

 practice of reducing all their observations, 

 both of right ascension and of declina- 

 tion, to some common and convenient 

 epoch. These corrections, or equations, 

 are necessary in consequence of the 

 elliptic figure of the moon's orbit, the 

 sun's attraction, and other causes. 



1 . By equating the observation for nuta- 

 tion, is meant the getting rid of a periodi- 

 cal cause of fluctuation, and present- 

 ing a result, not as it was observed, but 

 as it would have b^en observed, had 

 that cause of fluctuation had no exist- 

 ence. 



2. Equation of time. In popular lan- 

 guage, this expression denotes the differ- 

 ence between the times indicated by an 

 accurately constructed sun-dial and a 

 well-regulated clock. Astronomically, it 

 signifies the difference, expressed in 

 mean solar time, between the true or 

 apparent right ascension of the sun and 

 its mean right ascension. There are only 

 four days in the year, when the apparent 

 and the mean time coincide, and the 

 equation o/^ime is nothing, viz. April 15th, 

 June 15th, September 1st, and Decem- 

 ber 24th. Between April 15th and June 

 15th, the clock is earlier than the sun, and 

 the equation is sub tractive ; between 

 June 15th and September 1st, the clock 

 is later than the sun, and the equation 

 is additive. 



EQUATION (in Mathematics). An 

 assertion of the equality of two magni- 

 tudes, represented by the symbol =. It 

 is of two kinds, viz. an equation of con- 

 dition, which will be true only on the 

 supposition of certain conditions, as 

 2a -j- 1 = 13, which requires that a should 

 represent 6 ; or an identical equation, 

 which is true independently of all condi- 

 tions, i. e. whatever may be the value of 

 the symbols employed, as a -f a = 2a. 



EQUATION, ALGEBRA'IC. An 

 expression of two algebraic quantities, 

 when connected together by the sign of 

 equality. The solution of an equation Is 

 the operation by which the values of the 

 unknown quantities are found in terms 

 of the known quantities. If the equation 

 contains no power of the unknown quan- 

 tities, but these quantities merely in 

 their simplest form, it is called a simple 



