vacuum filtration — A method of extracting phy- 

 toplankton and bacteria from a water sample. 

 See Millepore* filter. 



valence electron — An electron which is gained, 

 lost, or shared in a chemical reaction. (70) 



valley — A relatively shallow, wide depression with 

 gentle slopes, the bottom of which grades con- 

 tinuously downward. The term is used for fea- 

 tures that do not have canyonlike characteristics 

 in any significant part of their extent. (62) 



valley iceberg — (also called drydook iceberg) . An 

 iceberg eroded in such a way that a large U- 

 shaped slot, which may be awash, extends 

 through its surface separating pinnacles or 

 slabs. (59) 



valve — 1. One of the pieces forming the shell of 

 a diatom. 



2. Any of the pieces forming the shell of cer- 

 tain invertebrates, such as the mollusks and 

 barnacles. 



vanishing tide — When a high and low water 

 "melt" together into a period of several houre 

 with a nearly constant water level, the tide is 

 in the diurnal category but is known as a van- 

 ishing tide. 



DAYS 



VANISHING TIDE 



vapor — ^Any substance existnig m the gaseous state 

 at a temperature lower than that of its critical 

 point ; that is, a gas cool enough to be liquified 

 if sufficient pressure were applied to it. 



If any vapor is cooled sufficieiitly, say at con- 



stant pressure, it ultimately reaches a state of 

 saturation such that further removal of heat is 

 accompanied by condensation to the liquid 

 phase. (5) 



vaporization — See evaporation. 



vapor pressure — (or 'oafor tension). The pres- 

 sure exerted by the molecules of a given vapor. 

 For a pure, confined vapor, it is that vapor's 

 pressure on the walls of its containing vessel; 

 and for a vapor mixed with other vapors or 

 gases, it is that vapor's contribution to the total 

 pressure (that is, its partial pressure). 



In the atmosphere, vapor pressure is used al- 

 most exclusively to denote the partial pressure 

 of water vapor. Care must be exercised in inter- 

 preting the term's meaning as used in other 

 branches of science. (5) 



vapor tension — See vapor pressure. 



variability of waves — 1. The variation of heights 

 and periods between individual waves within a 

 wave train. (Wave trains are not composed of 

 waves of equal height and period, but rather of 

 waves with heights and periods which vary in a 

 statistical manner.) 



2. The variation in direction of propagation 

 of waves leaving the generating area. 



3. The variation in height along the coast, usu- 

 ally called "variation along the wave." 



(61) 



variable — A quantity that may assume a number 

 of values. (22) 



variable depth sonar — (abbreviated VDS). A 

 shipborne sonar system whereby the transducer 

 can be lowered below the thermal layer. 



variance — A measure of variability denoted by 

 cr- and defined as the mean-square deviation from 

 the mean, that is, the mean of the squares of the 

 differences between individual values of the vari- 

 able and its mean value. 



variation — 1. The range within which ^'alues of a 

 variable lie, as in the diurnal or annual varia- 

 tion. 



2. See declination. 

 (5) 



varve — A sedimentary deposit, bed, or lamination 

 deposited in one season. It is usually distin- 

 guished by color or composition and used as an 

 index to changes in the depositional environ- 

 ment. 



vast ice floe — See ice floe. 



vector mean — See resultant current. 



174 



