26 University of California Puhlications in Zoology [Vol. 15 



tciul to make its density equal to that of the surrounding water, and 

 it will continue to rise until this condition is realized. Suppose this 

 to occur at the level h : then, owing to the effect of diffusion and con- 

 duction, its density when at a must have been less than its density 

 after it reached b by an amount exceeding the decrease in its density 

 caused by the difference in pressure between the two levels a and 6. 

 In other words, if the densities at the levels a and b be referred to a 

 common pressure, the former must be less than the latter or else the 

 mass of water could not rise, w^hence the densities ut situ, wiiich are 

 those at the two levels referred to atmospheric pressure, must be 

 related to each other in the same way. Hence, it follows that a pro- 

 gressive increase of the densities in situ with inerea.sing depth is the 

 necessary condition of stability. 



We have therefore used this relation, supplemented by the fact that 

 our observations have never revealed a "reversal" in temperature, 

 as a criterion for detecting errors in salinity due to evaporation as 

 well as to blunders in computation. In some instances, how^ever, this 

 criterion was insufficient for, though it revealed an impossible relation 

 between the densities in situ, it failed to indicate which partievdar 

 salinity determinations were erroneous. In order to isolate these, each 

 salinity determination was compared with the average and extreme 

 values for the corresponding depth, month, and general locality. All 

 the erroneous values thus detected are either excluded from tabulation 

 or questioned, depending upon the certainty with which the values 

 could be entirely attributed to evaporation and blunders in compu- 

 tation. 



D. An.\lysis op Se.\ -Water 



1. GENER.A.L PROPERTIES OF SE.^-WATER AND MEASUREMENTS OF 

 SALINITY 

 (a) Composition of sea-water. — Sea-water is a dilute solution of 

 various solid chemical compounds or salts. The weight of these salts 

 in a thoiLsand grams of sea-water is called its salinity and is commonly 

 denoted by the symbol S%o. A large niimber of observations have 

 shown that, while the salinity varies widely, the proportion of each 

 dissolved salt to all the others is nearly the same. The following 

 list (see Kriimmel, 1907, p. 219) gives the amount of each salt and its 

 proportion to the total in a sample of sea-water whose salinity is 35.00. 



