191 °] McEwen: Preliminary Hydrographic Report. 193 



The water samples are designated by numbers 1. 2. 3, etc., in 

 the order taken, and the results tabulated as follows: 



Water sample number; Date; Haul number; Position; Depth in fathoms 

 from which sample was taken. I> ; Temperature (Centigrade) of water — At 

 the depth D; T, During observation, t; Hydrometer — Number of instrument, 

 Reading. B; Specific gravity — Observed at t° c, St, Reduced to its value at 

 15.56° C, S, T° C, St; Chlorine in grammes per kilogram of water. 



The readings and instrument numbers refer either to the 

 specific gravity bottle used and its apparent weighl in grams 

 when full of water or to the number of the hydrometer and its 

 reading. The reduction of the densities to the last two temper- 

 atures was made in accordance with the method described by 

 Dittmar, Reports of Challenger Expedition. Physics and Chem- 

 istry. Vol. 1, page 70. The chlorine, in grammes per kilogram, 

 was computed from the density according to Dittmar \s method, 

 described in the same volume, page 80. 



From this collected data, the physical conditions for that 

 region, at the time they were determined can be obtained. bu1 

 these conditions are not constant: therefore they must be re- 

 peatedly determined at different times, and these results must 

 be studied for the purpose of obtaining a knowledge of the 

 manner in which the changes proceed, so that from a compar- 

 atively few discontinuous observations taken at various times 

 and places, the conditions at intervening times and places can 

 be deduced. 



The density of the water depends upon three independent 

 conditions: its temperature, the pressure which it supports, and 

 the amount of material dissolved in it. If the density is re- 

 duced to a standard temperature and pressure, the only variable 

 affecting it is the amount of dissolved matter, and the density 

 denned as above is a measure of the salinity, or quantity of 

 matter dissolved in unit mass of the water. If the density of 

 a given sample is represented by S, at any pressure, and by S p 

 at a pressure p in excess of the first value, the temperature re- 

 maining constant, then the following equation holds approxi- 

 mately: S P =S (1-f 3X 10~ 6 p) where p is expressed in pounds 

 per square inch. If. then, we know the density of a water 

 sample under atmospheric pressure and at any given temper- 

 ature, and want to know what its density would be at the same 



