44 THE BOTTOM OF THE SEA. 



heavier than fresh-water of equal volume. Thus, a 

 litre of fresh- water (nearly 1| pint) weighs less than 

 a kilogramme (exactly 0'^-998) at the temperature of 

 68° Fahrenheit ; the same volume of fresh-water under 

 the same conditions would weigh l'^-027. But the 

 weight of sea-water, like its composition, is far from 

 being constant ; it varies according to times and 

 places, and even according to the depth from which it 

 is taken. Southern sea-water is heavier than that of 

 the northern hemisphere in the proportion of 1*0272 

 to 1*0262 of specific gravity, owing to the greater 

 quantity of salt it contains. Off Cape Horn its specific 

 gravity is 1*028, the heaviest known. The upper 

 current of the Gulf Stream is lighter than the current 

 below, because it is less saturated with salt ; but these 

 facts will be further elucidated by-and-by. 



As the air, at whatever height we take a sample, 

 is found to be of the same composition, an interesting 

 question arises whether or not the sea partakes of this 

 property ? Are the gases which enter into its com- 

 position always in the same proportions? These 

 questions are not easy to solve, because the enor- 

 mous pressure exercised by a column of water many 

 thousands of yards in depth causes extraordinary 

 difficulties in the construction of the necessary ap- 

 paratus. It must be remembered that it is essential 

 to obtain the water at the required deptli, and brinoj 



