chap, x.] CONTINUITY OF THE CHALK. 505 



average of thirty-two observations upon water at a sufficient 

 distance from land to be unaffected by local disturbances was 

 T02779, the maximum being 1'0284 and the minimum l - 0270. 



It was almost always noticed that, during a high wind, the 

 specific gravity of surface-water was above the average. 



The average of thirty observations upon the specific gravity 

 of intermediate water was 10275, the maximum being 1-0281 

 and the minimum 1'0272. 



The specific gravity of bottom-waters at depths varying 

 from 77 to 2,090 fathoms, deduced from an average of forty- 

 three observations, was 1*0277, the maximum being 1-0283 and 

 the minimum 1'0267. 



It will be noticed that the average specific gravity of bottom- 

 water is slightly less than that of surface-water. In several 

 instances the specific gravities of surface- and of bottom-waters 

 taken at the same place having been compared, that of the 

 bottom-water was found to be appreciably less than that of the 

 surface-water. Thus — 



And 



At 1425 fathoms' depth (Station 17) it was . . 1-0269 



Surface at the same T0280 



At 664 fathoms' depth (Station 26 6) it was . . 1 -0272 



Surface at the same 1 '0280 



According, however, to a series of observations made at the 

 same spot (Station 42) at intervals of fifty fathoms, from 50 to 

 800, the specific gravity increased with the depth from 1-0272 

 at 50 fathoms to 1-0277 at 800 fathoms. 



Several series of specific-gravity observations were made near 

 the mouths of rivers and streams ; showing the gradual mixture 

 of fresh and salt water, and the floating of lighter portions above 

 the denser sea-water, as well as the reverse effect produced by 

 the influence of tidal currents. Thus outside Belfast Lough a 

 rapid stream of water of specific gravity 1-0270 was found above 

 water which, at a depth of seventy-three fathoms, had a specific 

 gravity of 1-0265. 



Gases of Sea-water.— The analyses of the gaseous constituents 

 of sea- water may be divided into two groups : (1) Analyses of 



