76 THE VOYAGE OF H.M.S. CHALLENGER. 



result given in the text ; and the others show how it is modified by taking account 

 of the diminished compressibility at the higher pressures. 



Of course we might have employed the more exact formulae, (A) or (B) as the 

 case may be, but for all practical applications the rough formula suffices. 



It might be interesting to study the effect on the mean level of a lake due to the 

 indirect as well as the direct results of change of temperature. Heating of the water 

 throughout, if there be a case of the kind, would increase the depth not only in 

 consequence of expansion (provided the temperature were nowhere under the maximum 

 density point), but also in consequence of the diminution of compressibility which it 

 produces. Thus there would be an efficient cause of variation of depth with the 

 seasons, altogether independent of the ordinary questions of supply from various 

 sources and loss by evaporation. 



If the temperature be not constant for all depths, p Q , p, and A are functions of £ 

 Substituting their values in the hydrostatic equation, we must integrate it and 

 determine the constant by the same conditions as before. 



The condition for stable equilibrium is merely that dpjd% shall not be anywhere 

 positive. Until some definite problem is proposed, no more can be done with the 

 equation. 



[29/10/88. — At Dr. Murray's request I have calculated, from the data given in his 



paper : On the Height of the Land, and the Depth of the Ocean (Scottish Geographical 



Magazine, vol. iv. pp. 1-41, 1888), that the whole depression of the ocean level, due to 



compression, is about 



116 feet only. 



If water ceased to be compressible, the effect would be to submerge some 2,000,000 

 square miles of land, about 4 per cent, of the whole.] 



