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



the points marked # in Flatc I. Drawing smooth curves through these, I obtained 

 parabolic formulae for the apparent compressibility. These gave the following results 

 when compared with the data from observation : — 



Apparent Compressibility of Sea-Water. 



Adding the correction for glass, the formula? became, for 1, 2, and 3 tons 



respectively — 



462 -3-20t + 0-QU 2 , \ 



447-5- 3-05/ + -Oof 2 , '. . . (B) 



437-5 -2-95C + 0-05/ 2 , j 



which may be compared with (A) for fresh water ; and which may be approximately 

 expressed in the form (very nearly correct forp = 2) — ■ 



0-00179 ^ / f- , 



38+jA 150 10,000 J 



with sufficient accuracy for most purposes of calculation. 



Of course it is easy to deduce from formula? (B) the points of minimum compressi- 

 bility, etc., for different jjressures ; but the data are scarcely accurate enough to warrant 

 such a proceeding. We may, however, extend the formula? tentatively to the case of 

 very low pressures, for which we obtain 



4Sl-3-4* + 0-03f 2 . 



[The term independent of t in the formulae (B) is of the form 



481-21-25^ + 2-2-V-] 



The second series of observations gave, when reduced, the points marked i on the 

 plate. The curves which I have drawn, and which evidently suit them very closely, 

 are imrallel respectively to the curves drawn through the * points. The interval 

 between them is throughout about 7 for 1 ton, 4 for 2 tons, and 3 for 3 tons, which 

 must be subtracted from the first terms of (B) respectively. The corresponding 

 intervals for the fresh water curves in the two series were 5, 2, 1. The differences of 

 corresponding intervals between the sets of curves are 2, 2, 2 ; the same for all the 

 groups of four curves each. 



