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



approximations to the truth, probably about as close as the apparatus and the method 



employed are capable of furnishing. 



They show that the ratio of compressibilities of sea-water and fresh water varies 



but little from 



0-92 



throughout a range of temperature from 0° to 15° C. 



[The doubts as to the behaviour of the indices, which have been more than once 

 alluded to above, Lave just led me to make a series of experiments (at one temperature 

 but at different pressures) by the help of the silvering process. The results with fresh 

 water were not much more concordant than when the hair-indices were used. When 

 means were taken, exactly as before, it was found that the results for 1 ton were almost 

 identical with the former. For 2 tons the average value was usually greater than 

 before by a unit (and in some cases two units) in the third place. For 3 tons it was 

 also greater, but now by one or two (and sometimes three) units. Hence it is probable 

 that the hair-indices do behave as I suspected, but that the effect is small, — not at the 

 worst (i.e. at the highest pressure) more than about - 5 per cent, of the mean value 

 found. With sea water there was a complex reaction, which made it difficult to read 

 the indications of the silver film. The ratio of the true compressibilities of sea-water 

 and fresh water was now found to lie about 0'925, the value which I gave from my 

 earliest experiments. 30/6/88.] 



Dr. Gibson has furnished me with the following data regarding specimens of sea- 

 water taken from two of the Winchester quarts filled off the Isle of May. One of these 

 had remained unopened ; the other had been often opened, and not closed with special 

 care. These correspond (at least closely) to the materials used in the first and second 

 series of experiments respectively : — 



Taking the reciprocals in the last three columns, we have 



Expressing these volumes as parabolic functions of the temperature, we find, for the 

 maximum density points, — 5°7 and — 4°-9 respectively. 



