18 



THE VOYAGE OF H.M.S. CHALLENGER. 



A few larger instruments were made for very accurate comparisons of fresh water 

 and sea-water at about 1 ton weight per square inch, and at different temperatures. 



The mercury contents of their bulbs, &c, were over 1000 grm. The content of 

 250 mm. of stem in mercury was about 7 grm. ; and the log. factor, for pressures about 

 150 atm., nearly = "8. 



For the compressibility of mercury, the annexed form of piezometer 

 was employed, as in this case the recording index could not be put in 

 contact with the liquid to be compressed. The bulb A and stem to B 

 contain mercury, and so does the U-tube CD. Between B and C there 

 is a column of water, whose length is carefully determined. The recording- 

 index rests on the mercury column at C. Thus, obviously, its displace- 

 ment is due to 



Compression of mercury A B + Compression of water B C - Compression of 

 vol. of glass vessel from A to C. 



The measurements of this apparatus are : — 



Mercury Piezometer. 25/7/87. 



Hg. and vessel, ..... 

 Vessel, ...... 



Weight of mercury whose compression is measured, 



Hg. and dish, ..... 

 Dish, ...... 



Weight of mercury in 210 mm. of tuhe 1! C, 

 Length of water column B C, 



The observations made with this apparatus were as follows, the results calculated 

 being added, enclosed in square brackets : — 



