28 



THE VOYAGE OF H.M.S. CHALLENGER. 



" It consisted of a narrow and a wide glass tube, forming as it were the stem and 

 bulb of a large air-thermometer. The stem was made of the most uniform tube which 

 could be procured, and was very accurately gauged ; and the weight of the content 

 of the bulb in mercury was determined. Thus the fraction of the whole content, 

 corresponding to that of one millimetre of the tube, was found. 



" This apparatus had the interior of the narrow tube very carefully silvered ; and 

 while the whole, filled with the liquid to be examined, was at the temperature of the 

 water in the compression apparatus, the open end was inserted into a small vessel 

 containing clean mercury. Four instruments of this kind were used, all made of the 

 same kind of glass. [They were numbered, as in the headings of the columns below, 

 1, 2, 3, 4, respectively. 20/6/88.] 



" The following are the calculated apparent average changes of volume per ton 

 weight of pressure per square inch (i.e. about 150 atmospheres) : — 



Fresh Water, at 12° C. 



Note. — The first two experiments with No. 2 failed in consequence of a defect in the silvering. 



The compressibility of glass was not directly determined. It may be taken as 

 approximately 0"000386 per ton weight per square inch. 



" From these data, which are fairly consistent with one another, we find the 

 following value of the true compressibility of water per ton, the unit for pressure (p) 

 being 1 ton-weight per square inch, and the temperature 12° C, 



0-0072 (1-0-034^); 

 showing a steady falling off from Hooke's Law. 



Sea- Water, at 12° C. 



Note. — The sea-water employed was collected about 1| miles off the coast at Portohello. 



These give, with the same correction for glass as before, the expression 



0-00666 (1 - 0-034 p). 



