84 PETER GUTHRIE TAIT 



determined, the value being 0-0000036 per atmosphere. These data were 

 necessary for the accurate determination of the compressibilities of the various 

 kinds of water and solutions. Within a range of temperature o to 1 5 C. and 

 a range of pressures from 1 50 to nearly 460 atmospheres, the compressibility 

 of fresh water was approximately represented by the empirical formula 



0-00186 / _ 31 



36 + / \ 400 10,000. 

 The corresponding formula for sea-water was 



0-OOI79/ / f \ 



38 + / \ 150 lo.ooo/' 



In these t is Centigrade temperature and p is pressure in tons weight per 

 sq. inch. The point of minimum compressibility of fresh water is about 60 C. 

 at atmospheric pressure, and that of sea- water at about 56 C. ; both are 

 lowered by increase of pressure. 



The average compressibility of solutions of NaCl for the first p tons 

 of additional pressure, at o C., s being the amount of NaCl in 100 parts 

 by weight of water, could be represented very accurately by the formula 

 o-oo 1 86/(36 +/ + .$). 



The maximum density point of water was found to be lowered about 3 C. 

 by 1 50 atmospheres of pressure ; and from the heat developed by compression 

 of water Tait calculated that this lowering of the maximum density point 

 should be 3 per ton weight per square inch, (i ton weight per sq. inch =152-3 

 atmospheres.) 



In most of his experimental work Tait did not apply his mind specially 

 to the invention of elaborate apparatus ; but that he could when the necessity 

 arose devise useful and ingenious forms appears very clearly in his compression 

 work. 



Consider for example his high pressure gauge, constructed of a steel 

 cylinder, the measured change of volume of which under hydrostatic pressure 

 gives by a simple elastic formula the value of the pressure. In this instrument 

 the pressure is applied to the outside of the cylinder, and the change of 

 volume is measured by the alteration in level of mercury which fills the 

 cylinder and the narrow glass tube fitted to it above. This glass tube which 

 is in continuous connection with the interior of the steel cylinder is open above 

 and is not itself exposed to pressure. It projects through the top of the 

 outer vessel which surrounds the steel cylinder and within which the pressure 



