HHIDGM.VX. — TIIKKMODYXAMIC PROPERTIES OF WATER. 813 



of the freezing of tlie oil. Parsons and Cook were able to secure 

 entire freedom from leak up to 4000 kgm. hy the employment of a 

 cupped leather washer comhined with a brass disc of special design. 

 It has been the experience of all those who have worked with high 

 pressures, however, that no leather washer is capable of standing 

 pressures very much in excess of the limit of 4500 kgm., since the 

 leather rapidly disintegrates under the pressure. In the present 

 work the same form of packing was used which was used in the pre- 

 vious work on the freezing of water and mercury under pressure. 

 This has been proved in the previous paper to be absolutely free from 

 leak up to the highest pressures wliich can be sustained by the steel 

 containing vessels. In the present work this same packing has 

 proved itself to be reliable for the purposes of this method. 



The question of the method of measuring pressure is also of con- 

 siderable importance in using this method, since the usual measuring 

 devices, such as a Bourdon gauge, cannot be applied, for reasons to be 

 discussed later, and attempts to calculate the pressure directly from 

 the force required to produce motion of the piston are likely to be in 

 error because of the friction of the packing. Parsons and Cook did, 

 however, adopt this latter method, and computed the pressure from 

 the known force required to move the piston. The effect of the 

 friction of the packings was allowed for in as large a degree as possible 

 by taking the mean of the readings during increasing and decreasing 

 pressure, assuming that the friction remained constant. The results 

 obtained by Parsons and Cook in this way were surprisingly good. 

 That the friction did remain fairly constant was indicated by the 

 constancy of the results and the fact that the curve nearly always 

 returned to the starting point; but it is doubtful if the method would 

 work at very much higher pressures because of the increase of friction 

 due to the flow of the softer parts of the piston. The brass washers 

 used by Parsons and Cook would almost certainly have upset under 

 two or three thousand more kgm., and it is the experience of the 

 author that it is difiicult to obtain even steel washers which will 

 stand much more than 8000 kgm. without taking some set. In fact, 

 at high pressure there must necessarily be some plastic yield, in order 

 to follow the expansion of the cylinder. The result of this set in the 

 washers is that the friction becomes very irregular, and cannot be 

 assumed to be the same during increasing and decreasing pressure. 

 Variations in the amount of friction due to this cause of as much as 

 200 or 300% have been found at the higher pressures of this work. 

 . The only escape from the difficulty seems to be to measure the 



