358 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Inverted position 



Ac 



H,0 



_AFH,+ FA.;Fe-FH,Ar>H, 



Vnfi + A V, 



Hg 



A, _ A T-Hg + y^VTe - A^H^o ( I ^h,0 + ^ Vj,^) 



* Hg 



The following gives a discussion of the minor experimental details 

 that it was necessary to observe. 



Considerable care is necessary in filling the piezometer to ensure 

 complete exclusion of air. The procedure in filling was as follows. 

 The piezometer A with the cover B held loosely over it in a suitable 

 frame was placed in a test tube containing distilled water. The test 

 tube was then connected to the air pump and the water boiled under 

 reduced pressure at room temperature, removing the occluded air. 

 The piezometer was then removed from the test tube, the weighed 

 quantity of mercury introduced, and then replaced in the test tube 

 which was again exhausted as before. The mercury had been cleaned 

 with acid and was freshly distilled. Finally, before raising the piezom- 

 eter above the surface of the water, the cap B was screwed into place 

 with a simple key. Removing from the test tube, the cap B was tight- 

 ened home by applying a spanner. 



This tightening of the cap must be done with especial care, and to 

 irregularities in this most of the irregularities in the data could be 

 traced. Since the piezometer is made of a very mild steel, it is a com- 

 paratively easy matter to screw the cap in too far, stripping the thread 

 or shearing off the shoulder. This was avoided by graduating around 

 the top of A, and always screwing B in to the same mark. But even 

 at best, to secure a tight joint, it is necessary to force the cap in pretty 

 tightly, exceeding the elastic limit locally, and thereby introducing 

 probable irregularities as explained before. When the method was 

 first used, a great deal of trouble was found from such irregularities. 

 This was shown most convincingly by the fact that the volume of the 

 piezometer did not remain constant, but changed very slightly after 

 each application of pressure, usually becoming less. This change of 

 volume is not entirely due to the effect of the joint, because it could 

 be mostly removed by annealing. Nevertheless, since the effect of 

 annealing is to remove internal strains, and since internal strains are 

 doubtless introduced at the joint, there is probably some irregular 

 action introduced at the joint. This source of error remains the 

 chief objection to the method, although it can be greatly reduced by 

 proper manipulation. This consists in careful previous annealing, and 



