PHYSICS: C. BARUS 
341 
question is introduced into a long steel tube TT, figure 1, and that 
the tubulure p conveys the increments of pressure dp. The end p is 
rigidly fijced. The other end q of the tube is free to move. By aid 
of the stylus, e, the elongation is registered on the plate n of a, con- 
tact lever read by interferometry, the lever being identical in con- 
struction with the apparatus described in my paper on magnetic elonga- 
tion.^ Thus the interferometer will indicate the elongations due both 
to the pressure increment and to the corresponding temperature incre- 
ment of the suddenly compressed liquid, and it becomes a question to 
what degree the two may be adequately separated. If Al, Ap, Ad 
are corresponding increments of the length, /, of the tube and the 
kb' zr zf i>r 
pressure and temperature of its liquid content, we may write successively, 
if Al = Al' + Al", 
Al'/l = {rVmi - r\) ) Ap = ^Ap, say, (2) 
AF/l = aAd (3) 
where a is the coefficient of expansion, k the bulk modulus, ri and ^2 
the internal and external radius of the steel tube of length /. Hence 
aAd = Al/l - ^Ap (4) 
and equation (1) becomes 
de/dp = - ^ = ^ (5) 
a JpC 
or 
C = (ayjp) (a/{Al/(lAp) - ^) (6) 
