1906-7.] On the Application of a Differential Densimeter. 217 
The Densimeter used Differentially. 
Before the instrument can be used differentially, its error must be 
determined. This may be done by introducing into each cistern 70 c.c. of 
any given sea- water, and charging the tubes ; the lengths of the water 
columns are then adjusted, and micrometer readings taken as already de- 
scribed. If, now, the two cisterns have the same form and capacity, and 
rest upon the same horizontal plane, and if, also, the two tubes are of the 
same diameter, then the micrometer readings for both tubes will be 
identical. In practice, the readings will differ somewhat, because it is so 
difficult to attain to strict equality. We therefore content ourselves by 
approximating to the ideal conditions, and then determine any difference 
as outlined above, and allow for it subsequently. 
We are now in a position to measure the density of a water by the 
differential method. A sea- water is selected, and its density determined 
by the method of reversal (page 216) ; this water then becomes the standard 
with which others may be compared. The cisterns are then charged, the 
one with the normal volume of the standard water, and the other with an 
equal volume of the water to be examined. After observing the usual 
preliminaries, micrometer readings are taken, and from these the difference 
in the lengths of the two water columns is found, due allowance being at 
the same time made for the densimeter error ; finally, the density of the 
water is calculated. 
The following examples will, it is hoped, make this clear : — 
Table II. — Determination of Densimeter Error. 
70 c.c. of the same sea-water in each cistern. 
Exper. 
1 
Micrometer Readings. 
Diff. for 
Tube K. 
Standard 
Tube S. 
Comparison 
Tube K. 
1 . 
42*95 mm. 
43*14 mm. 
+ 0*19 mm. 
2. 
39*95 m 
40*14 M 
+ *19 i. 
3. 
36*92 „ 
37*13 n 
+ *21 „ 
4. 
33*78 m 
33*98 ii 
+ *20 ii 
5. 
30*85 m 
31*04 ,, 
+ *19 
6. 
27*86 m 
28*06 .. 
+ *20 it 
Mean = 
+ *20 mm. 
