280 Miss Sarah Marks on the 
The same physicists have quite recently made a determina- 
tion, not of the Volta-effect, but of the true contact-force 
between two liquids, by the use of mercury electrodes ; em- 
ploying for this purpose a theorem of Helmholtz, that if by 
judicious polarization the capillary constant of a mercury- 
liquid surface be made a maximum, then between the mercury 
and the liquid there is no difference of potential*. 
Their account of this last determination has not yet been 
published, but it will appear shortly (before this is printed 
probably) in the Comptes Rendus. 
The theorem gives evidently an admirably simple method 
of measuring liquid / liquid contact-force, if it can be made to 
work practically. 
MM. Bichat and Blondlot apparently hope, by a comparison 
of the two methods, the results of which are quite different 
(in accordance with the views expressed in the present com- 
munication), to be able to obtain values for air / liquid contacts. 
This would be a most important and useful piece of work : the 
only difficulty in accomplishing it is the difficulty of getting 
both sets of measurements thoroughly dependable. I hope 
they may succeed in overcoming all obstacles. 
[To be continued. | 
XXXI. The Uses of a Line-Divider. 
By Miss Saran Marxsf. 
are ordinary method of dividing a given straight line AB 
into any number of equal parts (fig. 1), without using a 
special instrument, is to draw a line AC at any angle to it, to 
cut off from AC the given number of equal parts AD’, D/H’, &c., 
to join M, the last of the points so taken, to B, and from D’, Hi’, 
&c. to draw lines parallel to MB, meeting AB in D, EH, &e. 
These points D, H, &. divide AB into the required number 
of equal parts. This is exactly the method adopted in using 
the line-divider. 
It consists of a hinged rule with a firm joint, having the 
left-hand limb fitted to slide in an undercut groove upon the 
plain rule. Both limbs are bevelled on their inner edges, and 
the left one is divided both on the bevel and on the top into 
eighths, quarters, half-inches, and inches, which are consecu- 
tively numbered, beginning at the hinged end, so that any 
* Monatsberichte der Berliner Akad. November 3, 1881. 
+ Communicated by the Physical Society. Read February 14, 1885, 
