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Proceedings of the Royal Society of Edinburgh. [Sess. 
limit of Bouty’s law. For this purpose I have, in the case of each of the 
curves in hg. 2, obtained values of the constants A, B, C, and D by selecting 
four points on the curves, reading off the corresponding values of p and t, 
substituting into the above expression, and solving the four equations thus 
obtained. The four points selected had to be on a part of the curve for 
which the values of t were, at the one end, large enough to give trustworthy 
observations of p, and, at the other end, small enough to be within the assumed 
time limit of the applicability of the formula. They had, consequently, 
to be taken from comparatively small portions of the curves. It is clear, 
however, that according to the expression under consideration we must 
have p = 0 for t = 0, and that the origin in thus necessarily a point on the 
curve. 
Tables I., II., and III. give the values of the polarisation calculated 
by the above formula, the values observed, and the differences between 
them. 
It will be seen that in all three tables (especially in the first and third ) 
the observed and calculated values of the polarisation show considerable 
