42 Proceedings of Royal Society of Edinburgh. [sess. 
If we now compare the relative deviations given in this table 
with the relative densities given in Table B, we at once observe 
that the differences in the former are of practically the same 
magnitude as those exhibited in the latter ; therefore, if we proceed 
to arrange the waters according to their degrees of total salinity 
or “ total salts ” per kilogram, the value obtained for the several 
relative deviations will enable us to differentiate the waters as 
sharply and decisively as the corresponding values for the relative 
densities, and the use of either method would lead us to arrange 
the waters according to the following descending order of salinity : 
l v , 5 V , 2 V , 4 V , 3 V . The waters 2 V and 4 V are practically identical, 
as both methods place the former in the higher position by only 
• 00001 . 
The second or refractive index method for comparing waters, 
requires, in addition to the minimum deviation observation, an 
accurate determination of the refracting angle of the prism : when 
these are known, the refractive index //, may be calculated from 
the well-known formula 
_ sin | (A + D) 
sin J A. 
A being the angle of the prism and D the minimum deviation. 
This method would, however, probably prove to be far less con- 
venient in practice than the relative deviation method ; and since 
the value for fi increases or decreases with the deviation, one 
would be led to adopt the simpler or relative deviation method, 
rather than the other. It should also be observed that when the 
refractive indices for similar samples of average sea-water are com- 
pared with one another, the “ total salinity ” of one water is gener- 
ally distinguished from that of another by a change in the value 
of the 5th decimal figure only ; occasionally the 4th figure changes 
by 1 ; the relative deviations, on the other hand, may and do show 
well-marked differences in the 4th decimal. 
In Table G the relative densities, relative deviations, and re- 
fractive indices of the five samples of water examined are grouped 
together, so that the results obtained by the different methods 
under consideration may be conveniently inspected and compared. 
