624 On the Method of judging by the Ear 
matical planes, with a view to assist his imagina« 
tion while he peruses the sequel. 
The first of these planes lies parallel to the 
horizon, when the hearer stands in an erect 
posture ; and contains in it the imaginary right 
line that joins the ears, which in future will be 
called, the axis of hearing. This plane, there- 
fore, divides the head into two dissimilar solids, 
the one superior, and the other inferior in 
respect to itself, The other plane must be 
conceived to stand perpendicular to the last, 
and to bisect the axis of hearing at right angles. 
On these accounts, it evidently passes from the 
front to the back part of the head, dividing it 
vertically into two portions; which are so nearly 
alike in most men, that they may be considered, 
without danger of error, as two equal and 
similar geometrical solids. The preceding de- 
scription, which bears the face of a mathematical 
construction, may perhaps assist my readers in 
conceiving what was meant above, when I spoke 
of combining the perception produced by the 
comparative elevation or depression of a sound- 
ing body, with the sensation which arises from 
its place on the horizon, relative to the axis of 
hearing: for every man judges, with some 
degree of accuracy, of the angles which are 
made with the two planes described above, as. 
well as their common section, by the right line 
