®? 
re ee we 
and on our Perception of Distance. * 369 
sider its influence, our up and down, our hofizon and our general 
geometrical circumscription, essentially as matters @f course, as 
intuitional perceptions, and as subjects with which reason has 
little concern. It is doubtless true however that we acquire such — 
fundamental ideas from continuous experience, just as positively, 
as we do any other elements of our practical knowledge. ; 
Our ideas of the vertical and horizontal and our sense of distance, 
have thus become so firmly established by our life-long experi- 
ence, that their origin, antecedent to our philosophising exercise 
of reason has become much obscured. Yet they are clearly capable 
of rational analysis, and their component elements can be e 
subjects of experiment and variation. As introductory to some 
observations of this nature which I have made, I will present 
such general views as these and other experiences have suggested. 
The vertical and horizontal are our habitual codrdinate axes 
axes. We combine a mental estimate of distance with an angu- 
lar reference to the vertical and horizontal axes and plane. In 
a central line in our field of view, by angular coérdinates. When 
we attempt to embrace the whole horizon in one mental sweep, 
we for the most part refer objects to the North and other compass 
lines; this however isa more deliberate process, and our con- 
sciousness of its steps is comparatively complete. 
In the attempt to define the sources of our perception of the 
horizontal and vertical, two stages of causation have oceurred to 
nution of effective gravity due to the increasing rapidity of bodily 
Seconp Seams, Vol. XX, No. 60.—Noy., 1855, 47 
