I4o0 THE HAMILTON ASSOCIATION. 
g. Perpendicular lines are not parallel. Two perpendicular 
lines may form an angle of 45 degrees, or an angle of any size, or 
two perpendicular lines may make one straight line, because of the 
sphericity of the earth. 
10. In digging canals an increase depth of 8 inches to the mile 
is required to maintain uniform depth. 
11. Surveyors in laying out large tracts of territory have 
‘““excess measurement” to deal with, because of the earth’s sphericity. 
12. The angles at the intersection of parallel and longitudinal 
lines are not right angles, because the earth is a sphere. 
a Z 
os i eae 
