Relief of Earth's Surface. — Curtis. 179 
purpose and use of the rational model, the representation of the 
real features of the land in their characteristic expression, that 
attention should be called to the error. 
It is as reasonable to expect satisfactory results from the 
mechanical models if truth is wanted, as it is, from giving, for 
example, several contours from a live figure to a mechanic and 
permitting him with this data alone to attempt to carve out 
an expressive or "living" statue. This product would stand 
at the right hight and would be correct according to the data 
furnished at several levels, but in other respects could one 
"speaking likeness" be expected? Were the usual methods of 
constructing mechanical models employed, neither arms, "parts" 
nor head could be distinguished. If the scale employed were 
the same throughout, the model would as near resemble a stack 
of straw as a man, and if the vertical element were exagger- 
ated according to the frequent custom in "reliet maps" the 
statue would resemble the flag pole. 
The two distinct classes of work have fields of their own, 
and since the 'ruthful model embodies all the correct data of 
the mechanical model the two bear the relation of part and 
whole. 
The use of the mechanical corresponds to that of the con- 
ventional or exaggerated engineering section or "blackboard 
diagram" which does not assume to be natural and therefore 
may not be depended upon except for special purposes. Under 
the direction of a skilful demonstrator this may convey ideas 
of relative bights which would not be so apparent on the 
truthful representation of the facts, or upon the field. It does 
not burden one with the multiplicity of facts which crowd the 
landscape, and liberty may be taken with it, from covering 
over with letters, figures and colors to twisting one of its 
scales any number of times out of its true form. It has the 
value of the arbitrary diagram representing graphically some 
points to be illustrated. 
The mechanical model is cheaper since it can be rapidly 
produced by inexpensive labor while the truthful model re- 
quires the direction of a scientist and the skill and appreciation 
of the landscape sculptor and painter. 
One of the purposes of resorting to topographic reliefs is 
to present to the general public the facts of diction which 
