58 Captain Vetch on the Utility of Contour Lines on Plans. 
of their absolute heights above a fixed common point or 
datum, or the relative height between any two sites, on the 
plan; but when the horizontal plan exhibits these contour 
lines, drawn, say at every four feet, and marked 0, 4, 8, 12, 16, 
feet, &c. (Plate I.), we see at one glance all the places situated 
at these respective elevations above datum, and know their 
relative heights to each other, and with a little practice, the 
eye will be able to intercalate (within one foot or less) the in- 
termediate heights between the contour lines; and to facilitate 
this operation, vertical distances of four feet between the 
contour lines will prove most advantageous, as most readily 
admitting of subdividing. The contour lines on the Ordnance 
plan of Windsor are drawn at every four feet of altitude, and 
on the low flat ground subject to inundation at every two feet. 
The engineer or other person who therefore consults such 
a map for practical purposes, obtains a correct notion of the 
height of every part, on the plan, and the declivities or 
slopes ; and for engineering purposes a knowledge of the 
heights is equally necessary with a knowledge of the dis- 
tances ; and if the plans do nct afford him that information, 
he must commence a number of levellings, to shew what is 
practicable, and what is not. These levellings occupy much 
time, expense, and delay in making, and at last only give 
the engineer the means of judging between the two or three 
or more lines which he has levelled, while the contour lines 
enable him to judge of every possible line he may wish to 
examine or compare. 
By means of contour lines, in any street, or lane, or build- 
ing site, the direction of the fall of the ground is known at 
sight ; and the amount of that fall, in proportion to the dis- 
tance, may be ascertained between any two points situated 
on the contour lines. Thus the fall from a to 4 on the diagram 
is known, by the contour lines, to be eight feet ; and the dis- 
tance measured on the plan being 400 feet, the slope would 
be one foot in 50. An engineer can, therefore, see, without 
any trial levels, the undulations and descent of each street 
from one contour line to another, and he knows the amount 
of cutting or filling, to reduce the street to a level or a re- 
gular incline. He knows also what descent the kennels and 
