60 Captain Vetch on the Utility of Contour Lines on Plans. 
of Government, the facilities and economy afforded by such 
a previous work would be manifest, and cannot be too highly 
appreciated, both for the saving of time and money, and for 
insuring accurate judgment and execution in the works of 
improvement; and I have no doubt that were such going 
forward to any extent, the expense of the Ordnance levels 
and contours would be saved in three years, in the mere em- 
ployment of surveyors, independent of the errors and ex- 
penses that might be entailed by less general and accurate 
views of the field to be operated on; but, having served for 
any particular purpose and occasion, the Ordnance contours 
and levels would be ready for use again and again, and one 
datum would be established, to which all altitudes would be 
referred. For the purpose of applying the liquid manure to 
irrigation, the contour lines would be essential; and at Leeds 
they would prove particularly serviceable for that purpose, 
and also for the extension of the buildings, and for the 
drainage on the low flat part of the town south of the river. 
By referring to the diagram plan already noticed, the ad- 
vantages of contour lines will be more readily perceived. 
The contour lines are there drawn at every four feet of alti- 
tude, above a fixed datum, and have a slight shade on their 
under side, the more readily to point out the direction of the 
fall of the ground; and this in some cases is essential, as 
otherwise it would be difficult to distinguish at sight between 
an isolated hill and isolated hollow. 
From such a contour plan a section may be procured in 
any direction in a few minutes. Suppose a section is required 
of the ground between the points A and B, we have merely 
to draw the line A B on the plan, and a corresponding line 
A’ B’ on section, and under the last to draw a number of 
parallel lines at equidistances of four feet, to whatever ver- 
tical scale we think most appropriate. On the diagram plan 
the scale is 400 feet to an inch, and the horizontal scale on 
the section is the same, but the vertical scale, to render the 
subject more distinct, is 40 feet to one inch.* The next opera- 
* The original diagram has been reduced, which accounts for an ap- 
parent discrepancy in the plate.—EDI?. 
