ee 
Captain Vetch on the Utility of Contour Lines on Plans. 59 
sewers may have, and how they must join each other, with- 
out wasting time and money on a number of trial levels. If 
the town is situated on the banks of a river, and if the floods 
of the river rise, say four feet above datum, he will perceive 
at once the extent of such floods, as shewn by the tinted 
ground on the diagram, enclosed within the contour lines 
marked with the number (4) or 4 feet ; and the engineer then 
knows over what extent to provide for such floods, in laying 
out roads, streets, or drains, &c. 
If new streets are to be laid out, the engineer will per- 
ceive at once, from such a plan, the declivity and aspect of 
the building ground, and the best line of drainage adapted 
for them. 
The contour lines being drawn on the map, and identified 
by corresponding marks on the buildings, serve as a record 
and reference for all past and future purposes. It is true 
that if no contour plan exists, the engineer may get what 
he wants by having recourse to extensive levellings for each 
particular object; but this expense is generally lost for any 
future object, or for any other occasion, and thus the same 
trouble and expense may have to be incurred frequently for 
levelling the same piece of ground; but having the whole 
extent of the town and district before him on a contour plan, 
the engineer can study his subject to better advantage, and 
observe how improvements can be best and most economi- 
cally effected. In waiting for trials of level, time is lost, 
and the Engineer is always loth to employ more time and 
money on these than can be helped, and he concludes with- 
out having the case sufficiently before him. 
When levels are executed piece-meal, and at various times, 
each engineer selects his own datum, and confusion and 
mistakes are thence likely to occur; so that no cautious en- 
gineer would trust to any previous work, but would level up 
to some fixed point, however distant ; but even if otherwise, 
he might not choose to risk his reputation on the previous 
levellings of others, of whose qualification he could not judge. 
But if the levels or contours were executed by the Ordnance 
Survey, his confidence would be complete. 
If contour plans and marks were made under the authority 
