226 On the Topography and Soil of Paris. 
concurred to raise the soil of Paris and to give the appears 
ance to its surface which it now has, it may be necessary 
to describe this appearance with the greatest possible ac- 
curacy. 
The simplest and most expeditious mode of judging, in 
. order to obtain certain results capable of being graphically 
represented, has been to indicate on a plan of Paris the 
height of the different points of its surface above or below 
a determinate horizontal plane. 
The scale of the plan drawn by Verniquet was large 
enough to admit of the smallest differences in point of 
height being rendered perceptible. ‘This great topographi- 
cal work, therefore, presented us with the means of facili- 
tating the task which we undertook. 
We began with levelling the right bank of the Seine: 
we divided, by great lines directed from cast to west and 
from north to south, the whole space contained between 
the river and the new Boulevards: we thus obtained profiles ' 
which were connected with each other and with a common 
index. When we were well assured by the necessary veri- 
fications of the accuracy of these profiles, we divided by 
new lines the great spaces contained between the directions 
of the former, and we obtained still closer outlines. All 
these having been verified in their turn, we divided by 
more multiplied lines the space comprehended between the 
second lines of operation, and so on, multiplying the strokes 
of the level more and more until we obtained the line of 
intersection of all the streets of these quarters. In addi- 
tion to all this, we chose a great number of points, the 
heights of which were Jaid down in a separate sheet, and 
which were distinguished on Verniquet’s plan by a particu- 
lar notation ; precautions .by means of which these points 
will. serve not only to verify the heights already found, 
but to multiply their number if that were necessary. 
The heights of the soil at the intersections of all the 
streets being determined, we searched between two conse- 
culive intersections, and supposing the slope of the ground 
to be uniform, for one or more points which were at heights 
determined with respect to the plan of the level, which is 
supposed to be raised 50 feet above the surface of the water 
of the basin of Ja Villette, or, what is the saine thing, about 
26 metres above the Seine at low water. 
We afterwards joined by straight Jines all the points 
which were found at the saine height. This gave the trace 
of a polygon, which evidently represents the intersection of 
the surface of Paris by a horizontal plane. 
We repeated this operation for every metre in ee 
and 
