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Remarks on re-meafuring the Lines with two rectangular 
Levels, or meafuring Frames. • 
The levels ufed in this work were, each, 20 feet in length, and 4 
feet in height. They were made of pine, an inch thick, and in form of 
a retftangle ; the breadth of the bottom board was 7! inches, that of the 
top ~ 3 inches, of the ends =z 4! inches, and the bottom and top were 
ltrengthened with boards firmly hxed to them at right angles. The joints 
were fecured with plates of iron, and the ends were plated with brafs. The 
plumb lines ufed in fetting them level, were := 3 feet and 2 inches in 
length, and hung in the middle of the levels, being fecured in a tube from 
the wind, in the manner of carpenters levels 3 wherefore we called thefe by 
the fame name. 
^Vhen the plumb-line bifedted a point at the bottom, the ends were 
perpendicular. 
Where the ground was not horizontal, or there were logs, See. to pafs 
over, one end of the level was raifed by a winch and pully. 
The level being fet, a fliort ftaff was drove into the ground (very near 
and oppofite the plumb-line), in the top of which moved a thin plate of 
iron, about 12 inches long; at the ends of which were points, which were 
diredled to the interfeclions of lines, drawn on the board that covered the 
plumb-line. By bringing the points in a line W'ith one of the faid inter- 
fedlions, if the level was by accident moved, it might be difeovered, and 
brought again to its place. 
A level being thus marked, the end of the other was brought in contact 
with it, and marked in the fame manner, before the firft was moved ; the 
firft was then taken up, and fet before the laft. And fo the operation was 
continued. Mr. Dixon attended one plumb-line and ftaff, and I the other. 
The meafure was carried on in a ftrait line, and in the proper direction, 
by pointing the levels to the fartheft part of the vifto that could be feen j 
this was readily and accurately done, on account of their lengths. The 
levels were frequently compared with the brafs ftandard, of 5 feet, provided 
for that purpofe, and the difference was noted between 8 times the brafs 
ftarrdard, and the length of the two levels taken togerher ; as may be feen 
in the 3d and 4th columns of the following table. This difference ferves 
for reducing the meafure taken with the levels, to what it would have been 
if it had been taken with the brafs ftandard itfelf ; fee column 6th. For fa- 
cilitating this comparifon of the levels with the brafs ftandard, pieces of brafs 
were fixed into the bottom boards of the levels, on each of which was 
drawn a faint line. And one tenth of an inch at the end of the brafs ftan- 
darej being divided into ten parts or hundredths of an inch, the difference 
between eight times the brafs ftandard, and the two levels joined together, was 
with the help of a magnifying glafs of a fliort focus, determined to great 
accuracy. Moreover, the brafs ftandard being liable to alter with the changes 
of heat and cold, a further correction becomes neceffary on that account, in 
order 
