ANEROID BAROMETER IN GEOLOGICAL SURVEYING. 137 
the task was completed in sixteen months, with an average force 
of ten men, and at a total cost of twenty-five cents per square 
mile. The degree of accuracy attained can be inferred by the 
following facts. Each county was surveyed separately. Asa 
rule the observers who surveyed contiguous counties were unac- 
quainted with each other’s results. When the county maps were 
brought together it was found that the average difference in the 
elevation assigned to common points was less than ten feet. A 
difference of twenty-five feet was occasionally met with, but was 
not frequent. These errors were corrected, and the maps re-drawn, 
omitting most of the elevations, but retaining the contours. Plate 
II. is a reproduction on a much reduced scale of the map of one 
of the rougher counties, while Plate III. represents one of the 
more level ones. 
Better results could, of course, have been reached had it been 
possible to devote more time to the work. 
Co WW. INO, 
UNIVERSITY OF ILLINOIS, 
December 20, 1894. 
