List of Altitudes in Illinois. 87 



cheeked by them ; aad the relations of the railroads at 

 intersecting points were ascertained, and the profiles of 

 the roads checked on each other, using those that had 

 been corrected by U. S. data as master systems. 



Barometric profiles were made, with stationary and 

 moving barometers, of such roads as had no profiles, 

 and after the profiles of the other roads had been brought 

 into agreement, these were added and corrected in the 

 same manner. 



To the outline thus established, the details of the surface 

 between the roads were added by traverses, with bar- 

 ometer and telescopic hand-level, arranged to intersect 

 railroads as often as possible, and practicallj^ to bring 

 the observer within sight of every section of land in his 

 district. Prominent points either of elevation or depres- 

 sion were visited, and observations made upon them. 

 Many cross checks and other means of correction were 

 applied to overcome errors due to changes in atmospheric 

 pressure, instrumental irregularities, and observation. 



In regard to the accuracy of the barometric work, it 

 may be said that a county was made the unit of work 

 in these surveys, and that the results obtained in each 

 county in no way depended upon those arrived at in ad- 

 joining territory ; in fact, the observ^er was generally un- 

 informed as to what had been done in the surrounding 

 counties. When the results of these independent surveys 

 were brought together, it was found that a difference of 

 more than twenty feet in the elevation of points along 

 county lines but rarely occurred, the average difference 

 being less than ten feet. The cost of the survey was 

 twenty-four cents per square mile. 



The figures at the right of the page indicate, in feet, 

 the elevation above the level of the sea. Wliei'e the lo- 

 cality name in the left-hand column is followed by the 

 name of a railroad, the locality is a station on that road. 

 Where it is followed by a personal name in the middle 

 column, it is the name of a small town or other rural 

 locality, and the personal name is that of the observer. 



