GEOGRAPHY OF NORTH AMERICA. 221 



The highest elevation above Lake Superior which the lines 

 traversed was one hundred and ninety meters. The result- 

 ing- height of the mean surface of Lake Superior above that 

 of Lake Michigan is 6.249 meters, or 20.5 feet. In a similar 

 way, the line from Lake Erie to Lake Huron has been relev- 

 elled, and a preliminary reduction gives the height of Lake 

 Superior above mean tide at New York as six hundred and 

 two feet. Water-level observations have been continued at 

 two stations each on Lakes Ontario, Erie, and Michigan ; one 

 eacli on Lakes Superior and Huron, and in the straits con- 

 necting Lake Huron with Lake Superior on the one side and 

 with Lake Erie on the other. Six lines of soundings have 

 been carried across Lake Erie, besides many shorter lines to 

 complete the hydrography, for which, to mention one only 

 of many details in this branch of the work, over 40,000 casts 

 of the lead are reported. 



The extension of the survey to the Mississippi was begun 

 in November, 1876 ; its object is to construct a good map of 

 the river, and to obtain data for any improvements needed 

 in its navigation, and for the location of levees. The party 

 undertaking it was instructed to base the topography on a 

 secondary triang illation, so that the work might be the reli- 

 able base for all future partial surveys; to erect numerous 

 permanent stone marks, determined in position and height ; 

 to run continuous lines of level alon^ the river-bank, and 

 also back from the river once in each mile, so as to locate 

 contour lines for every three feet of elevation. The survey 

 was begun at Cairo, 111., and pushed southward for fifteen 

 miles. A base-line a little over a mile in length was meas- 

 ured, and from it the triangulation was carried about seven 

 miles, fourteen stations being occupied : the topography 

 was carried back one mile from the shore-line. Permanent 

 reference marks were set in three straight lines across the 

 river, five or six miles apart, each consisting of two stones 

 on either side of the river, placed respectively one and three 

 half-miles from the river-bank. From old surveys made by 

 the government in 1809-10, it appears that portions of the 

 banks have been washed away to the depth of nearly half a 

 mile. The character of the bottom of the river was care- 

 fully examined, and borings made near Cairo to study the 

 old bed. A line of levels of precision was also run from 



