GARDNER.] 



GEOGEAPHY ELEVATION OF DATUM-POINTS. 



643 



MEMPHIS TO CHICAGO— Continued. 



Feet. 



Various datum-planes. 



.2 c o a 



Chicago directrix 



From the figures given above vre have also Memphis, 



Tenn., H. W. = city datum + 100 feet. 

 Memphis, H. W. 1858 



Do 



Hictman, Ky. , H. "W. 1858 



Columbus, Ky., H. "W. 1858 



Cairo, H. W. 1858 



Cairo City datum=ordinary L. "W. 



Above M. T. 'New Orleans. 

 Above New Orleans M. T . 



Above previous extreme 



H.W. 

 Above New Orleans M. T . 



590, 01 

 218. 65 



Below H. W. 1858 



219. 65 

 313. 01 

 320. 01 

 331. 51 

 291. 13 



The adopted elevation of the Chicago directrix being 587.15, this line 

 of levels, nine hundred and sixty miles long, reaches Chicago with an 

 error of only 2.86 feet. The error of cpniiection, due to using the H. W. 

 slope of the Mississippi River for fourteen miles, would not probably 

 exceed a foot or two. At Memphis, the line is checked to within 1,8 feet 

 with the levels from Mobile Bay. At Columbus and at Cairo, the line 

 is checked by the M. & O. E.. R. ; but the only reports of this line which 

 I can find give the elevations as determined by an experimental survey, 

 the results of which were reported to the second meeting of stockhold- 

 ers, in 1850. The character of reconnaissance-surveys is such, and the 

 results have proved so inaccurate when I have been able to compare 

 them with the construction-levels, that I have not felt justified in giving- 

 the results of the M. & O. E. E. preliminary survey to Columbus and 

 Cairo any weight as compared with tlie profiles of constructed lines. It 

 is, however, upon this preliminary survey that Humphreys and Abbot, 

 in their Hydraulics of the Mississippi River, base their elevations of 

 Saint Louis, Cairo and Columbus, and consequently their slope of the 

 river from Cairo to Memphis. 



RESULTS BT M. & 0. E. K., PEELIMINAET, COMPAEED WITH ADOPTED LEVELS. 





Feet. 



Various datum-planes. 



Elevation in feet 

 above mean sur- 

 face of Atlantic 

 Ocean. 



H W. at Columbus, by M. & 0. preliminary survey 



M. T. Mobile Bay ,.... 



308. 50 



1.70 



306. 80 



320. 01 



320. 



Above M. L.W. Mobile Bay 

 Above M L T 







Above M. T. Mobile Bay . . 

 Above M. T. New Orleans. 



Above L. T. Mobile Bay. . . 



306. 80 



H. W. 1858, Columbus, by N. 0. J. & G. N., M. C, M. & 

 T., M. & L., N. & N. W. E. E. 



320. 01 





Above M. T. Mobile Bay . . 



318. 30 



H. W. 1858, Cairo, by N. 0. J. & G. N., M. C, M. & T., 

 M. & L., N. & N. W. E. E. 





331. 51 









It will be seen further on that the elevation of the Cairo City datum, 

 as brought from New Orleans, is within a foot of that brought from the 

 Cleveland directrix, via Cincinnati and Indianapolis. In summing up 

 all evidence on the elevation of the Cairo City datum, on page G17, it 

 will be shown why I reject the levels of the M. & O. R. R. 



If we consider now the line from Portland to Chicago and from Chi- 

 cago to New Orleans as one, we have a connected chain of railroad-levels 



