GAEDNEE.] 



GEOGRAPHY ^ELEVATION OF DATUM-POINTS. 



641 



C. O. & 1. E. E. 1152.98; the P. F. W. & 0. being probably too low. 

 At Chicago, the P. P. W. & C. E. E. levels are 1.74 feet too low. The 

 mean of these checks at Alliance, Crestline, and Chicago would make 

 the whole of the western part of this line 1.27 feet too low. I therefore 

 think it not improbable that the elevation of Pittsburgh by the Pa. E. 

 E. may be about 1 foot too low. Considering all the evidence, I should 

 be inclined to adopt 746 feet as the elevation of Pittsburgh Union-depot 

 track instead of the Pa. E. E. result of 744.913. 



The longest connected line of railroad-levels that we have an opportu- 

 nity to check at Chicago is that of the G. T. E. W. of Canada, from 

 Portland, Me., to Detroit, Mich., and thence to Chicago by the M. C. E. 

 E. The details of the line and its results are as follows : 



Elevation of Chicago directrix by G. T. B. W. of Canada and M. C. E. B. 



Feet. 



Various datum-planes. 



=« CO a 

 .2 00 a 



Ills 



Mean high tide at Portland 



Mean tide, G. T. E. E. datum at Three Eivers . 



Toronto 



Detroit junction 



Chicago depot 



Chicago diiectris 



Do 



Distance from Portland to Chicago 1,142 miles. 



4.5 



6.77 



239. 78 



340. 22 



4.70 



6.50 



Above M. T. Portland 



Above M. H. T. Portland. 

 Above G. T. E. W. datum 



Above Toronto 



Below Detroit junction. . . . 

 Below depot of M. C. E. E. 



580. or 



My adopted elevation is 587.15 feet 5 therefore, this line of levels, 

 eleven hundred and forty-two miles long, appears to be in error only 

 7.08 feet, and the greater part of this error is in the M. C. E. E. line in 

 the last three hundred miles. The quality of the line in different parts 

 is shown by the checks at various points. The first check is at Toronto, 

 six hundred and thirty miles from Portland, where the line is connected 

 with the surface of Lake Ontario, the state of the water unknown, but 

 assuming it to be the mean surface, the levels are 3.72 feet too low. The 

 next check is at Sarnia, on Lake Huron, eight hundred miles from 

 Portland, where the line is connected with the surface of this lake; the 

 state of the water is again unknown, but, if assumed it to be the mean 

 surface, the levels are 1.88 feet too low. At Detroit junction we have 

 an approximate check by the height of that point, by the M. C. E. E., 

 above H. W. in river at Detroit, June 1, 1869. The M. C. E. E. gives 

 the height of Grand Trunk junction at 17.90 feet above H. W. 1869. 

 This point we have supposed, by the geological-survey report, to be 

 about 4 feet above the mean surface of Lake Erie; heace the junction 

 would be 21.90 above Lake Erie, or 594.98 feet above M. T. The eleva- 

 tion of this point by the G. T. E. W. is 59L27, a discrepancy of 3.71 

 feet. From this, and from the ninth determination of elevatiou of Lake 

 Michigan, it appears that there is an error of about 4 feet in the M. C. 

 E. E. profile, in the difference of elevation in its termini. The leugth 

 of this line is two hundred and eighty-four miles. 



Establishing the elevation of the Chicago directrix gives us the means 

 of checking another very long line of levels, extending from M. T. at 

 New Orleans to Chicago, a distance of nine hundred and sixty miles. 

 The details of the line are as follows : 

 41 a s 



