GAKDXEK.] GEOGRAPHY ELEVATION OF DATUM-POINTS. 639 



Hence, if the height 589.15 feet is adopted as the mean surface, 587.15 

 feet must be the elevation of the city-directrix. I have given their respect- 

 ive relative weights to the different determinations for the following rea- 

 son : The fourth, fifth, sixth, and ninth are of least value, because they 

 depend on the height of the water at Detroit, being 3 feet above Lake 

 Erie, as reported by the State geological survey. The original data upon 

 which this report depends cannot be found, and I therefore consider it 

 open to much doubt. The first, second, and third determinations are 

 given the value (2) because they are first-class railroad-lines, run from a 

 base at Toronto, called surface of Lake Ontario, but which I do not 

 linow to be the mean surface, but simply assume it to be so. They run 

 directly to Lake^ Huron, but only in one case is the state of the water 

 given 5 and then I do not know of how many years it is the mean. 

 These results should, of course, be far better than the fourth, fifth, sixth, 

 and ninth, but are not nearly so probable as the seventh and eighth, 

 which depend on railroad-lines run directly from the Cleveland direct- 

 rix to the Chicago depots, which in this case seem well connected 

 with the Chicago directrix. The height of the Cleveland directrix, as 

 brought through on these same railroad-lines, the L. S. & M. S., the 

 P. F. W. & C, C. & P., and C. C. C. & L R. Es. had checked so closely with 

 the canal and lake surface result that they are entitled to great weight 

 in their westward extension to Chicago. In connection with these 

 directrices of Cleveland and Chicago, the fluctuations of the lakes have 

 been obvserved, and the mean surfaces determined. For accounts of 

 these fluctuations see Smithsonian Contributions, 1860: Fluctuations of 

 Level in the ISTorth American Lakes, by Charles Whittlesey; also a 

 recent report from the Dudley observatory, Albany, IST. Y. 



Determinations 1, 2, 3, 7, and 8 are the only ones which rest upon 

 sufficient evidence to make them of much value, and it will be noticed 

 that the range among these five is only 2.53 feet, with the lake at an. 

 unknown stage of the water. The range among those three that refer 

 to the mean surface of Lakes Huron and Michigan is only 1.53 feet. I 

 think therefore that the elevations of the mean surface of Lake Michigan 

 and of the Chicago directrix will probably not be open to a change of 

 over one foot. We have here at the Chicago directrix an opportunity 

 for comparing the results of two very long and independent lines of 

 railroad-levels, those of the X. Y. C. and L. S. & M. S. R. Es., and of the 

 Pa. E. K and P. F. W. & C. E. E. I ^ive the results in detaih 



