24 BULLETIN OF THE 



points along all great rivers were located, and cities at the foot 

 of the Rocky Mountains. 



The general result of the investigation is to show that we did 

 not know accurately the elevatioji of any points except on our 

 seaboard. The elevation of our lakes is changed by it ; the 

 fall of the Ohio, Mississippi, and Missouri Rivers is materially 

 altered ; and the mean surface of the continent is found to be 

 higher above the ocean than was supposed. 



The Saint Louis directrix is fixed at 428.29 feet, a change of 

 23 feet from the old determination ; while Kansas City and all 

 the railroads leading westward from it are shown to have an 

 error of 115 feet. One of the most striking instances of accuracy 

 in long lines of American railroad levels is given by connecting 

 the New York Central and Lake Shore and Michigan Southern 

 Roads to Chicago, and thence southward by the Illinois Cen- 

 tral and other roads to New Orleans, making a line 1800 miles 

 long, which reaches the gulf with an error of only 2^ feet. 



By the Iowa railroads and the Union Pacific R. R., the ele- 

 vation of Denver is deduced from Chicago; and by the Missouri 

 roads and Kansas Pacific, the height of Denver is gotten from 

 Saint Louis, These independent i-esults differ less than five feet, 

 and the elevation of Denver is established at 5196.58 feet. The 

 heights of the principal Rocky Mountain peaks were measured 

 ^ above Denver; and now, that this point is fixed, we can know 

 with certainty the elevations of our great mountains. 



Considerable discussion followed, mainly on the effect of local 

 attractions in determinations by the spirit-level on mountain 

 slopes, in which JNIessrs. Hilgard, Abbe, Gardner, and Cutis 

 participated. 



Mr. Abbe said that, for the sake of clearness, he would state 

 the hypsometric problem as it presented itself to him. 



We desire to deduce the exact figure of the earth's surface, or 

 the position of each point of its surface, relative to a system of 

 co-ordinate axes, whose origin is an assumed approximate centre. 

 To this end we assume for the whole earth Bessel's, or some 

 other, ellipsoid, that agrees well with a limited known portion, 

 and must then determine, as deviations from this geometrical 

 figure, the irregularities of the actual surface. 



If by levelling operations we seek to determine the relative 

 elevations of the surface, we obtain thereby the altitudes not 

 above the assumed spheroid, but above an irregular level surface 

 which is the result of the irregularities in the direction of gravity. 



