OF ARTS AND SCIENCES. 285 



in seconds of arc. The mean altitude of the nearer point, C, was 

 found by these observations to be —317", and the next column gives 

 the residual from this mean. The last column gives the object ob- 

 served, C being employed to indicate a second point on C, which 

 with certain illuminations was more readily seen. C was found to be 

 277" below C, and the measures of its altitude have been corrected 

 by that amount. 



The average value of the residuals in the last column but one is 

 3. "4, which shows the degree of accuracy that may be attained with 

 this instrument in determining absolute altitudes. The accuracy, port- 

 ability, and cheapness of the micrometer level ought to render it use- 

 ful for many purposes. Valuable work could be done with it from 

 any station commanding a distant view. Observations of distant points 

 in different azimuths, under varying meteorological conditions, are still 

 much to be desired. 



From the differential character of the observations described above, 

 it has not been necessary to consider the curvature of the earth, or its 

 variation in different azimuths, in the present discussion. 



