68 WISCO]S"SI]S" ACADEMY SCIENCES, AETS, AND LETTERS. 



LEVELING, AND USE OE THE BAROMETER. 



BY JOHN" NADER, C. E. 



The term leveling is used to denote the art of determining the 

 difference of level between two objects or of one object with refer- 

 ence to some fixed or known object. 



Leveling is one of the most difficult branches of surveying, in as 

 much as it is impossible to detect or correct an error as may be 

 done in some other branches without again repeating the whole 

 work. 



The term level is also applied to the position of an apparent hor- 

 izontal plane. The object desired may be attained in various ways 

 depending upon the purpose of the work and the amount of accur- 

 acy required, and also upon the instruments which may be avail- 

 able. 



The art of leveling is based upon invariable natural laws, i. e., 

 the horizontality of a body which yields to the fullest extent to the 

 force of gravit}', such as a liquid under favorable circumstances, or 

 a body freely suspended and submitted to the action of gravity. 



It would hence be an easy matter to assume a horizontal plane 

 and refer objects to the same were it not that a number of influ- 

 ences come to bear upon the results which often differ widely from 

 the truth. 



The principal influences are the following: Mechanical imper- 

 fections; errors of observation; effects of temperature; curvature 

 of the earth and atmospheric refraction. 



I will here notice the latter. When an object is viewed obliquely 

 through a transparent medium of any nature whatever, it does not 

 occupy the position in which it appears, the rays being bent by re- 

 fraction. If the medium be of a uniform density throughout the 

 ray will pass through in a straight line, but if the density of the 

 medium is variable, the line will be irregular, and as in the case of 

 the atmosphere, whose density is as its height, the line of refraction 

 will be a curve, and since the denser medium will have the gre ater 



