THE WYE LEVEL 



Hi: Y- Level was invented by Jonathan Sissons of 

 London in 1740. The general design has never 

 been changed, for revision of details which make 

 for improvement is necessarily restricted by the 

 fundamental principles of construction. Our latest 

 improvements include a new method of interior 

 focusing, which reduces the possibility of derange- 

 ments in collimation, and a torsionless mounting for 

 the level vial, which prevents undue strain in varying temperature. 

 The routine of adjustment is comparatively simple, but perma- 

 nency and stability are so dependent upon mechanical conditions 

 in the collars that, for the more particular work, adjustments should 

 be tested and rectified frequently. The prime accomplishment 

 sought in the adjustment of any leveling instrument, of whatever 

 design, is parallelism between the bubble axis and the sight-line. 

 All details of construction and all methods of adjustment are only 

 a means to this end, for, by its attainment, the sight-line can be 

 relied upon as lying in the true horizon when the bubble occupies 

 the center of its scale. 



The vertical axis should be regarded only as means by which 

 the sight-line, so adjusted, may be directed into any position of the 

 field; but for convenience sake it is highly desirable that the vertical 

 axis should be perpendicular to the sight-line and that it should be 

 otherwise provided with those means by which it can be adjusted 

 to perfect verticality. 



The collars determine the geometrical axis of the tube and are 

 at the basis of the principal adjustments. The sight-line is colli- 

 mated, or the cross hairs are brought into the optical axis, by longi- 

 tudinal revolution in the collars, on the one hand, and the bubble- 

 axis is adjusted to the same collar-axis by end-for-end reversals, on 

 the other. On the assumption that things which are equal to the 

 same thing are equal to each other, we must feel satisfied that the 

 collimated line of sight will be parallel to the bubble 



This is a safe speculation in a new instrument which is also 

 free from eccentricity of mounting in the objective; but when the 

 collars become either coniral or eccentric through continued wear, 

 this method of comparison should not be considered as infallible. 

 In such a case the peg method, with a test base at least 100 feet 

 each side of the center, is the only field method upon which the 

 engineer can place dependence. (See pap. 



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