Mr. Nixon's Theoty of the Spirii- Level. -'563 



The measurement of the angular inclination of a line lying 

 in a vertical plane does not differ essentially h-om that ot a 

 line of maximum inclination described on an mclmed plane, 

 but is susceptible of more evident demonstration. Let A 15 

 and CD be two straight lines lying in the same vertical plane 



and equally inclined to the horizontal line HH. Place the 

 under edge of either (vertical) side of a level on AB, and, 

 havincr marked the ends of the bubble, reverse the level, and 

 makelhe (opposite) under edge coincide with DC; when the 

 bubble, as the same end of the level is eijually elevated as be- 

 fore, will come to rest at the initial marks. Depress this ele- 

 vated end until the under edge coincides with AB, w-lien the 

 bubble must have passed over a distance on the scale equal 

 to the anMe CDB, evidently the stm of the two equal angles 

 CDH and HDB, either of which is equal to the inclination 

 of the line AB (or that of CD). In depressing the reversed 

 level from its position on DC to that on AB, its under edge 

 must have coincided with, or been parallel to the horizontal 

 Hue HH when the bubble had run over halfihe distance an- 

 swerinfr to the double inclination of AB, equal angle CDB. 

 HencAhe under surface of the level will be horizontal when 

 (its sides being vertical) the middle of the bubble settles at a 

 point on the tube, or division on the scale corresponding to 

 the point of bisection of the space run over by the bubble, on 

 reversing the level on any inclined line. When this pomt lalls 

 exactly between the two marks drawn on the tube, or on the 

 zero of the scale, the level is properly adjusted. When this 

 is not the case, turn the adjusting screws until the bubble set- 

 tles on the elevated side of zero at a distance from it equal to 

 the iuclinalion of the line on which the level rests. Should the 

 error of adjustment be trivial, it is more advisable to ascertain 

 and register its value every time the level is made use ot, than 

 to attempt to correct it by the screws ; for variations ot tempe- 

 rature not only cause the vertex of the tube to alter its position 

 3 A 2 relatively 



