450 Experiments to. determine variations in length 
graduated surfaces of the bars are brought into a horizontal 
plane within 1’ by means of a long striding level. The micro- 
scopes are focused. on the graduations at the ends of a level bar, 
made vertical within 1’ and their readings on the lines of colli- 
mation determined. Micrometer readings are then made on 
the bars alternately, these readings being near the lines of col- 
limation. The values of the micrometer screws are determined 
at intervals by reading on spaces of known value. No account 
ing microns or millionths of a meter. The revolutions increas¢ 
in the direction F—W; so that if F, and W, are the readings 
on Z,and F, and W, the readings on S,, we have from these 
readings 
Z,—S,=(F ,—F,) 92°1¢—(W, — W,) 95°83" 
