Extracts from Field Reports 



137 



difference of these two measures is the angle between the mirror surface and compass-bowl 

 surface, and is the combined effect of y and w. So that, if m represents this effect, then 



m = y + w OT y = tn — w 



which, substituted in equation (2), gives 



m tan h — w tan h + z tan h tan 9 + t*^ tan /i sec ^ = D„ — D — a;„ (3) 



which contains but two unknowns, when m has been determined. 



b = z tan h tan „ • — If the only error in the mirror adjustment is that which results 



Fia. 6. 



when the mirror axis is not perpendicular to the line "peep-sight and vertical thread," 

 then as the mirror is rotated, a normal to its surface moves in a plane FOGZ, Figure 6, 

 passing through the zenith and making a constant angle, z, with the Une "peep-sight and 

 vertical thread." It intersects the celestial sphere in a great circle FPZG. For a par- 

 ticular altitude, /(, the normal, OP, to the mirror surface, intersects this circle in a point P, 



at a distance from the zenith Z equal to ^. The Sun is found in a plane, EPSH, contain- 

 ing the "peep-sight and vertical thread," £0, and the normal, OP. The error, b, is the angle 

 SZH between the vertical plane ZS, through the Sun, and the vertical plane EOH, 

 containing the line "peep-sight and vertical thread." 

 The quadrantal triangle, PZE, gives 



tan PEZ = sin PZE tan PZ = sin z tan x 



