The construction of the accompanying diagram is based on the geometrical principle that a perpendicular dropped from the rectangular vertex of a right triangle upon the hypothenuse is a mean proportional between the segments of the hypothenuse so formed.
In example 1 on the accompanying diagram it is evident that
(OD’)2=OB’ x OX, or OB’=(OD’)2/OX=(OD’)2x1/OX
which is of the form,—
Correction to observed altitude= t2x a of the reduction to meridian sight.
OX the reciprocal of "a" (from Bowd., Table 26) expressed in minutes of arc.
OD'= t in minutes of time.
OB'= Correction in minutes of arc.
One-tenth of an inch equals one minute of time on the vertical or one minute of arc on the horizontal. This scale could be increased if desired. In the diagram as printed the scale is reduced.
Construct the scale at the bottom of the blue print as follows:
From Table 26 take the value of "a" for 0° Lat and 10° Dec.