The shadow of the edge of the
triangular plate moves around the northern part of the dial from
morning to afternoon, and thus supplies a rough measurement of the
hour of the day.
The style or gnomon, as it always equals the latitude of the
place, can be laid out as follows: Draw a line AB, Fig. 1, 5 in.
long and at the one end erect a perpendicular BC, the height of
which is taken from table No. 1. It may be necessary to
interpolate for a given latitude, as for example, lat. 41
degrees-30'. From table No. 1 lat. 42 degrees is 4.5 in. and for
lat. 40 degrees, the next smallest, it is 4.2 in. Their difference
is .3 in. for 2 degrees, and for 1 degrees it would be .15 in. For
30' it would be 1/2 of 1 degrees or .075 in. All added to the
lesser or 40 degrees, we have 4.2+.15+.075 in.= 4.42 in. as the
height of the line BC for lat. 41 degrees-30'. If you have a table
of natural functions, the height of the line BC, or the style, is
the base (5 in. in this case) times the tangent of the degree of
latitude. Draw the line AD, and the angle BAD is the correct angle
for the style for the given
[Illustration: Details of Dial]
TABLE No.
Pages:
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633