195 
1915-16.] Note on Captain Weir’s Azimuth Diagram. 
The scale x = 0, y = cot c becomes x' = — l,y'( — y)=cotc. The line (5) 
becomes 
2?/' = (1 +ic')(cos a cos B — sin a cot c)/sin B + (l — x) cot c . . (10) 
while the parallel line y=—x cot C becomes 
2y = — (1+#') cot C . • (11) 
These lines (10) and (11) intersect in the point C given by the scale 
x — 1 y = — cot C, 
so that in the x', y' plane the new network point (a, B) is collinear with 
the point c on the scale x' = — 1, and the point C on the scale x' = + 1. The 
resulting straight-line nomogram is reproduced in fig. 3. It was first 
drawn byPerret* in 1904, following a method due to Professor d’Ocagne of 
Paris. To use it we have merely to draw a straight line across it joining 
up the three points c, (a, B), and C. When any three of the variables are 
given, the fourth is then read from the diagram. We thus see that Captain 
Weir was the first to construct and use this type of nomogram, for fig. 3 is 
merely a projection of fig. 2. D’Ocagne’s form has, of course, the advantage 
of dispensing with the parallel line. 
* Annales Hydrographiaues, Paris, 1904. 
