The publisher apologises for the errors in Paragraph 1 of the Introduction. Lines 6–10 of paragraph one should read as follows:

Finding the infinitesimal distance dD _O that the vessel must cover in Great Circle sailing for the orthodromic course to change by a small value dC _O illustrates the problem. Referring to Figure 1, from a spherical right angled triangle TVP with any two parts given any third can be found as follows:

And in Table 1, there are errors in Line 1 and Line 5. Lines 1 and 5 should read as follows:

The corrected Table is given below.

Table 1.

Middle Latitude Rule (Sphere).