2513
The calculation in the upper left corner shows the number of seconds, thirds, fourths, and fifths in a minute of a degree.
The number of fifths (12960000) is then doubled to give the number of half-fifths (25920000).
In the upper right corner, Harriot calculates that if 1 minute is represented by 1 mile, a half-fifth of a degree would be represented by of an inch, a distance that, as he observes, would be invisible. ]
In the upper right corner, Harriot calculates that if 1 minute is represented by 1 mile, a half-fifth of a degree would be represented by of an inch, a distance that, as he observes, would be invisible. ]