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As the proper Distance upon the Rhumb is to the Radius;
So is the proper Difference of Latitudes to the Co-sine of the Rhumb from the Meridian:
And
So is the Sine of the Rhumb from the Meridian to the Diffe∣rence of Longitude.
Thus, if one of the Places be in the Latitude of 50 degr. and the other in 55 degr. and their proper Distance upon the Rhumb 6 degr. or 120 Leagues; their Difference of Longi∣tude will be found to be 5 degr. 30 min.
DRaw A D and B C, two Parallels of Latitude, through 50 degr. and 55 degr. which were the two given Lati∣tudes. Then out of the Meridian Line take the proper Distance upon the Rhumb (having respect to both Lati∣tudes) from K to L: the Compasses being opened to this Distance, one foot being set in A, the lesser Latitude, the other will cross the Parallel of the greater Latitude in C. So the Distance B C, being measured at the bottome of the Chart from E, will reach to 5 degr. 30 min. And such is the Diffe∣rence of Longitude between the two Places.
But if this Difference of Longitude were to be found by the Plain Chart, it would be but 3 degr. 20 min. which is no less then 2 d. 10 min. less then the truth; as by the Triangle T V. E drawn upon the Plain Chart may appear.