The Operations of the Geometric and Military Compass

ate the same rule with very small numbers, even though on the instrument the points from 15 downwards could not be marked due to the hinge original: "nocella" that joins and connects the legs of the instrument. But on this occasion, we will use the tens of the points as if they were units, so that, by saying for example: if 10 gives 7, what will 13 give? Since we cannot take 7 to throw it to 10, we will take 70, that is, 7 tens, and we will throw it to 10 tens, that is, to 100. And immediately taking 13 tens, we will return to measure this distance directly, and we will find it to contain 91 points, which are 9 and one tenth, proceeding as has been said, so that every ten counts as one. And from all these warnings, when one has them well in practice, one will easily be able to investigate the solution of all the difficulties that could occur in any case.

Rule of Three inverse resolved by means of the same lines. Oper. V.

With not dissimilar operation, one will resolve the questions of the inverse rule of three. Here is an example: that provision which would suffice to maintain 100 soldiers for 60 days, to how many would it suffice for 75 days? These numbers, arranged for the rule, would stand in this order: 60, 100, 75.

And the operation of the instrument requires that you take the first number directly, that is, 60, and apply it transversally to the third number, that is, 75. And without moving the instrument, take the 100, which is the second, transversally,