The mariners magazine, or, Sturmy's mathematical and practical arts containing the description and use of the scale of scales, it being a mathematical ruler, that resolves most mathematical conclusions, and likewise the making and use of the crostaff, quadrant, and the quadrat, nocturnals, and other most useful instruments for all artists and navigators : the art of navigation, resolved geometrically, instrumentally, and by calculation, and by that late excellent invention of logarithms, in the three principal kinds of sailing : with new tables of the longitude and latitude of the most eminent places ... : together with a discourse of the practick part of navigation ..., a new way of surveying land ..., the art of gauging all sorts of vessels ..., the art of dialling by a gnomical scale ... : whereunto is annexed, an abridgment of the penalties and forfeitures, by acts of parliaments appointed, relating to the customs and navigation : also a compendium of fortification, both geometrically and instrumentally

Sturmy, Samuel, 1633-1669.

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The ART of Surveying of Land By the SEA-COMPASS: The DESCRIPTION of the COMPASS, and STAFF, and CHAIN. The Fifth Book.

CHAP. XII. The most Excellent Gunners Scale, Which resolves the Chief Principles of the whole Art of Gunnery, in a very brief and Compendious form, never by any set forth in the like nature before; with divers Excellent Conclusions both Arithmetical, and Geometrical, and Instrumental; and by Tables being framed both with, and without the help of Arithmetick. As also divers Artificial Fire-Works, both for Recreation, and for Sea and Land-Service.

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Page 58

SECT. XI. How to know whether your Piece be Chamber-bored.

FIrst you may Dispert your Piece the three first ways, and when they agree in one, take that for the true Dispert; then with your Priming Iron take the Dispert this last way; which done, compare it with the other Dispert first found, and what it wants, is the just difference of the Chamber from the Bore of the Piece.

Admit the Dispert truly found by the two first ways be three Inches, as by this last way is but two Inches, it shews that the Chamber differs from the true Bore on each side one Inch; so that if the Bore of the Piece be five Inches high, the Chamber be∣ing one Inch on each side lower, is, but three Inches high: the like Observation we would always have you to make, that you may not afterwards be deceived in making Cartredges of Canvas or Paper to load the same.

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