Thesaurarium mathematicae, or, The treasury of mathematicks containing variety of usefull practices in arithmetick, geometry, trigonometry, astronomy, geography, navigation and surveying ... to which is annexed a table of 10000 logarithms, log-sines, and log-tangents / by John Taylor.

PROP. III. To three numbers given, to find out a fourth in a Duplicate proportion.

The nature of this proposition is to discover the proportion of Lines, to Superficies, and Superfi∣cies, to Lines; for like Plains are in a duplicate Ratio; that is as the Quadret of their Homologal sides; therefore to Operate any Example in this proportion, Square the third term, and its square multiply by the second Term, their product di∣vide by the square of the first Term, the Quotient is the 4th. term sought; Examp. Admit there be two Geometrical squares; now if the side of the grea∣ter

square be 50 feet, and require 3000 Tiles to pave it; what number shall the lesser square require, whose side is 30 feet? To operate this according to the Rule, I square the third Term 30, whose square is 900: then I multiply it by the second Term 3000, its product is 2700000, which divided by 2500, the square of the first Term 50, the Quotient is 1080, and so many Tiles will pave the lesser square, whose side is 30 feet.