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628 according to the weight it was halfed. But this would better appear in an example. Suppose a stone intire to be worth 8l. Now if the same be cut in two halfs, each half reckoned by the weight alone would be reduced to 40s. and the two halfs to 4l. But if the stone be reckoned according to the extent and superficies only, then the two halfs would be worth two eight pounds or 16l. But forasmuch as the rule of weight alone and the rule of extent alone are each of them insufficient, you must joyne them both together and take the medium. For joining 4l.: the value by weight, to 16l., the value by measure, the total is 20l., the half whereof is 10l.; and thus you see the stone which intire is worth but 8l., being divided is worth 10l., yielding an advantage of 40s., which is more than the charge of dividing it doth commonly amount to.

B. Your answer is very satisfactory & ingenious & from whence I now understand the use of your glass or horne table. For I suppose that by applying the flat section to the squared table you may with diligence measure the difference of any superficies almost exactly.

A. You apprehend it right & when I have measured so the extent of two several stones, I cast up their values by the aforementioned rule of duplicate proportion, & having cast them up both by weight & by measure, I take the medium.

B. Lord bless me, what a fool was I wholy to omit those two guides neither of which could I make use of whilest the stone was set, & how easy is it for the best jeweler in the world to mistake one grain or one square in 20, nay, to mistake one in 100 where the value of one grain is above 200l., and how doe the workmen who doe set diamonds indeavour so to set them as to make them look 5 grains or 5 squares in 100 bigger then they are. I am very well pleased with this discourse by which in a quarter of an houre one may learn to get or save 2 or 300l., & to learn an art which is so little the worse for the wearing.

A. I am glad you accept my advice. Some men would have made a frivolous objection against it, or have received it with a scornfull smile as a prety useless fancy and no more. But because you are so candid, I will proceed to the other points.