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the weight is compared with the National Standard Weight in air at the temperature 65°.66 F (18°.7 C), and under the barometric pressure 29.75 in (755.64 mm). In order to compare the weight in vacue, it is necessary to hear in mind that the specific gravity of the National Standard Weight at 32°F (0°C), as compared with water at its greatest density, is 21.157, and that of the copy is 8.151.
I am, dear Sir,
Yours very faithfully
GB Airy
