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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1747 Excerpt: ...in the faid Line, let Gunpowder be placed in the Focus of the other Mirror in the fame Line; if you make the Coal burn fiercer with a Pair of Bellows, the Gunpowder will be fet on Fire; for by the fecond Reflexion the Rays are collected, which after the firft are parallel. For when 3306. the Focus, in "which the parallel Rays are collected by the concave Speculum, becomes the radiant Point, the Rays being a little difperfed, are reflected parallel to one another. 3296-3244 What has been faid-j-concerning parallel Rays a little difpcrfed 337and direttt may alfo be applfd to oblique Rays. For if there be fuch Plate 105. Rays DA, da, thefe being reflected mutually, interfecl: one another Fig. 6. atF in fuch manner, that AF may be equal to a Quarter of the-f-3296. Cord A B j whic...