[291] These figures are to be found in the following editions of Vegetius:--Lutetiæ apud C. Wechelum, 1532, fol. p. 180. Fegetius, vier Bücher von der Rytterschafft. Erfurt, Hans. Knappen, 1511, fol. These figures are inserted also in Leupold’s Theatrum Pontificale, p. 11, tab. ii. fig. 6. [292] Le Fortificationi di Bounaiuto Lorini. Venet. 1609, fol. p. 232. [293] Fran. Kessleri Secreta. Oppenheim, 1617, 8vo. [294] Bartholini Acta Hafn. 1676, p. i. obs. 17. [295] Scheeps-bouw, ut supra. [296] See vol. i. p. 222, edit. Hag. Com. 1743. [297] Acta Eruditorum, 1683, Decemb. p. 553. Jac. Bernoulli Opera. [298] Phil. Trans. 1736.--Martin Triewald’s Konst at lefwa under watnet. Stockholm, 1741, 4to. [299] Brewster’s Edinburgh Encyclopædia, Art. Diving-bell. [300] Reports of the late John Smeaton, F.R.S., vol. iii. p. 279. [301] Phil. Trans. 1717 and 1721. The art of living under water, by Halley. [302] Martin’s Philosophia Britannica, vol. iii. p. 180. [303] For further information on this important subject the reader is referred to the article Diving-bell in the Encyclopædia Britannica and its Supplement, also the Encyclopædia Metropolitana, Brewster’s Edinburgh and the Penny Cyclopædia, Halley’s papers in the Phil. Trans. for 1716 and 1721, Triewald’s in the same for 1736, Healy in the Philosophical Magazine, vol. xv., and Leopold’s Theatrum Machinarum Hydraulicarum. COLOURED GLASS.--ARTIFICIAL GEMS. It is probable that there was no great interval between the discovery of the art of making glass, and that of giving it different colours. When the substance of which it is formed contains, by accident, any metallic particles, the glass assumes some tint; and this happens oftener than is wished; nay, a considerable degree of foresight is necessary to produce glass perfectly colourless; and I am of opinion that this skill has not been attained till a late period in the progress of the art. Even in Pliny’s time the highest value was set upon glass entirely free from colour, and transparent, or, as it was called, crystal[304]. From the different colours which glass acquired of itself, it was easy to conceive the idea of giving it the tinge of some precious stone: and this art, in ancient times, was carried to a very great extent. Proofs of this may be found in Pliny, who, besides others, mentions artificial hyacinths, sapphires, and that black glass which approached very near to the obsidian stone, and which in more than one place he calls _gemmæ vitreæ_[305]. Trebellius Pollio relates in how whimsical a manner Gallienus punished a cheat who had sold to his wife a piece of glass for a jewel[306]: and Tertullian ridicules the folly of paying as dear for coloured glass as for real pearls. The glass-houses at Alexandria were celebrated among the ancients for the skill and ingenuity of the workmen employed in them. From these, the Romans, who did not acquire a knowledge of that art till a late period,