[Footnote 17: The tribuli are small engines with four
spikes, one fixed in the ground, the three others erect or
adverse, (Procopius, Gothic. l. iii. c. 24. Just. Lipsius,
Poliorcetwv, l. v. c. 3.) The metaphor was borrowed from the
tribuli, (land-caltrops,) an herb with a prickly fruit,
commex in Italy. (Martin, ad Virgil. Georgic. i. 153 vol.
ii. p. 33.)]