BOMB  , a

term formerly used for an explosive shell (see AMMuNrT1oN) fired by artillery . The word is derived from the Gr. fbµ(3os, a hammering, buzzing noise, cf . " bombard " (q.v.) . At the present day it is most frequently used of a shattering or incendiary grenade, or of an explosive vessel actuated by clock-work or trip mechanism, employed to destroy life or property . In naval warfare, before the introduction of the shell gun, ex-plosive projectiles were carried principally by special vessels known as bomb-vessels, bombards or, colloquially, bombs . In geology, the name "bomb " is given to certain masses of lava which have been hurled forth from a volcanic vent by explosive action . In shape they are spheroidal, ellipsoidal or discoidal; in structure they may be solid, hollow or more or less cavernous; whilst in size they vary from that of a walnut to masses weighing several tons . It is generally held that the form is partly due to rotation of the mass during its aerial flight, and in some cases the bomb becomes twisted by a gyratory movement . According, however, to Dr H . J . Johnston-Lavis, many of the so-called bombs of Vesuvius are not projectiles, but merely globular masses formed in a stream of lava; and in like manner Professor J . D .

Dana showed that what were regarded as bombs in Hawaii are in many cases merely lava-balls that have not been hurled through the air . Certain masses of pumice ejected from Vulcano have been called by Johnston-Lavis " bread-crust bombs," since they present a coating of obsidian which has been bent and cracked in a way suggestive of the crust of a roll . It is probable that here the acid magna was expelled in a very viscous condition, and the crust which formed on cooling was burst by the steam from the occluded water . Some of the bombs thrown out during recent eruptions of Etna consist of white granular quartz, encased in a black scoriaceous crust, the quartz representing an altered sandstone . The bombs of granular olivine, found in some of the tuffs in the Eifel, are represented in most geological collections (see VOLCANO) .