3. (Mech.) A spiral cam, or a flat piece of metal of spirally curved
outline, used for giving motion to, or changing the position of,
another part, as the hammer tail of a striking clock.
4. A tortoise; in ancient warfare, a movable roof or shed to protect
besiegers; a testudo. [Obs.]
They had also all manner of gynes [engines] . . . that needful is
[in] taking or sieging of castle or of city, as snails, that was
naught else but hollow pavises and targets, under the which men,
when they fought, were heled [protected], . . . as the snail is in
his house; therefore they cleped them snails.
Vegetius (Trans.).