Coal mining generates significant adverse environmental impacts. Historically, coal mining has been a very dangerous activity and the list of historical coal mining disasters is a long one. Underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions.
The term 'OVD' (Optically Variable Device) is used to describe both laser generated hologram images and electron beam generated images. These optically variable devices are created in highly-secure facilities and are at the forefront of overt asset and brand protection programmes.
Cartesian oval; An ovoid is the 2-dimensional surface generated by rotating an oval curve about one of its axes of symmetry. The adjectives ovoidal and ovate mean having the characteristic of being an ovoid, and are often used as synonyms for egg-shaped. Projective geometry