Wegener also noted that the continents move up and down to maintain equilibrium in a process called isostasy. As an example he cited the sinking of Northern Hemisphere lands under the weight of continental ice sheets in the last ice age, and their rise since the ice melted some 10,000 years ago.
Drill bits are cutting tools used to create cylindrical holes, almost always of circular cross-section. Drill bits come in many sizes and have many uses. Bits are usually connected to a mechanism, often simply referred to as a drill, which rotates them and provides torque and axial force to create the hole.
The bow drill is an ancient form of drilling tool. It commonly was used to make friction fire, and in this function it also was called a fire drill. However, the same principle also was used widely in drilling for purposes of woodworking and dentistry. Bow drill is used to get an ember (left), the ember makes tinder smoke (center), until a fire is lit (right). Whilst a v is most common, other methods to create a cavity to contain the dust whilst it is being ignited can be used.