Importance of Leverage in Crushing Power.--Nut-crackers a Lever of the Second Order.--The Chaff-cutting and Tobacconists’ Machines.--Jaws of various Animals.--The Wolf-fish or Sea-wolf.--The Rolling-mill and its Action.--Gunpowder-mills and Granulating Machine.--The “Jacob’s Ladder.”--The Mangle and its various Adaptations.--The Grindstone.--Primitive Grindstones of the Savage Races.--The Kafirs and the Inhabitants of Palestine.--Ceasing of the Millstone.--“Facing” of Millstones.--Tusk of the Elephant and its Structure.--Its Facings always preserved.--Power of Self-renewal.--Pressure of Atmosphere.--The Napier Coffee Machine.--The Cupping Instrument.--The Pneumatic Peg.--The Magdeburg Hemispheres.--Plane Surfaces of Glass or Metal.--Suckers of the Cuttle-fish.--Foot of the Water-beetle.--The Limpet.--The Star-fish and its Mode of Progression.--The Sucking-fish and the Fables connected with it.--Its real Structure.--Modification of the Dorsal Fin.--The Gobies and Lump-fish.--The Gecko and Tree-frog.--The Lampern and the Medicinal Leech.--Seed Dibbles and Drills.--Labourers versus Machinery.--Natural Dibble of the Grasshopper.--The Daddy Long-legs.--Drills and Dibbles of the Ichneumon-flies.--A wonderful Specimen from Bogotá.--The Pelecinus and its Mode of laying Eggs. CRUSHING INSTRUMENTS. As we are on the subject of leverage, we will take some examples of levers in Art and Nature, without, however, even attempting to exhaust the topic. On the right hand of the illustration is shown a very familiar example of a lever, namely, nut-crackers, with a nut between them. This useful implement is simply an adaptation of levers of the second kind, the power being represented by the human hand, the weight by the nut, and the fulcrum being the joint of the instrument. The common chaff-cutter, which is worked by hand, is another familiar example of this kind of lever, and so is the knife used by tobacconists in cutting cake Cavendish into threads, and by druggists for similar purposes. In these instruments the point of the knife is jointed to some fixed object, and becomes the fulcrum; the hand of the cutter supplies the power, and the weight is the object which is being cut. It will be seen that, by increasing the length of the handle, very great power can be obtained. [Illustration: JAWS OF WOLF-FISH.] [Illustration: NUT-CRACKERS.]