USEFUL AKTS. 



CHAPTER II. 



CRUSHING INSTRUMENTS.— THE NUT - CRACKERS, ROLLING- 

 MILL, AND GRINDSTONE.— PRESSURE OF ATMOSPHERE.— 

 SEED DIBBLES AND DRILLS. 



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 Pales- 

 tine. — Ceasing of the Millstone. — " Facing " of Millstones. — Tusk of the 

 Elephant and its Structure. — Its Tracings always preserved. — Power of 

 Self-renewal. — Pressure of Atmosphere. — The Napier Coffee Machine. — 

 The Cupping Instrument. — The Pneumatic Peg. — The Magdeburg Hemi- 

 spheres. — 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 Bogota. — 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, 



