TECHNICAL INDEX AND GLOSSARY 



Foundation of Mechanics," Vol. I, p. 196; and "Inclined Planes 

 and Derricks," Vol. IX, p. 37. 



Cross-staff. A primitive apparatus for measuring the angle 

 between two visual objects, such as two stars, or a star and the 

 horizon-line. It was used by navigators from an early period, 

 but was ultimately superseded by astrolabe (fifteenth century) 

 and quadrant (eighteenth century). See "The Development of 

 the Sextant," Vol. VII, p. 18. 



Crowbar. A simple lever, which, now made of iron or steel 

 instead of wood, performs the same service for the modern 

 workman that it did for his prehistoric ancestor. 



Cupping-glass. A glass cup, which becomes a suction appa- 

 rtus through the exhaustion of air within it; much used by- 

 physicians in the days when venesection was in vogue, but now 

 practically obsolete. 



Cyclone. A revolving atmospheric current, describing a circle 

 that may be a few feet or many miles in diameter. The uni- 

 versality of cyclonic air currents was first prominently taught 

 by H. W. Dove, about 1827. See "Cyclones and Anti-cyclones," 

 Vol. Ill, p. 199- 



Daguerreotype. The form of photograph (using a metal sur- 

 face, and giving a reversed image) invented by Louis J. M. 

 Daguerre about 1839. The process is still much employed for 

 the cheap photographs called tin-types. See "Photography in 

 its Scientific Aspects," Vol. VIII, p. 224. 



Darwinian Theory. The theory of evolution (which owed its 

 development to Charles Darwin, though independently con- 

 ceived by Alfred Russell Wallace), which explains organic evo- 

 lution as due in a large measure to the preservation of favorable 

 varieties or stocks through "natural selection." The essentials 

 of the theory, stated in a phrase, are spontaneous (i.e., unex- 

 plained) variation, and (in Herbert Spencer's phrase) the "sur- 

 vival of the fittest." Perhaps no other theory in the entire his- 

 tory of thought ever had so important an influence on the ideas 

 of a generation as this theory exercised. See "Theories of 

 Organic Evolution," Vol. IV, p. 140. 



Derrick. See Crane. 



Diamond Drill. A drill for boring through rock, the cutting 

 edge being made of a ring of black diamonds. See "The Mineral 

 Depths," Vol. VI, p. 247, and "Other Sources of Diamonds; 

 Practical Uses," Vol. IX, p. 317. 



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