TECHNICAL INDEX AND GLOSSARY 



Lamp, Safety. A lamp invented by Humphry Davy, for use 

 in mines, to give protection against danger from explosion due 

 to "fire-damp." The device consists essentially of enclosing the 

 lamp in wire gauze. See "Conditions to be Considered in Min- 

 ing," Vol. VI, p. 247. 



Latent Heat. A term introduced by Dr. Joseph Black to apply 

 to the modicum of heat which seemingly disappears or becomes 

 latent when a substance changes its physical state, as, for ex- 

 ample, when water is transformed into steam. The heat reap- 

 pears when the steam is retransformed into water. Black's 

 studies in this field were probably a source of inspiration to his 

 friend Watt, in connection with the perfecting of the steam 

 engine. See Vol. VI, p. 94. 



Latitude. Distance north or south from the equator, measured 

 in degrees, minutes and seconds of arc. Latitude is determined 

 with comparative ease by the mariner through observation of 

 the height of the sun at meridian, as measured with the sextant 

 See "The Development of the Sextant," Vol. VII, p. 18. 



Law, Natural. The expression of an observed sequence of 

 phenomena in nature, so often repeated that we are justified in 

 regarding the sequence as inevitable. 



Lever. An instrument which, in various modified forms, is 

 the most universally employed of all instruments with which 

 work is performed. The principles of the lever were formulated 

 by Archimedes. See "How Work is Done," Vol. VI, p. 29, 



Leyden Jar. Name given a simple apparatus for storing elec- 

 tricity generated by a friction machine, as discovered inde- 

 pendently by Dean Von Kleist and Pieter von Musschenbrock in 

 1745. The original apparatus of Von Kleist consisted simply of 

 a nail or a piece of brass wire placed in a glass bottle. See 

 "The Leyden Jar Discovered," Vol. II, p. 280. 



Light. The phenomena interpreted by the eyes as light and 

 color consists essentially of undulations in the ether, having 

 clearly defined upper and lower limits of frequency. This ex- 

 planation of light was demonstrated by Young and Fresnel; 

 and their theory supplanted the corpuscular or emanation theory 

 that had been championed by Newton. The undulations in the 

 ether that are interpreted as light are induced by vibrations of 

 molecules of matter ordinarily produced by excessive heat; in 

 other words, light is a manifestation of energy. See "Modern 

 Theories of Heat and Light," Vol. Ill, p. 206. 



Lightning. A manifestation of atmospheric electricity, as wa 



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