KEY AND INDEX 



Elevator. The contrivance (in England called a "lift") for 

 conveying passengers and freight to the upper stories of build- 

 ings, through which the modern skyscraper has been made 

 tenantable. The earlier forms were operated by a piston-rod 

 resting on a water cushion, but for very high buildings the elec- 

 tric elevator has practical monopoly. See "Elevator or 'Lift,' " 

 Vol. IX, p. 169. 



Embryology. The science that deals with the development of 

 the embryo while in the egg or womb. The study is highly 

 important in its bearing on physiological and evolutionary 

 problems. See "The Cell Theory Elaborated," Vol. IV, p. 122, 

 for the studies of Schwann, von Baer, Miiller, and Carpenter. 



Energy. The capacity to do work. Energy may be potential, 

 as represented by a stone held in the hand; or kinetic (operative), 

 as when the stone is dropped. Various manifestations of energy 

 (as molar motion, molecular activity, heat, electricity) may be 

 transmuted one into another; but energy can be neither created 

 nor destroyed (doctrine of the conservation of energy; see 

 Mayer, Joule, Helmholtz). There is, however, a seeming loss 

 of energy from the solar system, through the constant sending 

 out of radiant heat. See "The Conservation of Energy," Vol. 

 Ill, p. 253; "Lord Kelvin and the Dissipation of Energy," Vol. 

 Ill, p. 274; "How Work is Done," Vol. VI, p. 29. 



Engines. See Atmospheric e., Electric e., Gas e., Hot-air e., 

 Piston e., Steam e., and Water e., and "Captive Molecules: The 

 Story of the Steam Engline," Vol. VI, p. 79; "The Master 

 Worker," Vol. VI, p. no; "Gas and Oil Engines," Vol. VI, 



P- 133- 



Epicycles, Theory of. A theory invented or elaborated by 

 Hipparchus (second century B.C.) to explain the observed fact 

 that the sun spends more time on one side of the equator than 

 on the other, and that the moon and planets show similar irreg- 

 ularities of action. The theory supposes that the circling 

 bodies describe minor circles about invisible centers. These 

 fictitious epicycles continued to be evoked by astronomers until 

 Kepler discovered that the true explanation of the observed 

 anomalies is, not that the bodies describe minor circles 

 (epicycles), but that their orbit is elliptical. The theory of 

 epicycles was thus shown to be utterly untenable; yet it had 

 seemed to offer a valid explanation of observed phenomena. 

 See "Hipparchus, 'The Lover of Truth,'" Vol. I, p. 233; "The 

 New Cosmology," Vol. II, p. 74- 



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