hypotheses concerning the internal structure of the 

 earth. 



Whether the intensity of gravity is sought in absolute 

 or relative measure, the most widely used instrument 

 for its determination since the creation of classical 

 mechanics has been the pendulum. In recent dec- 

 ades, there have been invented gravity meters based 

 upon the principle of the spring, and these instru- 

 ments have made possible the rapid determination of 

 relative values of gravity to a high degree of accuracy. 

 The gravity meter, however, must be calibrated at 

 stations where the absolute value of gravity has been 

 determined by other means if absolute values are 

 sought. For absolute determinations of gravity,' the 

 pendulum historically has been the principal instru- 

 ment employed. Although alternative methods of 

 determining absolute values of gravity are now in use, 

 the pendulum retains its value for absolute determina- 

 tions, and even retains it for relative determinations, 

 as is exemplified by the Cambridge Pendulum Ap- 

 paratus and that of the Dominion Observatory at 

 Ottawa, Ontario. 



The pendulums employed for absolute or relative 

 determinations of gravity have been of two basic 

 types. The first form of pendulum used as a physical 

 instrument consisted of a weight suspended by a fiber, 

 cord, or fine wire, the upper end of which was attached 

 to a fixed support. Such a pendulum may be called 

 a "simple" pendulum; the enclosure of the word 

 simple by quotation marks is to indicate that such a 

 pendulum is an approximation to a simple, or mathe- 

 matical pendulum, a conceptual object which con- 

 sists of a mass-point suspended by a weightless 

 inextensible cord. If / is the length of the simple 

 pendulum, the time of swing (half-period in the sense 

 of physics) for vibrations of infinitely small amplitude, 

 as derived from Newton's laws of motion and the 

 hypothesis that weight is proportional to mass, is 



The second form of pendulum is the compound, or 

 physical, pendulum. It consists of an extended 

 solid body which vibrates about a fixed axis under 

 the action of the weight of the body. A compound 

 pendulum may be constituted to oscillate about one 

 axis only, in which case it is nonreversible and 

 applicable only for relative measurements. Or a 

 compound pendulum may be constituted to oscillate 

 about two axes, in which case it is reversible (or 

 "convertible") and may be used to determine absolute 

 values of gravity. Capt. Henry Kater, F.R.S., 



during the years 1817-1818 was the first to design, 

 construct, and use a compound pendulum for the 

 absolute determination of gravity. He constructed 

 a convertible pendulum with two knife edges and 

 with it determined the absolute value of gravity at 

 the house of Henry Browne, F.R.S., in Portland 

 Place, London. He then constructed a similar 

 compound pendulum with only one knife edge, and 

 swung it to determine relative values of gravity at a 

 number of stations in the British Isles. The 1 9th 

 century witnessed the development of the theory and 

 practice of observations with pendulums for the 

 determination of absolute and relative values of 

 gravity. 



Galileo, Huygens, and Newton 



The pendulum has been both an objective and an 

 instrument of physical investigation since the founda- 

 tions of classical mechanics were fashioned in the 

 17th century. 1 It is tradition that the youthful 

 Galileo discovered that the period of oscillation of a 

 pendulum is constant by observations of the swings 

 of the great lamp suspended from the ceiling in the 

 cathedral of Pisa. 2 The lamp was only a rough 

 approximation to a simple pendulum, but Galileo 

 later performed more accurate experiments with a 

 "simple" pendulum which consisted of a heavy ball 

 suspended by a cord. In an experiment designed to 

 confirm his laws of falling bodies, Galileo lifted the 

 ball to the level of a given altitude and released it. 

 The ball ascended to the same level on the other side 

 of the vertical equilibrium position and thereby 

 confirmed a prediction from the laws. Galileo also 

 discovered that the period of vibration of a "simple" 

 pendulum varies as the square root of its length, a 



1 The basic historical documents have been collected, with a 

 bibliography of works and memoirs published from 1629 to 

 the end of 1885, in Collection de mimoires relalifs a la physique, 

 publiis par la Societe Francaise de Physique [hereinafter referred 

 to as Collection de mimoires]: vol. 4. Mi-moires sur le ptndule, precedes 

 d'une bibliographie I Paris: Gauthier-Villars, 1889); and vol. 5, 

 Mimoires sur le pendule, part 2 (Paris: Gauthier-Villars, 1891). 

 Important secondary sources are: C. Wolf, "Introduction 

 historique," pp. 1-42 in vol. 4, above; and George Biddell 

 Airy, "Figure of the Earth," pp. 165-240 in vol. 5 of Encyclo- 

 paedia metropolitana (London, 1845). 



llileo Galilei's principal statements concerning the pen- 

 dulum occur in his Discourses Concerning Two Ketv Sciences, 

 transl. from Italian and Latin into English by Henry Crew and 

 Alfonso de Salvio (Evanston: Northwestern University Press, 

 1939), pp. 95-97, 170-172. 



304 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



