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Galileo and the Problem of Free Fall

  • R. H. Naylor (a1)


There can be little doubt that 1973 will remain notable as a year in which knowledge of Galileo's mechanics increased dramatically. Professor Stillman Drake's publication, in May, of some of Galileo's early work on the law of free fall was followed in the autumn by the publication of a number of important manuscripts clearly indicating Galileo's use of precise measurement. From a discussion of these manuscripts and Thomas Settle's performance of Galileo's inclined plane experiment, Drake implies that a clear view of Galileo's use of experiment is now emerging. Added emphasis was given to Drake's thesis that doubts concerning Galileo's use of experiment were largely unfounded, by James MacLachlan's realization of a Galilean experiment which was previously described as ‘imaginary’ by Koyré. The purpose of this paper is to suggest that, while it cannot be doubted that Galileo used experiment and precise measurement, his attitude to observation may well have been far more complex than Drake has supposed. My point of departure is James MacLachlan's remark that continuing disagreement over Galileo's use of experiment should lead to further examination of Galileo's experimental claims. I shall indicate that more than one view of Galileo's use of experiment may prove capable of explaining our present knowledge—a corollary of this being that alternative explanations may be proposed for the manuscripts recently published by Drake.



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The reconstruction of Galileo's inclined plane experiment was only made possible by the support and assistance received from my Faculty (Social Sciences and Humanities) and the Faculties of Science and Engineering at Thames Polytechnic. For this support, I am particularly grateful to Miss V. Pitt, Mr M. Yolles, Mr I. Bittle, and Dr R. A. M. Scott. The many discussions of this and other of Galileo's experiments with Dr Scott were of considerable help in the preparation of this paper.

1 Drake, Stillman, ‘Galileo's discovery of the law of free fall’, Scientific American, ccxxviii (1973). 8492.

2 Drake, Stillman, ‘Galileo's experimental confirmation of horizontal inertia: unpublished manuscripts’, Isis, lxiv (1973), 291305.

3 Settle, Thomas, ‘An experiment in the history of science’, Science, cxxxiii (1961), 1923.

4 MacLachlan, James, ‘A test of an “imaginary” experiment of Galileo's’, Isis, lxiv (1973), 374–9.

5 Koyré, Alexandre, ‘An experiment in measurement’, Proceedings of the American Philosophical Society, xcvii (1953), 224.

6 MacLachlan, , op. cit. (4), p. 375.

7 MSS. Galileiani, , Biblioteca Nazionale di Firenze, vol. lxxii, folios 114, 116v. See Drake, , op. cit. (2), pp. 297, 301.

8 Galileo, , Dialogue concerning the two chief world systems, trans. Drake, S. (Berkeley, 1967), pp. 144–5; Opere [Edizione Nazionale], ed. Favaro, A. (20 vols., Florence, 18901909), vii. 171.

9 Galileo, 1967, op. cit. (8), trans. Drake, p. 145.

10 Galileo, , Dialogues concerning two new sciences, trans. Crew, H. and de Salvio, A. (New York, 1963), p. 163.

11 Drake, , op. cit. (2), p. 300.

12 Drake, , op. cit. (1), pp. 8492.

13 Drake, , op. cit. (2), p. 300.

14 Galileo, , Opere, viii. 373. The translation appears in Drake, Stillman, ‘Galileo's 1604 fragment on falling bodies’, The British journal for the history of science, iv (19681969), 342.

15 Drake, , op. cit. (14), p. 349.

16 Galileo, , op. cit. (10), p. 171; Opere, viii. 213.

17 Settle, , op. cit. (3), p. 20, and Drake, , op. cit. (13), p. 349.

18 Galileo, , op. cit. (10), p. 171.

19 Drake, , op. cit. (14), p. 349, and op. cit. (1), p. 91.

20 Drake, , op. cit. (14), pp. 340–3, and op. cit. (1), pp. 8492.

21 Galileo, , Opere, x. 97100, and op. cit. (10), p. 181.

22 Galileo, , Dialogue, op. cit. (8), trans. Drake, p. 26.

23 Mersenne, M., Harmonie universelle (Paris, 1636), pp. 111–38. Quoted in Koyré, A., Metaphysics and measurement, trans. Maddison, R. E. W. (London, 1968), pp. 113–15.

24 Mersenne, 1636, op. cit. (23), p. 138.

25 Drake, , op. cit. (2), pp. 291–2.

26 Settle, , op. cit. (3), pp. 1923.

27 Settle, , op. cit. (3), p. 21, and Drake, Stillman, ‘Free fall in Galileo's Dialogue’, Isis, lvii (1966), 269–71.

28 Galileo, , Opere, xviii. 75–7.

29 Lindberg, David, ‘Galileo's experiments on falling bodies’, Isis, lvi (1965), 352–4.

30 Galileo, , Dialogue, op. cit. (8), trans. Drake, pp. 22 and 30.

31 Ibid., p. 223. See also the translator's note, pp. 484–5. Galileo's copy of the first edition is owned by the Library of the Seminary in Padua. The value appears as a footnote in Opere, vii. 54.

32 Mersenne, 1636, op. cit. (23), p. 112.

33 Galileo, , On motion, trans. Drabkin, I. E. and Drake, S. (Madison, 1960), p. 69.

34 Galileo, , op. cit. (10), pp. 171–2.

35 Galileo's units cannot be identified precisely. The braccio, translated variously as cubit or yard, ranged between 20 and 25 inches. Most Northern Italian cities appear to have had a standard braccio which seems to have been essentially a measure for cloth. In my view, Koyré was over-confident in stating that Galileo's braccio was ‘doubtless’ 20 inches (or 49 cms); it was probably not very different from this figure, though a value of around 22 inches seems most likely. The uncertainty on this issue is really of marginal significance, however. Mersenne had no doubts about Galileo's braccia; it was the measurement he suspected. Settle's comments on the doubts concerning the exact value of the units cannot explain Galileo's lack of knowledge of the rate of free fall. Settle believes that Galileo's braccia was close to 22 · 7 inches; see Settle, , op. cit. (3), p. 19. For Drake's views on Galileo's units, see Galileo, , Dialogue, op. cit. (8), p. 471. See also Skinner, F. G., Weights and measures (London, 1967).

36 Koyré, , op. cit. (5), p. 224.

37 Settle, , op. cit. (3), p. 20.

38 Galileo, , Opere, xviii. 77.

39 Bernal, J. D., The extension of man. A history of physics before 1900 (London, 1972), p. 183. Bernal was unable to achieve success with the experiment, and he ascribed this to the fact that Galileo used better equipment. Bernal's interpretation of Galileo's description agrees well with mine; it seems clear that Bernal considered the parchment lining a refinement likely to lead to greater accuracy.

40 Drake, Stillman, Discoveries and opinions of Galileo (New York, 1957), p. 227.

41 Santillana, G. de, The crime of Galileo (Chicago, 1967), p. 157, and Broderick, J., Galileo (London, 1964), p. 117.

42 Galileo, , The assayer, trans. Drake, Stillman, in The controversy on the comets of 1618 (Philadelphia, 1950), p. 252.

Galileo and the Problem of Free Fall

  • R. H. Naylor (a1)


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