Figure 2. — Astronomical Clock of dc Dondi, 

 showing gearing on the dial for Mercury and 

 escapement crown wheel. Each of the seven side 

 walls of the structure shown in figure r was fitted 

 with a dial. 



B.C. have been preserved.'' It might be remarked 

 that these "machine" gear wheels are characterized 

 by having a "round number" of teeth (examples with 

 16, 24 and 40 teeth arc known) and a shank with a 

 square hole which fits without turning on a squared 

 shaft. Another remarkable feature in these early 

 gears is the use of ratchet-shaped teeth, sometimes 

 even twisted helically so that the gears resemble 

 worms intermeshing on parallel axles. ^ The existence 

 of windmills and watermills testifies to the general 

 familiarity, from classical times and through the 

 middle ages, with the use of gears to turn power 

 through a right angle. 



Granted, then, this use of gears, one must guard 

 against any conclusion that the fine-mechanical use of 



' A general account of these impoi tant archaeological objects 

 will be published by J. Needham, Science and civilisation in China, 

 Cambridge, 1959(?), vol. 4. The original publications (in 

 Chinese) arc as follows: Wang Chcn-to, "Investigations and 

 reproduction in model form of the south-pointing carriage and 

 hodometer," .National Peiping Academy Historical Journal, 1937, 

 vol. 3, p. 1. Liu Hsien-chou, "Chinese inventions in horo- 

 logical engineering," Ch'ing-Hua University Engineering Journal, 

 1956, vol. 4, p. 1. 



' For illustrations of intermeshing worms in Indian cotton 

 mills, sec Matschoss, op. cil. (footnote 3), figs. 5, 6, 7, p. 7. 



gears to provide special ratios of angular movement 

 was similarly general and widespread. It is custom- 

 ary to adduce here the evidence of the hodometer 

 (taximeter) described by \'itruvius (1st century B.C:.) 

 and by Hero of Alexandria (1st century A.D.) and 

 the ingenious automata also described by this latter 

 author and his Islamic followers. ° One may also cite 

 the use of the reduction gear chain in power machin- 

 ery as used in the geared windlass of Archimedes and 

 Hero. 



Uitfortimately, eveti the inost complex automata de- 

 scribed by Hero and by such authors as Ridwan con- 

 tain gearing in no more extensive context than as a 

 means of transmitting action around a right angle. 

 As for the windlass and hodo.meter. they do, it is true, 

 contain whole series (jf gears u.sed in steps as a reduc- 

 tion mechanism, usually for an extraordinarily high 

 ratio, but here the technical details are so etherial 

 that one must doubt whether such devices were actu- 

 ally realized in practice. Thus \'itruvius writes of a 

 wheel 4 feet in diameter and having 400 teeth being 

 turned by a 1 -toothed pinion on a cart axle, but it is 

 very doubtful whether such small teeth, necessarily 

 separated by about j!g inch, would have the requisite 

 ruggedness. Again, Hero mentions a wheel of 30 

 teeth which, because of imperfections, might need 

 only 20 turns of a single helix worm to turn it ! Such 

 statements behove caution and one must consider 

 whether we have been misled by the 16th- and 17th- 

 century editions of these authors, containing recon- 

 structions now often cited as authoritative but then 

 serving as working diagrams for practical use in that 

 age when the clock was already a familiar and com- 

 plex mechanism. At all events, even if one admits 

 without substantial evidence that such gear reduction 

 devices were familiar from Hellenistic times onwards, 

 they can hardly serve as more than very distant an- 

 cestors of the earliest mechanical clocks. 



Mechanical Clocks 



Before proceeding to a discussion of the controversial 

 evidence which may be used to bridge this gap be- 

 tween the first use of gears and the fully-developed 

 mechanical clock we must examine the other side of 

 this gap. Recent research on the history of early me- 



" It is interesting to note that the C^hincsc hodometer was con- 

 temporary with that of Hero and Vitruvius and very similar in 

 design. There is no evidence whatsoever upon which to decide 

 whether there may have been a specific transmis.'jion of this in- 

 vention or even a "stimulus diffusion." 



84 



BULLETIN 218: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



