Meclianics understand the making of spheres and know 

 how to produce a model of the heavens (with the courses 

 ol the stars moving in circles?) by mean of equal and circular 

 motions of water, and Archiincdcs the Syracusan, according 

 to some, knows the cause and reasons for all of these. 

 Pappus (3rd century, A.D.), \i'orks (Hultsch edition), 



\'III. J. I'pps" translation. 



A similar arrangement seems to be indicated in 

 another mechanized globe, also mentioned by Cicero 

 and said to have been made by Posidonius: 



But if anyone brought to Scythia or Britain the globe 

 (sphaerain) which our friend Posidonius [of Apameia, the 

 Stoic philosopher] recently made, in which each revolution 

 produced the same (movements) of the sun and moon and 

 far wandering stars as is produced in the sky each day and 

 night, who would doubt that it was by e.xertion of reason? . . . 

 Yet doubters . . . think that Archimedes showed more 

 knowledge in producing movements by revolutions of a 

 globe than nature (does) in effecting them though the copy 

 is so infinitely inferior to the original . . . 



Dc nutura deorum. II. .xxxiv-xxxv (88), 

 ^'onge"s translation. 



In spite ol" die lack of sullicient technical details in 

 anv case, these mechanized globe models, with or 

 without geared planetary indicators (which would 

 make them highly complex machines), bear a .striking 

 resemblance to the earliest Chinese device described 

 by Chang Heng. One must not reject the possibility 

 that transmission from Greece or Rome could ha\e 

 reached the East by the beginning of the 2nd century, 

 A.D., when he was working. It is an interesting 

 question, but even if such contact actually occurred, 

 very soon afterwards, as we shall see, the western and 

 eastern lines of evolution parted comj)any and 

 evolved so far as can be seen, quite independently 

 until at least the 12th century. 



The next Hellenistic source of which we must take 

 note is a fragmentary and almost unintelligible chapter 

 in the works of Hero of Alexandria. .-Mone and un- 

 connected with his other chapters this describes a 

 model which seems to be static, in direct contrast to 

 all other devices which move by pneumatic and hydro- 

 static pressures; it may well be conjectured that in its 

 original form this chapter described a mechanized 

 rather than a static globe: 



I he World represented in the Centre of the L'niverse: 

 The construction of a transparent globe containing air and 

 liquid, and also of a smaller globe, in the centre, in imitation 

 of the World. Two hemispheres of glass are made; one 

 of them is covered with a plate of bronze, in the middle of 

 which is a round hole. To fit this hole a light ball, of small 



size, is constructed, and thrown into the water contained 

 in the other hemisphere: the covered hemisphere is next 

 applied to this. and. a certain Cjuantity of the liquid having 

 been removed fioni the water, the intermediate space will 

 contain the ball; thus by the application of the second 

 hemisphere what was proposed is accomplished. 



Pneumatics, XLVI, Woodcrofl's translation. 



It will be noted that these earliest literary references 

 are concerned with pictorial, 3-dimensional models 

 of the universe, moved perhaps by hand, perhaps by 

 waterpower; there is no evidence that they contained 

 complicated trains of gears, and in the absence of this 

 we may incline to the view that in at least the earliest 

 such models, gearing was not used. 



The next developments were concerned on the one 

 hand with increasing the inathematical sophistication 

 of the model, on the other hand with its mechanical 

 complexity. In both cases we are most fortunate in 

 having archaeological e\i(k'nce which far exceeds any 

 literary sources. 



The mathematical process of mapping a sphere onto 

 a plane surface by stereographic projection was intro- 

 duced by Hipparchus and had much influence on 

 astronomical techniques and instruments thereafter. 

 In particular, by the time of Ptolemy {ca. A.D. 120) 

 it had led to the successi\e inventions of the anaphoric 

 clock and of the planispheric astrolabe.'- Both these 

 devices consist of a pair of stereographic projections, 

 one of the celestial sphere with its stars and ecliptic 

 and tropics, the other of the lines of altitude and 

 azimuth as set for an observer in a place at .some 

 yjarticular latitude. 



In the astrolabe, an o]3enwork metal reie con- 

 taining markings for the stars, etc.. may be rotated 

 by hand over a disc on which the lines of altitude 

 and azimuth arc inscribed. In the anaphoric clock 

 a disc engraved with the stars is rotated automatically 

 behind a fixed grille of wires marking lines of altitude 

 and azimuth. Power for rotating the disc is provided 

 by a float rising in a clepsydra jar and connected, 

 by a rope or chain passing over a |)ulley to a counter- 

 weight or by a rack and ])inion, to an axle which 

 supported the rotating disc and communicated this 

 motion to it.'' 



'- A, G. Drachmann, "The plane astrolabe and the anaphoric 

 clock," Centauriis, 1954, vol. 3, pp. 183-189. 



" .\ fuller description of the anaphoric clock and coqnate 

 waterclocks is given by A. G. Drachmann, "Klesibios, Philon 

 and Heron," Acta Hislorica Scientiarum Naluralium et Medicinalium, 

 Copenhagen, 1948, vol. 4. 



90 



BULLETIN 218: CONTRIBUTIONS FROM THE MUSEUM Ol- HISTOR\- AND TECHNOLOGY 



