A Postscript to Reti's Notes on Juanelo Turriano's Water Mills

Abstract

Historians of technology interested in early sources of mechanical power should be particularly grateful for Ladislao Reti's preliminary publication of extracts from Volume III of Juanelo Turriano's sixteenthcentury codex on engineering and machines.1 While I fully agree with Reti that the efficiency and power output of horizontal waterwheels generally have been underrated, his estimates of 18 h.p. for one of the mills and a corresponding grinding output of 510 kg/hour are difficult to reconcile with data that I recorded for free-jet mills in Persia. These mills are in almost every respect identical with the molino de bomba shown in Figure 1 of Reti's article. The data obtained for the Persian mills include the waterhead, the jet diameter, the rotor diameter, the millstone diameter, the grain ouput, and the speed of millstone and rotor. In order to calculate the actual power output of the mill cited by Reti, we might use the given data for the Persian mills, comparing the required power with a similar mill recently converted to dieselengine operation. The data are average values recorded in three mills at Qasr-e Dasht, 5 miles west of Shiraz (Fig. 1). There is little variation from the values obtained in the northern province of Azarbaijan and the eastern province of Khorasan, so that these three mills can be taken as typical. Such uniformity in performance is characteristic for machinery developed over many centuries by craftsmen to an optimal upper limit. The waterhead is 8 m. (24 feet) for one of the mills and 5 m. (15 feet) for the other two. All the mills are working in conjunction with an irrigation scheme, so that the water passes the mills before it is let by the water bailiff onto the fields. In good seasons with ample water