THE WORK OF AIR AND WATER 



of Antipater, who is supposed to have lived in the time 

 of Cicero. But though mills driven by water were 

 introduced at this early period, yet public mills did 

 not appear till the time of Honorius and Arcadius. 

 They were erected on three canals, which conveyed 

 water to the city, and the greater number of them lay 

 under Mount Janiculum. When the Goths besieged 

 Rome in 536, and stopped the large aqueduct and con- 

 sequently the mills, Belisarius appears to have con- 

 structed, for the first time, floating mills upon the Tiber. 

 Mills driven by the tide existed at Venice in the year 

 1046, or at least in 1078. 



The older types of water wheel are exceedingly simple 

 in construction, consisting merely of vertical wheels 

 revolving on horizontal axes, and so placed as to receive 

 the weight or pressure of the water on paddles or buck- 

 ets at their circumference. The water might be al- 

 lowed to rush under the wheel, thus constituting an 

 under-shot wheel; or more commonly it flows from 

 above, constituting an over-shot wheel. Where the 

 natural fall is not available, dams are employed to 

 supply an artificial fall. 



This primitive type of water wheel has been prac- 

 tically abandoned within the last generation, its place 

 having been taken by the much more efficient type of 

 wheel known as the turbine. This consists of a wheel, 

 usually adjusted on a vertical axis, and acting on what is 

 virtually the principle of a windmill. To gain a mental 

 picture of the turbine in its simplest form, one might 

 imagine the propelling screw of a steamship, placed 

 horizontally in a tube, so that the water could rush 



