HE same kind of wheels as those before written are also made in rivers. Around their 
fronts are affixed pinnae (4), which, when impelled by the current of the 
Fig. LXXXIV. 
river, force the wheel to revolve; and the buckets thus drawing up the water, 
discharge it at the top, without the operation of walking; the impulse of the river itself per- 
forming the whole work. 
By the same means also the hydraulae are turned; in which all the parts are the same, ex¬ 
cept that on one end of the axis they have a toothed tympanum (B) included, 
Fig. LXXXIV 
which, by being set perpendicularly on the edge, is turned equally with the 
wheel (C). Adjoining this tympanum a larger one (D), also toothed, is placed horizontally; 
(1*) Hydraulae is altered by Salmasius to hydromylae, 
water mills ; the former word may however have been 
used by the Romans, as the common name of all machines 
moved by water. 
(2*) Tympanum dentatum & inclusum.—Perrault objects 
to this phrase, as expressing that the tympanum was in¬ 
cluded within the axis, instead of the latter being included 
in the former: but it is evident Vitruvius means as he 
has expressed it; for he uses the same expression again 
in this chapter, and I imagine it is to be understood to 
signify that the tympanum is included within the extent 
of the length, not within the body, of the axis. The same 
expression is again repeated at the 14th chapter following, 
where it is evidently used in this sense. 
(3*) The words are, in cultrum, on the knife; a mode 
of expression used to signify the being situated on the side 
or edge : and a coltello is at this day used by the Italians 
in the same sense. 
(4*) Secundum id tympanum majus item, &c.—Mqjus 
(larger) is changed to minus (smaller) by Perrault and 
Galiani; for, say they, * unless this last-mentioned tym- 
panum was smaller than the former, the mill-stone which 
was adjoined to it would move slower than the water 
wheel, which may be too slow to answer the purpose, and 
is contrary to the present practice." 
B00 K . X. 
CHAPTER X. 
Of Wheels and Tympanums for grinding of Corn. 
All the manuscripts that I have examined, as well as 
the printed editions, agree in having majus, and not minus: 
and we ought to be cautious of altering the text in pas- 
sages where all the copies agree; the rather as the antient 
mills may have differed from the modern ones in this 
respect, and yet have performed their office as well. For, 
although by such a construction the mill-stone would 
be moved slower, it would however be moved easier, 
as being moved by a longer lever ; or, if the water wheel 
should have been smaller than ours, it would revolve 
quicker, and thus cause the mill-stone to revolve as 
quickly as ours does. Or, thirdly, if the mill-stone itself 
should have been larger, its velocity at the circumference 
may have been greater than that of a smaller mill-stone 
whose revolutions were quicker, and also would have 
contained a greater quantity of grain under action at the 
same time; so that the intention might be as well exe¬ 
cuted by a mill of such a construction, as by those in pre¬ 
sent use. For these reasons it appears unnecessary to follow 
the example of Perrault and Galiani in deviating from 
the text. 
Perrault has represented the mill-stone wheel as being 
composed of spindles, instead of teeth, as Vitruvius de¬ 
scribes ; because, he says, they are so at present: as it it 
were not possible for the antient mills to be made different 
from those of the moderns.