ON ROMAN AQUEDUCTS. 347 



It so happens that in modern Home the unit of distribution 

 -is a pipe of an uncia in diameter, the pressure on. the depth of 

 its centre and its length being each r of a palm. 



Now, remarks Prony, although Frontinus has given us no 

 information as to the pressure on the orifice, that is, as to the 

 depth of its centre below the surface of the water in the reser- 

 voir, yet the Romans must have had some rule, and as in mo- 

 dern Rome the rule is to make the depth in question equal to 

 the length of the adjutage, let us assume that this relation ex- 

 isted also in his time. The next step is to convert Frontinus's 

 rule, that the calix must not be less than 12 digits in length, into 

 a statement that an adjutage must be employed of that precise 

 length, and then the inference follows at once that the pipes 

 were placed 12 digits below the surface. But, however inge- 

 nious this is, there are several objections to it besides the one 

 already noticed. 



In the first place, the argument from tradition is worth little 

 or nothing in the case of a matter which nowise depends on 

 popular usage, but is settled at the will and pleasure of persons 

 in authority, who are free to adopt whatever may seem to them 

 to be an improvement. In the second place, Italy has for many 

 reasons long been remarkable for the degree of attention paid to 

 the subject of hydraulics. The reasons for this are connected 

 with the physical geography of that country. Not to dwell 

 upon them, it is sufficient to remark, that in a country in which 

 so much attention has been paid to the subject there is little 

 reason to suppose that any point of modern practice is founded 

 on old tradition; and, as we have seen, the most eminent Italian 

 writers confess that they cannot discover that the Romans had 

 any rule as to the pressure on the orifice. Moreover, no one 

 knew better than Prony how vague were Frontinus's views OD 

 hydraulics. 



Quid te exempta juvat spinis de pluribus una ] 



What is gained by getting over one difficulty by an ingenious 

 assumption, while others, and especially the question as to the 

 different directions in which water is distributed, remain un- 

 touched ? 



On this hypothesis Prony estimates the quantity of water 



