348 ON ROMAN AQUEDUCTS. 



supplied by a quinaria at about 56 cubic metres in 24 hours, 

 that is to say, at more than 1200 gallons, an estimate which 

 appears excessive, notwithstanding all we know of the magni- 

 ficence of Rome. It is about three times as much as that given 

 by Dureau de la Malle, who simply assumes, without assigning 

 any reason for doing so, that the quinaria was subject to the 

 same pressure as the Pouce de Fontainier in the French system 

 of distribution, that is, to a pressure of 7 lines on the centre. 

 The old French system of distribution (and I am not aware 

 what improvements have of late been introduced) was compara- 

 tively speaking rude and inartificial, and it is on that account 

 more likely to represent the Roman practice than the scientific 

 method to which Prony would compare the latter. No atten- 

 tion was paid to the presence or absence of an adjutage, and 

 allowance being made for the thickness of the lead, the upper 

 part of the outside surface of the pipe would be just at the sur- 

 face, a mode of placing it which is perhaps indicated by Fron- 

 tinus's phrases ad libram and ad lineam. The objection at once 

 occurs that pipes of different bores would have their centres at 

 different depths below the surface. Prony has remarked that 

 the Romans must always have subjected the centre of the orifice 

 to the same pressure (and therefore must have had a different 

 rule as to depth), because they estimated the discharge of pipes 

 of different sizes by simply comparing the area of their sections 

 with that of the quinaria. The inference would be valid if we 

 had any reason to believe that these estimates were either 

 founded on or verified by observation. In the present state of 

 our knowledge it involves a petitio principii, and it is remark- 

 able that Prony should have attached any weight to it, as he 

 admits that Frontinus estimated the product of a stream of water 

 as if it bore a constant ratio to its section. 



Several considerations may be suggested which make it 

 easier for us to believe that neither Frontinus nor any of his 

 contemporaries were sufficiently acquainted either theoretically 

 or practically with the principles of hydraulics to be able to 

 avoid enormous errors. Why, it may be said, was so much 

 care bestowed on a proper determination of the size of the pipes, 

 if other elements of the question of supply were neglected? 

 Surely they must have learned to give up making calculations 

 on which a little observation would have shown that no reliance 



