ON ROMAN AQUEDUCTS. 341 



the circumstance that the diameter, being as it appears a clear 

 internal diameter, was equal to J of a digit : and undoubtedly 

 in his time the names of a number of other pipes were formed 

 in analogy with that of quinaria in a way which indicates that 

 the magnitude of the bore was regarded as the foundation of 

 the nomenclature. Thus senaria was a pipe of six quarters bore, 

 and so on ; but it by no means follows that the received opinion 

 as to the origin of the word is true, and it seems impossible to 

 set aside the authority of Vitruvius, who speaks without doubt, 

 and must have been speaking of a matter with which he was 

 familiar : nor is there any thing improbable in the supposition 

 that in the interval between his time and that of Frontinus a 

 new nomenclature may have grown up linked with the old one 

 at one point only, namely, the word quinaria. Dureau de la 

 Malle brings an objection against the supposition that the width 

 of the lead is the cause of the name, that the circumference of 

 5 digits does not correspond to a diameter of one and a quarter, 

 adding sagely, what is by no means to any practical purpose, 

 that the problem of the rectification of the circle has hitherto 

 been found insoluble. But this objection, rightly considered, leads 

 to a curious confirmation of the statement of Vitruvius, which 

 I shall now attempt to explain. 



The diameter spoken of is the bore or lumen, whereas the 

 diameter corresponding to a circumference of 5 inches must be 

 that of a circle lying somewhere in the thickness of the lead 

 between its two surfaces, one of which, as Frontinus remarks, 

 is stretched and the other diminished, either by condensation or 

 puckering in the process of formation. If we assume, as we 

 may do without sensible error, that the lead is stretched to one 

 half of its thickness and no more, we have to calculate the 

 diameter corresponding to a circumference of five, and to di- 

 minish this by the thickness of the lead, that is, by twice the 

 half thickness, in order to get the bore. The question then is, 

 what data have we for determining the thickness of the lead 

 from which these pipes are made, and the answer to it is given 

 by a passage in Vitruvius, and which has been copied by Pliny 

 arid Palladius. 



Vitruvius, in speaking of aqueducts, says that leaden pipes 

 ought to be made in pieces of 10 feet long, their weight to vary 

 with the width of the lead of which they are made, from 1200 Ibs. 



