344 ON ROMAN AQUEDUCTS. 



was doubtless produced by casting with that which is now 

 made in a similar manner. Of the thickest kind of the latter 

 the weight is said to be lllbs. to the square foot, and we 

 have seen that Vitruvius' estimate gives a weight of 19 '2 Ibs. 

 to the square foot. Reducing this to English weights and 

 measures it would appear, speaking in round numbers, that an 

 English square foot of Roman lead would weigh about 15 Ibs. 

 avoirdupois. It is therefore considerably thicker than that 

 which is usually made in England. But it is to be under- 

 stood that the Romans may have had a thinner kind than 

 that which they were in the habit of using in the formation 

 of pipes. It has already been remarked that pipes of moderate 

 bore are now made by extension. The thickest kind of pipes 

 of an inch bore are made in lengths of 15 feet,, and weigh 56 Ibs. 

 Fifteen English feet of fistula quinaria would weigh about 68 Ibs. 

 avoirdupois. I have thought these details worthy of notice, 

 because they illustrate the economy which is the result of 

 improved modes of manufacture, and of our power of calcu- 

 lating the strength of materials. 



II. 



The most interesting consideration which the treatise of 

 Frontinus suggests relates to the state of knowledge current at 

 the time it was composed of what used to be called Hydraulics. 

 The details into which he enters are so precise (at least on some 

 points), and the provisions of the Senatus Consulta so strict, 

 that one finds it difficult to believe that he and his contempo- 

 raries were ignorant of any of the circumstances which it is 

 necessary to attend to in distributing a supply of water (neces- 

 sary, that is, unless considerations of justice are altogether to be 

 neglected) ; but then the difficulty arises, why, when so much is 

 said, so many details, not at all more likely to be generally 

 known than those mentioned, should have been omitted. 



Prony has remarked that it has been the universal practice 

 to estimate the supply of water by the quantity which in a given 

 time and under a given pressure is discharged through a pipe of 

 given bore from a reservoir whose surface is kept at a fixed 

 level. But although he conceives that this remark applies to 

 the Roman system, yet neither the text of Frontinus, nor any 

 inference which can be deduced from it, appears to justify us in 



