350 ON ROMAN AQUEDUCTS. 



the supply, lie attempted to re-measure it, and formed a new 

 estimate, greatly exceeding theirs. The supply being now in 

 excess, he explained the discrepancy by imputing frauds to the 

 aquarii. Of many they were probably guilty, but if any 

 scientific knowledge of the subject had existed in his time or 

 that of his predecessors, there could scarcely have been the 

 great difference between his estimate of the supply and theirs, 

 unless the supply varied greatly at different seasons of the year, 

 and that he and they omitted taking this variation into account. 



If I were to guess how the distribution of water was really 

 made, my conjecture would be something of this kind. The 

 distribution to private persons having at first been quite a secon- 

 dary object, their pipes were inserted so as not to receive any 

 water when the water in the reservoir fell below a certain level, 

 which was known as the libra or linea. If, therefore, the water 

 rose above this height, the grantees had the benefit of the in- 

 creased supply, whatever it may have been. When the pipe 

 first became full, the level of the water was higher than the 

 libra by the diameter of the tube, and at this state the phrase 

 implere mensuram may perhaps have been especially applied. 

 The determination of the libra was probably made on the prin- 

 ciple that when the water fell below it, the supply was observed 

 to be no more than adequate to the public purposes of the 

 aqueduct. Vitruvius speaks of a reservoir especially devoted 

 to private purposes, but it does not appear that this arrange- 

 ment was actually followed ; on the contrary, we know by the 

 practice being prohibited, that at one time private pipes were 

 placed, but also by the running streams. 



Two or three things would be explained by this hypothesis. 

 1st, The absence of any statement of the depth below the surface 

 of the water at which the pipe was inserted. This question 

 would of course not arise, if the rule were to place the pipe above 

 what was considered low-water level. 2ndly, The absence of 

 any statement in Vitruvius as to how the water in the Castella 

 was to be kept at a constant level. He certainly speaks of the 

 reservoirs overflowing that is, of two of them overflowing but 

 the third, which in his arrangement was that into which private 

 pipes were to be inserted, was fed by the other two, and from 

 the nature of things must have been fed unequally in different 

 seasons : nor, as I have said, do we know that his arrangement 



