Polytechnic Association. 673 



Hydraulic mortars are all mortars which, in contradistinction to 

 common or lime mortar, resist the action of water. These mortars 

 were already known by the Romans, and applied by them on a very 

 extensive scale. We will only call attention to .the harbor dams of 

 Puteoli, the aqueducts of Claudius in Rome, near Segovia in Spain, 

 of Metz, in the, Eifel (between Cologne and Treves), and to the Tibur- 

 tinian gate, which, to this day, have resisted the vicissitudes of time. 

 However, when it is considered that hydraulic mortars occur in the 

 volcanic districts of South Italy, the merit of this knowledge is con- 

 siderably lessened; a mere accidental observation of the same, it 

 being perhaps mixed with sand instead of lime, may have led to its 

 application. 



Yitruvius, in the fourth chapter, second volume, of his work, 

 De Architectura, says : " There exists a kind of dust which produces 

 strange things; it is found near Baja and the Vesuvius. When 

 mixed with lime, it forms a mortar, which not only imparts great 

 strength to buildings, but also to water-works." 



The natural cement in question is a volcanic earth, mostly of an 

 ash color but sometimes yellowish and brown, which is still found in 

 the environs of Naples. At a less remote period of time, when the 

 Romans invaded the valleys of the Lower Rhine, they easily recog- 

 nized the volcanic nature of the Brohl valley. Here, among the long 

 extinct Rhenish volcanoes, they found another natural cement, the 

 trass, in such considerable quantities that the quarries which they 

 opened are still in existence. The use of hydraulic cement in ancient 

 times could therefore have only been a limited one, as it was found 

 only at the places mentioned. Its artificial preparation was not 

 understood. However, the Romans had made very fine observations 

 on the properties of this natural product. They cast, for instance, 

 immense blocks of stone from it, which they applied as ballast for 

 the erection of docks. Upon this topic, Vitruvius expresses himself, 

 in the third chapter, fifth volume, of the above named work, as 

 follows : 



" If there is a place not suited by nature to protect vessels against 

 storms, it seems that it is the most proper to throw up walls and 

 dams on one side, provided there be no river in the way, and a good 

 anchoring ground on the other side. But, if piers are to be erected 

 in the water, I proceed in getting earth from the dominion that 

 extends from Cumse to the promontory of Minerva, which I mix with 

 lime in the proportion of two to one. Boxes are then submerged in 

 the selected spot, and are united by strong piles and clamps in order 

 [Inst.] 43 



