186 UNIVEKSITY OV A'IRGINIA I'UBLICATIONS 



Class 15. 



Channels in ashlar masonry not in good order. Aqueducts with floor 

 of bricks and sidewalls of good masonry. Channels whose roughness is 

 that of pebbles li/4 to 1% inches in diameter held in place with cement, 

 or of boards with laths lx% inches nailed crosswise to sides and bottom, 

 2 inches apart. Common rubble masonry. Roughly hammered stone 

 masonry. Canals, ditches and channels in earth, of uniform trapezoidal 

 section, straight alignment, ia loam or earth with fine gravel. 



Class 16. 



Channels of dry rubble masonry built of large stones, roughly but 

 well built, with rubble bed or bed of clay and gi-avel. Creeks or canals in 

 earth of very regular alignment and uniform section. Small rivers or 

 creeks with very regular grade and section, straight reach, in earth and 

 clean. 



Class 17. 



Rubble masonry irregular and somewhat damaged. Small rivers and 

 canals with side walls of rubble or paved slopes, bed of earth, sand and 

 gravel, slight detritus. Straight canals of trapezoidal cross-section with 

 stony bottom, or earth with little vegetation, sand and mud. 



Class 18. 

 Canals not in good order with vegetation. Rivers in coarse gravel. 



Class 19. 

 Canals in bad order. Rivers with coarse gravel and detritus. 



Class 20. 

 Rivers somewhat irregular, with very coarse gravel and coarse detritus. 



